[llvm] [BOLT] CDSplit Main Logic Part 1/3 (PR #73082)
via llvm-commits
llvm-commits at lists.llvm.org
Tue Nov 21 20:35:49 PST 2023
https://github.com/ShatianWang created https://github.com/llvm/llvm-project/pull/73082
The first diff in a series of 3 that implements the main logic of
CDSplit. Under X86, function splitting can lead to block size increase.
This is because conditional and unconditional branch instructions whose
offset is under 8 bits can be encoded with 2 bytes. If the offset is
greater than 8 bits, then they need 6 and 5 bytes respectively.
Splitting a short conditional / unconditional branch will thus increase
the size of the src basic block by 4 and 3 bytes respectively. CDSplit
takes into account the potential block size increase when it makes
splitting decisions. This diff implements a function
estimatePostSplitBBAddress in CDSplit that approximates the block level
size increase at the given split index of the given function.
>From ad9c91c2348ba426a830575946cd8748bbb4ef86 Mon Sep 17 00:00:00 2001
From: Shatian Wang <shatian at meta.com>
Date: Thu, 2 Nov 2023 12:26:49 -0700
Subject: [PATCH 1/6] [BOLT] Extend calculateEmittedSize for Block Size
Calculation
This commit modifies BinaryContext::calculateEmittedSize to update the
BinaryBasicBlock::OutputAddressRange for each basic block in the input
BF. The modification is done in place, where BB.OutputAddressRange.second
less BB.OutputAddressRange.first now gives the emitted size of the basic
block.
---
bolt/include/bolt/Core/BinaryContext.h | 3 +++
bolt/lib/Core/BinaryContext.cpp | 35 +++++++++++++++++++++-----
2 files changed, 32 insertions(+), 6 deletions(-)
diff --git a/bolt/include/bolt/Core/BinaryContext.h b/bolt/include/bolt/Core/BinaryContext.h
index ad1bf2baaeb5b1e..17e55a673e8b489 100644
--- a/bolt/include/bolt/Core/BinaryContext.h
+++ b/bolt/include/bolt/Core/BinaryContext.h
@@ -1230,6 +1230,9 @@ class BinaryContext {
///
/// Return the pair where the first size is for the main part, and the second
/// size is for the cold one.
+ /// Modify BinaryBasicBlock::OutputAddressRange for each basic block in the
+ /// function in place so that BB.OutputAddressRange.second less
+ /// BB.OutputAddressRange.first gives the emitted size of BB.
std::pair<size_t, size_t> calculateEmittedSize(BinaryFunction &BF,
bool FixBranches = true);
diff --git a/bolt/lib/Core/BinaryContext.cpp b/bolt/lib/Core/BinaryContext.cpp
index 06b68765909d20e..baf86333ce53206 100644
--- a/bolt/lib/Core/BinaryContext.cpp
+++ b/bolt/lib/Core/BinaryContext.cpp
@@ -2331,14 +2331,37 @@ BinaryContext::calculateEmittedSize(BinaryFunction &BF, bool FixBranches) {
MCAsmLayout Layout(Assembler);
Assembler.layout(Layout);
+ // Obtain fragment sizes.
+ std::vector<uint64_t> FragmentSizes(BF.getLayout().fragment_size());
+ // Main fragment size.
const uint64_t HotSize =
Layout.getSymbolOffset(*EndLabel) - Layout.getSymbolOffset(*StartLabel);
- const uint64_t ColdSize =
- std::accumulate(SplitLabels.begin(), SplitLabels.end(), 0ULL,
- [&](const uint64_t Accu, const LabelRange &Labels) {
- return Accu + Layout.getSymbolOffset(*Labels.second) -
- Layout.getSymbolOffset(*Labels.first);
- });
+ FragmentSizes.push_back(HotSize);
+ // Split fragment sizes.
+ uint64_t ColdSize = 0;
+ for (const auto &Labels : SplitLabels) {
+ uint64_t Size = Layout.getSymbolOffset(*Labels.second) -
+ Layout.getSymbolOffset(*Labels.first);
+ FragmentSizes.push_back(Size);
+ ColdSize += Size;
+ }
+
+ // Populate new start and end offsets of each basic block.
+ BinaryBasicBlock *PrevBB = nullptr;
+ uint64_t FragmentIndex = 0;
+ for (FunctionFragment &FF : BF.getLayout().fragments()) {
+ for (BinaryBasicBlock *BB : FF) {
+ const uint64_t BBStartOffset = Layout.getSymbolOffset(*(BB->getLabel()));
+ BB->setOutputStartAddress(BBStartOffset);
+ if (PrevBB)
+ PrevBB->setOutputEndAddress(BBStartOffset);
+ PrevBB = BB;
+ }
+ if (PrevBB)
+ PrevBB->setOutputEndAddress(FragmentSizes[FragmentIndex]);
+ FragmentIndex++;
+ PrevBB = nullptr;
+ }
// Clean-up the effect of the code emission.
for (const MCSymbol &Symbol : Assembler.symbols()) {
>From 273ee295a2bbc09a1500c867d1156ea4d29ead40 Mon Sep 17 00:00:00 2001
From: Shatian Wang <shatian at meta.com>
Date: Wed, 8 Nov 2023 08:20:11 -0800
Subject: [PATCH 2/6] [BOLT] Refactor SplitFunctions for Function Reuse
This commit updates SplitFunctions.h and SplitFunctions.cpp to enable
the reuse of createEHTrampolines, mergeEHTrampolines, hasFullProfile,
and allBlocksCold by a distinct function splitting pass (CDSplit).
---
bolt/include/bolt/Core/BinaryFunction.h | 14 ++++++++++
bolt/include/bolt/Passes/SplitFunctions.h | 32 +++++++++++------------
bolt/lib/Passes/SplitFunctions.cpp | 18 +++----------
3 files changed, 33 insertions(+), 31 deletions(-)
diff --git a/bolt/include/bolt/Core/BinaryFunction.h b/bolt/include/bolt/Core/BinaryFunction.h
index 72c360ca0c2db66..3723cccc50f040c 100644
--- a/bolt/include/bolt/Core/BinaryFunction.h
+++ b/bolt/include/bolt/Core/BinaryFunction.h
@@ -1272,6 +1272,20 @@ class BinaryFunction {
/// otherwise processed.
bool isPseudo() const { return IsPseudo; }
+ /// Return true if every block in the function has a valid execution count.
+ bool hasFullProfile() const {
+ return llvm::all_of(blocks(), [](const BinaryBasicBlock &BB) {
+ return BB.getExecutionCount() != BinaryBasicBlock::COUNT_NO_PROFILE;
+ });
+ }
+
+ /// Return true if every block in the function has a zero execution count.
+ bool allBlocksCold() const {
+ return llvm::all_of(blocks(), [](const BinaryBasicBlock &BB) {
+ return BB.getExecutionCount() == 0;
+ });
+ }
+
/// Return true if the function contains explicit or implicit indirect branch
/// to its split fragments, e.g., split jump table, landing pad in split
/// fragment.
diff --git a/bolt/include/bolt/Passes/SplitFunctions.h b/bolt/include/bolt/Passes/SplitFunctions.h
index 4058f3317dfbdbb..91b6d5518eaab26 100644
--- a/bolt/include/bolt/Passes/SplitFunctions.h
+++ b/bolt/include/bolt/Passes/SplitFunctions.h
@@ -50,6 +50,19 @@ class SplitFunctions : public BinaryFunctionPass {
/// Split function body into fragments.
void splitFunction(BinaryFunction &Function, SplitStrategy &Strategy);
+ std::atomic<uint64_t> SplitBytesHot{0ull};
+ std::atomic<uint64_t> SplitBytesCold{0ull};
+
+public:
+ explicit SplitFunctions(const cl::opt<bool> &PrintPass)
+ : BinaryFunctionPass(PrintPass) {}
+
+ bool shouldOptimize(const BinaryFunction &BF) const override;
+
+ const char *getName() const override { return "split-functions"; }
+
+ void runOnFunctions(BinaryContext &BC) override;
+
struct TrampolineKey {
FragmentNum SourceFN = FragmentNum::main();
const MCSymbol *Target = nullptr;
@@ -81,27 +94,14 @@ class SplitFunctions : public BinaryFunctionPass {
/// corresponding thrower block. The trampoline landing pad, when created,
/// will redirect the execution to the real landing pad in a different
/// fragment.
- TrampolineSetType createEHTrampolines(BinaryFunction &Function) const;
+ static TrampolineSetType createEHTrampolines(BinaryFunction &Function);
/// Merge trampolines into \p Layout without trampolines. The merge will place
/// a trampoline immediately before its destination. Used to revert the effect
/// of trampolines after createEHTrampolines().
- BasicBlockOrderType
+ static BasicBlockOrderType
mergeEHTrampolines(BinaryFunction &BF, BasicBlockOrderType &Layout,
- const TrampolineSetType &Trampolines) const;
-
- std::atomic<uint64_t> SplitBytesHot{0ull};
- std::atomic<uint64_t> SplitBytesCold{0ull};
-
-public:
- explicit SplitFunctions(const cl::opt<bool> &PrintPass)
- : BinaryFunctionPass(PrintPass) {}
-
- bool shouldOptimize(const BinaryFunction &BF) const override;
-
- const char *getName() const override { return "split-functions"; }
-
- void runOnFunctions(BinaryContext &BC) override;
+ const TrampolineSetType &Trampolines);
};
} // namespace bolt
diff --git a/bolt/lib/Passes/SplitFunctions.cpp b/bolt/lib/Passes/SplitFunctions.cpp
index 34973cecdf49161..223f8d17367845d 100644
--- a/bolt/lib/Passes/SplitFunctions.cpp
+++ b/bolt/lib/Passes/SplitFunctions.cpp
@@ -109,21 +109,9 @@ static cl::opt<SplitFunctionsStrategy> SplitStrategy(
} // namespace opts
namespace {
-bool hasFullProfile(const BinaryFunction &BF) {
- return llvm::all_of(BF.blocks(), [](const BinaryBasicBlock &BB) {
- return BB.getExecutionCount() != BinaryBasicBlock::COUNT_NO_PROFILE;
- });
-}
-
-bool allBlocksCold(const BinaryFunction &BF) {
- return llvm::all_of(BF.blocks(), [](const BinaryBasicBlock &BB) {
- return BB.getExecutionCount() == 0;
- });
-}
-
struct SplitProfile2 final : public SplitStrategy {
bool canSplit(const BinaryFunction &BF) override {
- return BF.hasValidProfile() && hasFullProfile(BF) && !allBlocksCold(BF);
+ return BF.hasValidProfile() && BF.hasFullProfile() && !BF.allBlocksCold();
}
bool keepEmpty() override { return false; }
@@ -434,7 +422,7 @@ void SplitFunctions::splitFunction(BinaryFunction &BF, SplitStrategy &S) {
}
SplitFunctions::TrampolineSetType
-SplitFunctions::createEHTrampolines(BinaryFunction &BF) const {
+SplitFunctions::createEHTrampolines(BinaryFunction &BF) {
const auto &MIB = BF.getBinaryContext().MIB;
// Map real landing pads to the corresponding trampolines.
@@ -501,7 +489,7 @@ SplitFunctions::createEHTrampolines(BinaryFunction &BF) const {
SplitFunctions::BasicBlockOrderType SplitFunctions::mergeEHTrampolines(
BinaryFunction &BF, SplitFunctions::BasicBlockOrderType &Layout,
- const SplitFunctions::TrampolineSetType &Trampolines) const {
+ const SplitFunctions::TrampolineSetType &Trampolines) {
DenseMap<const MCSymbol *, SmallVector<const MCSymbol *, 0>>
IncomingTrampolines;
for (const auto &Entry : Trampolines) {
>From 2215673c1d18aca198c095f6acbc251f1af0bd97 Mon Sep 17 00:00:00 2001
From: Shatian Wang <shatian at meta.com>
Date: Thu, 2 Nov 2023 20:51:52 -0700
Subject: [PATCH 3/6] [BOLT] Setup CDSplit Pass Structure
This commit establishes the general structure of the CDSplit
implementation without incorporating the exact splitting logic.
Currently, all functions undergo hot-cold splitting based on the
decisions made by the SplitFunctions pass. Subsequent commits
will introduce the precise splitting logic.
---
bolt/include/bolt/Passes/CDSplit.h | 63 ++++++++
bolt/lib/Passes/CDSplit.cpp | 208 +++++++++++++++++++++++++
bolt/lib/Passes/CMakeLists.txt | 1 +
bolt/lib/Passes/SplitFunctions.cpp | 12 ++
bolt/lib/Rewrite/BinaryPassManager.cpp | 10 ++
bolt/lib/Utils/CommandLineOpts.cpp | 6 +
6 files changed, 300 insertions(+)
create mode 100644 bolt/include/bolt/Passes/CDSplit.h
create mode 100644 bolt/lib/Passes/CDSplit.cpp
diff --git a/bolt/include/bolt/Passes/CDSplit.h b/bolt/include/bolt/Passes/CDSplit.h
new file mode 100644
index 000000000000000..96a982683a7ec26
--- /dev/null
+++ b/bolt/include/bolt/Passes/CDSplit.h
@@ -0,0 +1,63 @@
+//===- bolt/Passes/CDSplit.h - Split functions into hot/warm/cold
+// after function reordering pass -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef BOLT_PASSES_CDSPLIT
+#define BOLT_PASSES_CDSPLIT
+
+#include "bolt/Passes/SplitFunctions.h"
+#include <atomic>
+
+namespace llvm {
+namespace bolt {
+
+using BasicBlockOrder = BinaryFunction::BasicBlockOrderType;
+
+class CDSplit : public BinaryFunctionPass {
+private:
+ /// Overall stats.
+ std::atomic<uint64_t> SplitBytesHot{0ull};
+ std::atomic<uint64_t> SplitBytesCold{0ull};
+
+ /// List of functions to be considered.
+ /// All functions in the list are used to construct a call graph.
+ /// A subset of functions in this list are considered for splitting.
+ std::vector<BinaryFunction *> FunctionsToConsider;
+
+ /// Helper functions to initialize global variables.
+ void initialize(BinaryContext &BC);
+
+ /// Split function body into 3 fragments: hot / warm / cold.
+ void runOnFunction(BinaryFunction &BF);
+
+ /// Assign each basic block in the given function to either hot, cold,
+ /// or warm fragment using the CDSplit algorithm.
+ void assignFragmentThreeWay(const BinaryFunction &BF,
+ const BasicBlockOrder &BlockOrder);
+
+ /// Find the best split index that separates hot from warm.
+ /// The basic block whose index equals the returned split index will be the
+ /// last hot block.
+ size_t findSplitIndex(const BinaryFunction &BF,
+ const BasicBlockOrder &BlockOrder);
+
+public:
+ explicit CDSplit(const cl::opt<bool> &PrintPass)
+ : BinaryFunctionPass(PrintPass) {}
+
+ bool shouldOptimize(const BinaryFunction &BF) const override;
+
+ const char *getName() const override { return "cdsplit"; }
+
+ void runOnFunctions(BinaryContext &BC) override;
+};
+
+} // namespace bolt
+} // namespace llvm
+
+#endif
diff --git a/bolt/lib/Passes/CDSplit.cpp b/bolt/lib/Passes/CDSplit.cpp
new file mode 100644
index 000000000000000..cd67b24241a4249
--- /dev/null
+++ b/bolt/lib/Passes/CDSplit.cpp
@@ -0,0 +1,208 @@
+//===- bolt/Passes/CDSplit.cpp - Pass for splitting function code 3-way
+//--===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the CDSplit pass.
+//
+//===----------------------------------------------------------------------===//
+
+#include "bolt/Passes/CDSplit.h"
+#include "bolt/Core/ParallelUtilities.h"
+#include "bolt/Utils/CommandLineOpts.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/MC/MCInst.h"
+#include "llvm/Support/MathExtras.h"
+
+#define DEBUG_TYPE "bolt-opts"
+
+using namespace llvm;
+using namespace bolt;
+
+namespace opts {
+
+extern cl::OptionCategory BoltOptCategory;
+
+extern cl::opt<bool> UseCDSplit;
+extern cl::opt<bool> SplitEH;
+extern cl::opt<unsigned> ExecutionCountThreshold;
+} // namespace opts
+
+namespace llvm {
+namespace bolt {
+
+namespace {
+/// Return true if the function should be considered for building call graph.
+bool shouldConsider(const BinaryFunction &BF) {
+ return BF.hasValidIndex() && BF.hasValidProfile() && !BF.empty();
+}
+} // anonymous namespace
+
+bool CDSplit::shouldOptimize(const BinaryFunction &BF) const {
+ // Do not split functions with a small execution count.
+ if (BF.getKnownExecutionCount() < opts::ExecutionCountThreshold)
+ return false;
+
+ // Do not split functions with at least one block that has no known
+ // execution count due to incomplete information.
+ // Do not split functions with only zero-execution count blocks
+ // as there is not enough variation in block count to justify splitting.
+ if (!BF.hasFullProfile() || BF.allBlocksCold())
+ return false;
+
+ return BinaryFunctionPass::shouldOptimize(BF);
+}
+
+/// Initialize algorithm's metadata.
+void CDSplit::initialize(BinaryContext &BC) {
+ // Construct a list of functions that are considered for building call graph.
+ // Only those in this list that evaluates true for shouldOptimize are
+ // candidates for 3-way splitting.
+ std::vector<BinaryFunction *> SortedFunctions = BC.getSortedFunctions();
+ FunctionsToConsider.reserve(SortedFunctions.size());
+ for (BinaryFunction *BF : SortedFunctions) {
+ if (shouldConsider(*BF))
+ FunctionsToConsider.push_back(BF);
+ }
+}
+
+/// Find the best index for splitting. The returned value is the index of the
+/// last hot basic block. Hence, "no splitting" is equivalent to returning the
+/// value which is one less than the size of the function.
+size_t CDSplit::findSplitIndex(const BinaryFunction &BF,
+ const BasicBlockOrder &BlockOrder) {
+ // Placeholder: hot-cold splitting.
+ return BF.getLayout().getMainFragment().size() - 1;
+}
+
+/// Assign each basic block in the given function to either hot, cold,
+/// or warm fragment using the CDSplit algorithm.
+void CDSplit::assignFragmentThreeWay(const BinaryFunction &BF,
+ const BasicBlockOrder &BlockOrder) {
+ size_t BestSplitIndex = findSplitIndex(BF, BlockOrder);
+
+ // Assign fragments based on the computed best split index.
+ // All basic blocks with index up to the best split index become hot.
+ // All remaining blocks are warm / cold depending on if count is
+ // greater than 0 or not.
+ FragmentNum Main(0);
+ FragmentNum Warm(1);
+ FragmentNum Cold(2);
+ for (size_t Index = 0; Index < BlockOrder.size(); Index++) {
+ BinaryBasicBlock *BB = BlockOrder[Index];
+ if (Index <= BestSplitIndex)
+ BB->setFragmentNum(Main);
+ else
+ BB->setFragmentNum(BB->getKnownExecutionCount() > 0 ? Warm : Cold);
+ }
+}
+
+void CDSplit::runOnFunction(BinaryFunction &BF) {
+ assert(!BF.empty() && "splitting an empty function");
+
+ FunctionLayout &Layout = BF.getLayout();
+ BinaryContext &BC = BF.getBinaryContext();
+
+ BasicBlockOrder NewLayout(Layout.block_begin(), Layout.block_end());
+ // Never outline the first basic block.
+ NewLayout.front()->setCanOutline(false);
+ for (BinaryBasicBlock *BB : NewLayout) {
+ if (!BB->canOutline())
+ continue;
+
+ // Do not split extra entry points in aarch64. They can be referred by
+ // using ADRs and when this happens, these blocks cannot be placed far
+ // away due to the limited range in ADR instruction.
+ if (BC.isAArch64() && BB->isEntryPoint()) {
+ BB->setCanOutline(false);
+ continue;
+ }
+
+ if (BF.hasEHRanges() && !opts::SplitEH) {
+ // We cannot move landing pads (or rather entry points for landing pads).
+ if (BB->isLandingPad()) {
+ BB->setCanOutline(false);
+ continue;
+ }
+ // We cannot move a block that can throw since exception-handling
+ // runtime cannot deal with split functions. However, if we can guarantee
+ // that the block never throws, it is safe to move the block to
+ // decrease the size of the function.
+ for (MCInst &Instr : *BB) {
+ if (BC.MIB->isInvoke(Instr)) {
+ BB->setCanOutline(false);
+ break;
+ }
+ }
+ }
+ }
+
+ // Assign each basic block in NewLayout to either hot, warm, or cold fragment.
+ assignFragmentThreeWay(BF, NewLayout);
+
+ // Make sure all non-outlineable blocks are in the main-fragment.
+ for (BinaryBasicBlock *BB : NewLayout) {
+ if (!BB->canOutline())
+ BB->setFragmentNum(FragmentNum::main());
+ }
+
+ // In case any non-outlineable blocks previously in warm or cold is now set
+ // to be in main by the preceding for loop, move them to the end of main.
+ llvm::stable_sort(NewLayout,
+ [&](const BinaryBasicBlock *L, const BinaryBasicBlock *R) {
+ return L->getFragmentNum() < R->getFragmentNum();
+ });
+
+ BF.getLayout().update(NewLayout);
+
+ // For shared objects, invoke instructions and corresponding landing pads
+ // have to be placed in the same fragment. When we split them, create
+ // trampoline landing pads that will redirect the execution to real LPs.
+ SplitFunctions::TrampolineSetType Trampolines;
+ if (!BC.HasFixedLoadAddress && BF.hasEHRanges() && BF.isSplit())
+ Trampolines = SplitFunctions::createEHTrampolines(BF);
+
+ if (BC.isX86() && BF.isSplit()) {
+ size_t HotSize;
+ size_t ColdSize;
+ std::tie(HotSize, ColdSize) = BC.calculateEmittedSize(BF);
+ SplitBytesHot += HotSize;
+ SplitBytesCold += ColdSize;
+ }
+}
+
+void CDSplit::runOnFunctions(BinaryContext &BC) {
+ if (!opts::UseCDSplit)
+ return;
+
+ // Initialize global variables.
+ initialize(BC);
+
+ // Only functions satisfying shouldConsider and shouldOptimize are candidates
+ // for splitting.
+ ParallelUtilities::PredicateTy SkipFunc = [&](const BinaryFunction &BF) {
+ return !(shouldConsider(BF) && shouldOptimize(BF));
+ };
+
+ // Make function splitting decisions in parallel.
+ ParallelUtilities::runOnEachFunction(
+ BC, ParallelUtilities::SchedulingPolicy::SP_BB_LINEAR,
+ [&](BinaryFunction &BF) { runOnFunction(BF); }, SkipFunc, "CDSplit",
+ /*ForceSequential=*/false);
+
+ if (SplitBytesHot + SplitBytesCold > 0) {
+ outs() << "BOLT-INFO: cdsplit separates " << SplitBytesHot
+ << " hot bytes from " << SplitBytesCold << " cold bytes "
+ << format("(%.2lf%% of split functions is in the main fragment)\n",
+ 100.0 * SplitBytesHot / (SplitBytesHot + SplitBytesCold));
+
+ } else
+ outs() << "BOLT-INFO: cdsplit didn't split any functions\n";
+}
+
+} // namespace bolt
+} // namespace llvm
diff --git a/bolt/lib/Passes/CMakeLists.txt b/bolt/lib/Passes/CMakeLists.txt
index b8bbe59a64480c9..4cc4b4fa6ae345c 100644
--- a/bolt/lib/Passes/CMakeLists.txt
+++ b/bolt/lib/Passes/CMakeLists.txt
@@ -9,6 +9,7 @@ add_llvm_library(LLVMBOLTPasses
CacheMetrics.cpp
CallGraph.cpp
CallGraphWalker.cpp
+ CDSplit.cpp
DataflowAnalysis.cpp
DataflowInfoManager.cpp
FrameAnalysis.cpp
diff --git a/bolt/lib/Passes/SplitFunctions.cpp b/bolt/lib/Passes/SplitFunctions.cpp
index 223f8d17367845d..0c11d0fb49cd09c 100644
--- a/bolt/lib/Passes/SplitFunctions.cpp
+++ b/bolt/lib/Passes/SplitFunctions.cpp
@@ -60,6 +60,7 @@ extern cl::OptionCategory BoltOptCategory;
extern cl::opt<bool> SplitEH;
extern cl::opt<unsigned> ExecutionCountThreshold;
extern cl::opt<uint32_t> RandomSeed;
+extern cl::opt<bool> UseCDSplit;
static cl::opt<bool> AggressiveSplitting(
"split-all-cold", cl::desc("outline as many cold basic blocks as possible"),
@@ -231,6 +232,17 @@ bool SplitFunctions::shouldOptimize(const BinaryFunction &BF) const {
}
void SplitFunctions::runOnFunctions(BinaryContext &BC) {
+ if (opts::UseCDSplit &&
+ !(opts::SplitFunctions &&
+ opts::SplitStrategy == SplitFunctionsStrategy::Profile2)) {
+ errs() << "BOLT-ERROR: -use-cdsplit should be applied together with "
+ "-split-functions using default -split-strategy=profile2. "
+ "-split-functions 2-way splits functions before the function "
+ "reordering pass, while -use-cdsplit 3-way splits functions "
+ "after the function reordering pass. \n";
+ exit(1);
+ }
+
if (!opts::SplitFunctions)
return;
diff --git a/bolt/lib/Rewrite/BinaryPassManager.cpp b/bolt/lib/Rewrite/BinaryPassManager.cpp
index 37de3eabc6d235d..28983de11c3ae07 100644
--- a/bolt/lib/Rewrite/BinaryPassManager.cpp
+++ b/bolt/lib/Rewrite/BinaryPassManager.cpp
@@ -11,6 +11,7 @@
#include "bolt/Passes/Aligner.h"
#include "bolt/Passes/AllocCombiner.h"
#include "bolt/Passes/AsmDump.h"
+#include "bolt/Passes/CDSplit.h"
#include "bolt/Passes/CMOVConversion.h"
#include "bolt/Passes/FixRISCVCallsPass.h"
#include "bolt/Passes/FixRelaxationPass.h"
@@ -182,6 +183,10 @@ static cl::opt<bool>
PrintSplit("print-split", cl::desc("print functions after code splitting"),
cl::Hidden, cl::cat(BoltOptCategory));
+static cl::opt<bool> PrintCDSplit("print-cdsplit",
+ cl::desc("print functions after cdsplit"),
+ cl::Hidden, cl::cat(BoltOptCategory));
+
static cl::opt<bool>
PrintStoke("print-stoke", cl::desc("print functions after stoke analysis"),
cl::Hidden, cl::cat(BoltOptCategory));
@@ -430,6 +435,11 @@ void BinaryFunctionPassManager::runAllPasses(BinaryContext &BC) {
Manager.registerPass(
std::make_unique<ReorderFunctions>(PrintReorderedFunctions));
+ /// This pass three-way splits functions after function reordering.
+ Manager.registerPass(std::make_unique<CDSplit>(PrintCDSplit));
+
+ Manager.registerPass(std::make_unique<FixupBranches>(PrintAfterBranchFixup));
+
// Print final dyno stats right while CFG and instruction analysis are intact.
Manager.registerPass(
std::make_unique<DynoStatsPrintPass>(
diff --git a/bolt/lib/Utils/CommandLineOpts.cpp b/bolt/lib/Utils/CommandLineOpts.cpp
index a1df5de26234029..75d63e369c731e4 100644
--- a/bolt/lib/Utils/CommandLineOpts.cpp
+++ b/bolt/lib/Utils/CommandLineOpts.cpp
@@ -191,6 +191,12 @@ cl::opt<unsigned>
cl::init(0), cl::ZeroOrMore, cl::cat(BoltCategory),
cl::sub(cl::SubCommand::getAll()));
+cl::opt<bool>
+ UseCDSplit("use-cdsplit",
+ cl::desc("split functions into 3 fragments using the CDSplit "
+ "algorithm after function reordering pass"),
+ cl::init(false), cl::cat(BoltOptCategory));
+
bool processAllFunctions() {
if (opts::AggregateOnly)
return false;
>From af930a0da876b280c969bb9ea512a256c4439d4e Mon Sep 17 00:00:00 2001
From: Shatian Wang <shatian at meta.com>
Date: Tue, 31 Oct 2023 14:57:00 -0700
Subject: [PATCH 4/6] [BOLT] Introduce .text.warm for -use-cdsplit=1
This commit explicitly adds a warm code section, .text.warm, when the
-use-cdsplit=1 flag is set. This replaces the previous approach of using
.text.cold.0 as warm and .text.cold.1 as cold in 3-way splitting.
---
bolt/include/bolt/Core/BinaryBasicBlock.h | 4 +--
bolt/include/bolt/Core/BinaryContext.h | 2 ++
bolt/include/bolt/Core/BinaryFunction.h | 16 ++++++----
bolt/include/bolt/Core/FunctionLayout.h | 5 ++-
bolt/lib/Core/BinaryBasicBlock.cpp | 19 +++++++----
bolt/lib/Core/BinaryEmitter.cpp | 6 +++-
bolt/lib/Core/BinaryFunction.cpp | 13 ++++++++
bolt/lib/Passes/BinaryPasses.cpp | 4 ++-
bolt/lib/Passes/CDSplit.cpp | 1 -
bolt/lib/Passes/IndirectCallPromotion.cpp | 6 ++--
bolt/lib/Passes/SplitFunctions.cpp | 6 ++--
bolt/lib/Rewrite/RewriteInstance.cpp | 39 ++++++++++++++++-------
12 files changed, 84 insertions(+), 37 deletions(-)
diff --git a/bolt/include/bolt/Core/BinaryBasicBlock.h b/bolt/include/bolt/Core/BinaryBasicBlock.h
index bc95e2c4de3a11e..32e48b964d73933 100644
--- a/bolt/include/bolt/Core/BinaryBasicBlock.h
+++ b/bolt/include/bolt/Core/BinaryBasicBlock.h
@@ -677,9 +677,7 @@ class BinaryBasicBlock {
return isSplit();
}
- void setIsCold(const bool Flag) {
- Fragment = Flag ? FragmentNum::cold() : FragmentNum::main();
- }
+ void setIsCold(const bool Flag);
/// Return true if the block can be outlined. At the moment we disallow
/// outlining of blocks that can potentially throw exceptions or are
diff --git a/bolt/include/bolt/Core/BinaryContext.h b/bolt/include/bolt/Core/BinaryContext.h
index 17e55a673e8b489..80cbcc37d2a2184 100644
--- a/bolt/include/bolt/Core/BinaryContext.h
+++ b/bolt/include/bolt/Core/BinaryContext.h
@@ -927,6 +927,8 @@ class BinaryContext {
const char *getMainCodeSectionName() const { return ".text"; }
+ const char *getWarmCodeSectionName() const { return ".text.warm"; }
+
const char *getColdCodeSectionName() const { return ".text.cold"; }
const char *getHotTextMoverSectionName() const { return ".text.mover"; }
diff --git a/bolt/include/bolt/Core/BinaryFunction.h b/bolt/include/bolt/Core/BinaryFunction.h
index 3723cccc50f040c..31677fabae1d93a 100644
--- a/bolt/include/bolt/Core/BinaryFunction.h
+++ b/bolt/include/bolt/Core/BinaryFunction.h
@@ -355,6 +355,9 @@ class BinaryFunction {
/// Name for the section this function code should reside in.
std::string CodeSectionName;
+ /// Name for the corresponding warm code section.
+ std::string WarmCodeSectionName;
+
/// Name for the corresponding cold code section.
std::string ColdCodeSectionName;
@@ -1231,13 +1234,7 @@ class BinaryFunction {
/// Return internal section name for this function.
SmallString<32>
- getCodeSectionName(const FragmentNum Fragment = FragmentNum::main()) const {
- if (Fragment == FragmentNum::main())
- return SmallString<32>(CodeSectionName);
- if (Fragment == FragmentNum::cold())
- return SmallString<32>(ColdCodeSectionName);
- return formatv("{0}.{1}", ColdCodeSectionName, Fragment.get() - 1);
- }
+ getCodeSectionName(const FragmentNum Fragment = FragmentNum::main()) const;
/// Assign a code section name to the function.
void setCodeSectionName(const StringRef Name) {
@@ -1250,6 +1247,11 @@ class BinaryFunction {
return BC.getUniqueSectionByName(getCodeSectionName(Fragment));
}
+ /// Assign a section name for the warm part of the function.
+ void setWarmCodeSectionName(const StringRef Name) {
+ WarmCodeSectionName = Name.str();
+ }
+
/// Assign a section name for the cold part of the function.
void setColdCodeSectionName(const StringRef Name) {
ColdCodeSectionName = Name.str();
diff --git a/bolt/include/bolt/Core/FunctionLayout.h b/bolt/include/bolt/Core/FunctionLayout.h
index 904da3a4a93aade..94d71a84aae8334 100644
--- a/bolt/include/bolt/Core/FunctionLayout.h
+++ b/bolt/include/bolt/Core/FunctionLayout.h
@@ -62,7 +62,10 @@ class FragmentNum {
}
static constexpr FragmentNum main() { return FragmentNum(0); }
- static constexpr FragmentNum cold() { return FragmentNum(1); }
+ static constexpr FragmentNum warm() { return FragmentNum(1); }
+ static constexpr FragmentNum cold(bool Flag = false) {
+ return FragmentNum(Flag ? 2 : 1);
+ }
};
/// A freestanding subset of contiguous blocks of a function.
diff --git a/bolt/lib/Core/BinaryBasicBlock.cpp b/bolt/lib/Core/BinaryBasicBlock.cpp
index 984bc6dbd220ab0..40a655c0cd24b3a 100644
--- a/bolt/lib/Core/BinaryBasicBlock.cpp
+++ b/bolt/lib/Core/BinaryBasicBlock.cpp
@@ -15,17 +15,25 @@
#include "bolt/Core/BinaryFunction.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/MC/MCInst.h"
+#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Errc.h"
#define DEBUG_TYPE "bolt"
-namespace llvm {
-namespace bolt {
+using namespace llvm;
+using namespace bolt;
+namespace opts {
+extern cl::opt<bool> UseCDSplit;
+}
constexpr uint32_t BinaryBasicBlock::INVALID_OFFSET;
-bool operator<(const BinaryBasicBlock &LHS, const BinaryBasicBlock &RHS) {
- return LHS.Index < RHS.Index;
+bool bolt::operator<(const BinaryBasicBlock &LHS, const BinaryBasicBlock &RHS) {
+ return LHS.getIndex() < RHS.getIndex();
+}
+
+void BinaryBasicBlock::setIsCold(const bool Flag) {
+ Fragment = Flag ? FragmentNum::cold(opts::UseCDSplit) : FragmentNum::main();
}
bool BinaryBasicBlock::hasCFG() const { return getParent()->hasCFG(); }
@@ -611,6 +619,3 @@ BinaryBasicBlock *BinaryBasicBlock::splitAt(iterator II) {
return NewBlock;
}
-
-} // namespace bolt
-} // namespace llvm
diff --git a/bolt/lib/Core/BinaryEmitter.cpp b/bolt/lib/Core/BinaryEmitter.cpp
index fb1bf530c1974aa..9829c6bc107f03a 100644
--- a/bolt/lib/Core/BinaryEmitter.cpp
+++ b/bolt/lib/Core/BinaryEmitter.cpp
@@ -34,6 +34,7 @@ namespace opts {
extern cl::opt<JumpTableSupportLevel> JumpTables;
extern cl::opt<bool> PreserveBlocksAlignment;
+extern cl::opt<bool> UseCDSplit;
cl::opt<bool> AlignBlocks("align-blocks", cl::desc("align basic blocks"),
cl::cat(BoltOptCategory));
@@ -287,7 +288,10 @@ void BinaryEmitter::emitFunctions() {
// Mark the end of hot text.
if (opts::HotText) {
- Streamer.switchSection(BC.getTextSection());
+ if (opts::UseCDSplit)
+ Streamer.switchSection(BC.getCodeSection(BC.getWarmCodeSectionName()));
+ else
+ Streamer.switchSection(BC.getTextSection());
Streamer.emitLabel(BC.getHotTextEndSymbol());
}
}
diff --git a/bolt/lib/Core/BinaryFunction.cpp b/bolt/lib/Core/BinaryFunction.cpp
index e81d58ef0b1047b..0b89be26def39df 100644
--- a/bolt/lib/Core/BinaryFunction.cpp
+++ b/bolt/lib/Core/BinaryFunction.cpp
@@ -59,6 +59,7 @@ extern cl::opt<bool> EnableBAT;
extern cl::opt<bool> Instrument;
extern cl::opt<bool> StrictMode;
extern cl::opt<bool> UpdateDebugSections;
+extern cl::opt<bool> UseCDSplit;
extern cl::opt<unsigned> Verbosity;
extern bool processAllFunctions();
@@ -167,6 +168,18 @@ template <typename R> static bool emptyRange(const R &Range) {
return Range.begin() == Range.end();
}
+/// Return internal section name for this function.
+SmallString<32>
+BinaryFunction::getCodeSectionName(const FragmentNum Fragment) const {
+ if (Fragment == FragmentNum::main())
+ return SmallString<32>(CodeSectionName);
+ if (Fragment == FragmentNum::cold(opts::UseCDSplit))
+ return SmallString<32>(ColdCodeSectionName);
+ if (Fragment == FragmentNum::warm())
+ return SmallString<32>(WarmCodeSectionName);
+ return formatv("{0}.{1}", ColdCodeSectionName, Fragment.get() - 1);
+}
+
/// Gets debug line information for the instruction located at the given
/// address in the original binary. The SMLoc's pointer is used
/// to point to this information, which is represented by a
diff --git a/bolt/lib/Passes/BinaryPasses.cpp b/bolt/lib/Passes/BinaryPasses.cpp
index 4e1343e2c30be56..c60a03d3d984720 100644
--- a/bolt/lib/Passes/BinaryPasses.cpp
+++ b/bolt/lib/Passes/BinaryPasses.cpp
@@ -1244,8 +1244,10 @@ void AssignSections::runOnFunctions(BinaryContext &BC) {
else
Function.setCodeSectionName(BC.getColdCodeSectionName());
- if (Function.isSplit())
+ if (Function.isSplit()) {
+ Function.setWarmCodeSectionName(BC.getWarmCodeSectionName());
Function.setColdCodeSectionName(BC.getColdCodeSectionName());
+ }
}
}
diff --git a/bolt/lib/Passes/CDSplit.cpp b/bolt/lib/Passes/CDSplit.cpp
index cd67b24241a4249..509d250c3419183 100644
--- a/bolt/lib/Passes/CDSplit.cpp
+++ b/bolt/lib/Passes/CDSplit.cpp
@@ -24,7 +24,6 @@ using namespace llvm;
using namespace bolt;
namespace opts {
-
extern cl::OptionCategory BoltOptCategory;
extern cl::opt<bool> UseCDSplit;
diff --git a/bolt/lib/Passes/IndirectCallPromotion.cpp b/bolt/lib/Passes/IndirectCallPromotion.cpp
index 8b084c3b63d565a..a46df74d7fd7bbc 100644
--- a/bolt/lib/Passes/IndirectCallPromotion.cpp
+++ b/bolt/lib/Passes/IndirectCallPromotion.cpp
@@ -34,6 +34,7 @@ extern cl::OptionCategory BoltOptCategory;
extern cl::opt<IndirectCallPromotionType> ICP;
extern cl::opt<unsigned> Verbosity;
extern cl::opt<unsigned> ExecutionCountThreshold;
+extern cl::opt<bool> UseCDSplit;
static cl::opt<unsigned> ICPJTRemainingPercentThreshold(
"icp-jt-remaining-percent-threshold",
@@ -259,9 +260,10 @@ IndirectCallPromotion::getCallTargets(BinaryBasicBlock &BB,
MCSymbol *Entry = JT->Entries[I];
const BinaryBasicBlock *ToBB = BF.getBasicBlockForLabel(Entry);
assert(ToBB || Entry == BF.getFunctionEndLabel() ||
- Entry == BF.getFunctionEndLabel(FragmentNum::cold()));
+ Entry ==
+ BF.getFunctionEndLabel(FragmentNum::cold(opts::UseCDSplit)));
if (Entry == BF.getFunctionEndLabel() ||
- Entry == BF.getFunctionEndLabel(FragmentNum::cold()))
+ Entry == BF.getFunctionEndLabel(FragmentNum::cold(opts::UseCDSplit)))
continue;
const Location To(Entry);
const BinaryBasicBlock::BinaryBranchInfo &BI = BB.getBranchInfo(*ToBB);
diff --git a/bolt/lib/Passes/SplitFunctions.cpp b/bolt/lib/Passes/SplitFunctions.cpp
index 0c11d0fb49cd09c..a5eb19724c31458 100644
--- a/bolt/lib/Passes/SplitFunctions.cpp
+++ b/bolt/lib/Passes/SplitFunctions.cpp
@@ -115,12 +115,12 @@ struct SplitProfile2 final : public SplitStrategy {
return BF.hasValidProfile() && BF.hasFullProfile() && !BF.allBlocksCold();
}
- bool keepEmpty() override { return false; }
+ bool keepEmpty() override { return opts::UseCDSplit ? true : false; }
void fragment(const BlockIt Start, const BlockIt End) override {
for (BinaryBasicBlock *const BB : llvm::make_range(Start, End)) {
if (BB->getExecutionCount() == 0)
- BB->setFragmentNum(FragmentNum::cold());
+ BB->setFragmentNum(FragmentNum::cold(opts::UseCDSplit));
}
}
};
@@ -144,7 +144,7 @@ struct SplitRandom2 final : public SplitStrategy {
std::uniform_int_distribution<DiffT> Dist(1, LastSplitPoint);
const DiffT SplitPoint = Dist(Gen);
for (BinaryBasicBlock *BB : llvm::make_range(Start + SplitPoint, End))
- BB->setFragmentNum(FragmentNum::cold());
+ BB->setFragmentNum(FragmentNum::cold(opts::UseCDSplit));
LLVM_DEBUG(dbgs() << formatv("BOLT-DEBUG: randomly chose last {0} (out of "
"{1} possible) blocks to split\n",
diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index 81c9cbff726bb9a..6d0b3cd5d15322b 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -84,6 +84,7 @@ extern cl::opt<JumpTableSupportLevel> JumpTables;
extern cl::list<std::string> ReorderData;
extern cl::opt<bolt::ReorderFunctions::ReorderType> ReorderFunctions;
extern cl::opt<bool> TimeBuild;
+extern cl::opt<bool> UseCDSplit;
cl::opt<bool> AllowStripped("allow-stripped",
cl::desc("allow processing of stripped binaries"),
@@ -3478,11 +3479,21 @@ std::vector<BinarySection *> RewriteInstance::getCodeSections() {
if (B->getName() == BC->getHotTextMoverSectionName())
return false;
- // Depending on the option, put main text at the beginning or at the end.
- if (opts::HotFunctionsAtEnd)
- return B->getName() == BC->getMainCodeSectionName();
- else
- return A->getName() == BC->getMainCodeSectionName();
+ // Depending on opts::HotFunctionsAtEnd, place main and warm sections in
+ // order.
+ if (opts::HotFunctionsAtEnd) {
+ if (B->getName() == BC->getMainCodeSectionName())
+ return true;
+ if (A->getName() == BC->getMainCodeSectionName())
+ return false;
+ return (B->getName() == BC->getWarmCodeSectionName());
+ } else {
+ if (A->getName() == BC->getMainCodeSectionName())
+ return true;
+ if (B->getName() == BC->getMainCodeSectionName())
+ return false;
+ return (A->getName() == BC->getWarmCodeSectionName());
+ }
};
// Determine the order of sections.
@@ -3639,7 +3650,7 @@ void RewriteInstance::mapCodeSections(BOLTLinker::SectionMapper MapSection) {
"non-relocation mode.");
FunctionFragment &FF =
- Function.getLayout().getFragment(FragmentNum::cold());
+ Function.getLayout().getFragment(FragmentNum::cold(opts::UseCDSplit));
ErrorOr<BinarySection &> ColdSection =
Function.getCodeSection(FF.getFragmentNum());
assert(ColdSection && "cannot find section for cold part");
@@ -4423,9 +4434,15 @@ void RewriteInstance::updateELFSymbolTable(
Function.getLayout().getSplitFragments()) {
if (FF.getAddress()) {
ELFSymTy NewColdSym = FunctionSymbol;
- const SmallString<256> SymbolName = formatv(
- "{0}.cold.{1}", cantFail(FunctionSymbol.getName(StringSection)),
- FF.getFragmentNum().get() - 1);
+ SmallString<256> SymbolName;
+ if (opts::UseCDSplit)
+ SymbolName = formatv(
+ "{0}.{1}", cantFail(FunctionSymbol.getName(StringSection)),
+ FF.getFragmentNum().get() == 1 ? "warm" : "cold");
+ else
+ SymbolName = formatv(
+ "{0}.cold.{1}", cantFail(FunctionSymbol.getName(StringSection)),
+ FF.getFragmentNum().get() - 1);
NewColdSym.st_name = AddToStrTab(SymbolName);
NewColdSym.st_shndx =
Function.getCodeSection(FF.getFragmentNum())->getIndex();
@@ -4684,8 +4701,8 @@ void RewriteInstance::updateELFSymbolTable(
SmallVector<char, 256> Buf;
NewColdSym.st_name = AddToStrTab(
Twine(Function->getPrintName()).concat(".cold.0").toStringRef(Buf));
- const FunctionFragment &ColdFF =
- Function->getLayout().getFragment(FragmentNum::cold());
+ const FunctionFragment &ColdFF = Function->getLayout().getFragment(
+ FragmentNum::cold(opts::UseCDSplit));
NewColdSym.st_value = ColdFF.getAddress();
NewColdSym.st_size = ColdFF.getImageSize();
Symbols.emplace_back(NewColdSym);
>From 9bae1c55d905e115ca9be1a49916a8db6abe6ebe Mon Sep 17 00:00:00 2001
From: Shatian Wang <shatian at meta.com>
Date: Tue, 7 Nov 2023 06:36:42 -0800
Subject: [PATCH 5/6] [BOLT] Initialize Auxiliary Variables
This diff defines and initializes auxiliary variables used by CDSplit.
---
bolt/include/bolt/Passes/CDSplit.h | 19 +++++++
bolt/lib/Passes/CDSplit.cpp | 82 ++++++++++++++++++++++++++++++
2 files changed, 101 insertions(+)
diff --git a/bolt/include/bolt/Passes/CDSplit.h b/bolt/include/bolt/Passes/CDSplit.h
index 96a982683a7ec26..a8d162d22b56469 100644
--- a/bolt/include/bolt/Passes/CDSplit.h
+++ b/bolt/include/bolt/Passes/CDSplit.h
@@ -18,7 +18,14 @@ namespace bolt {
using BasicBlockOrder = BinaryFunction::BasicBlockOrderType;
+struct JumpInfo {
+ bool HasUncondBranch = false;
+ BinaryBasicBlock *CondSuccessor = nullptr;
+ BinaryBasicBlock *UncondSuccessor = nullptr;
+};
+
class CDSplit : public BinaryFunctionPass {
+
private:
/// Overall stats.
std::atomic<uint64_t> SplitBytesHot{0ull};
@@ -29,6 +36,18 @@ class CDSplit : public BinaryFunctionPass {
/// A subset of functions in this list are considered for splitting.
std::vector<BinaryFunction *> FunctionsToConsider;
+ /// Auxiliary variables used by the algorithm.
+ size_t TotalNumBlocks{0};
+ size_t OrigHotSectionSize{0};
+ DenseMap<const BinaryBasicBlock *, size_t> GlobalIndices;
+ DenseMap<const BinaryBasicBlock *, size_t> BBSizes;
+ DenseMap<const BinaryBasicBlock *, size_t> BBOffsets;
+ // Call graph.
+ std::vector<SmallVector<const BinaryBasicBlock *, 0>> Callers;
+ std::vector<SmallVector<const BinaryBasicBlock *, 0>> Callees;
+ // Conditional and unconditional successors of each BB.
+ DenseMap<const BinaryBasicBlock *, JumpInfo> JumpInfos;
+
/// Helper functions to initialize global variables.
void initialize(BinaryContext &BC);
diff --git a/bolt/lib/Passes/CDSplit.cpp b/bolt/lib/Passes/CDSplit.cpp
index 509d250c3419183..6b6f1454512aa02 100644
--- a/bolt/lib/Passes/CDSplit.cpp
+++ b/bolt/lib/Passes/CDSplit.cpp
@@ -39,6 +39,25 @@ namespace {
bool shouldConsider(const BinaryFunction &BF) {
return BF.hasValidIndex() && BF.hasValidProfile() && !BF.empty();
}
+
+/// Find (un)conditional branch instruction info of the basic block.
+JumpInfo analyzeBranches(BinaryBasicBlock *BB) {
+ JumpInfo BBJumpInfo;
+ const MCSymbol *TBB = nullptr;
+ const MCSymbol *FBB = nullptr;
+ MCInst *CondBranch = nullptr;
+ MCInst *UncondBranch = nullptr;
+ if (BB->analyzeBranch(TBB, FBB, CondBranch, UncondBranch)) {
+ BBJumpInfo.HasUncondBranch = UncondBranch != nullptr;
+ if (BB->succ_size() == 1) {
+ BBJumpInfo.UncondSuccessor = BB->getSuccessor();
+ } else if (BB->succ_size() == 2) {
+ BBJumpInfo.CondSuccessor = BB->getConditionalSuccessor(true);
+ BBJumpInfo.UncondSuccessor = BB->getConditionalSuccessor(false);
+ }
+ }
+ return BBJumpInfo;
+}
} // anonymous namespace
bool CDSplit::shouldOptimize(const BinaryFunction &BF) const {
@@ -67,6 +86,69 @@ void CDSplit::initialize(BinaryContext &BC) {
if (shouldConsider(*BF))
FunctionsToConsider.push_back(BF);
}
+
+ // Initialize auxiliary variables.
+ for (BinaryFunction *BF : FunctionsToConsider) {
+ // Calculate the size of each BB after hot-cold splitting.
+ // This populates BinaryBasicBlock::OutputAddressRange which
+ // can be used to compute the size of each BB.
+ BC.calculateEmittedSize(*BF, /*FixBranches=*/true);
+
+ for (BinaryBasicBlock *BB : BF->getLayout().blocks()) {
+ // Unique global index.
+ GlobalIndices[BB] = TotalNumBlocks;
+ TotalNumBlocks++;
+
+ // Block size after hot-cold splitting.
+ BBSizes[BB] = BB->getOutputAddressRange().second -
+ BB->getOutputAddressRange().first;
+
+ // Hot block offset after hot-cold splitting.
+ BBOffsets[BB] = OrigHotSectionSize;
+ if (!BB->isSplit())
+ OrigHotSectionSize += BBSizes[BB];
+
+ // Conditional and unconditional successors.
+ JumpInfos[BB] = analyzeBranches(BB);
+ }
+ }
+
+ // Build call graph.
+ Callers.resize(TotalNumBlocks);
+ Callees.resize(TotalNumBlocks);
+ for (BinaryFunction *SrcFunction : FunctionsToConsider) {
+ for (BinaryBasicBlock &SrcBB : SrcFunction->blocks()) {
+ // Skip blocks that are not executed
+ if (SrcBB.getKnownExecutionCount() == 0)
+ continue;
+
+ // Find call instructions and extract target symbols from each one
+ for (const MCInst &Inst : SrcBB) {
+ if (!BC.MIB->isCall(Inst))
+ continue;
+
+ // Call info
+ const MCSymbol *DstSym = BC.MIB->getTargetSymbol(Inst);
+ // Ignore calls w/o information
+ if (!DstSym)
+ continue;
+
+ const BinaryFunction *DstFunction = BC.getFunctionForSymbol(DstSym);
+ // Ignore calls that do not have a valid target, but do not ignore
+ // recursive calls, because caller block could be moved to warm.
+ if (!DstFunction || DstFunction->getLayout().block_empty())
+ continue;
+
+ const BinaryBasicBlock *DstBB = &(DstFunction->front());
+
+ // Record the call only if DstBB is also in FunctionsToConsider.
+ if (GlobalIndices.contains(DstBB)) {
+ Callers[GlobalIndices[DstBB]].push_back(&SrcBB);
+ Callees[GlobalIndices[&SrcBB]].push_back(DstBB);
+ }
+ }
+ }
+ }
}
/// Find the best index for splitting. The returned value is the index of the
>From f88459fb42dcdd6918feba8ea69bc54ac2f09525 Mon Sep 17 00:00:00 2001
From: Shatian Wang <shatian at meta.com>
Date: Mon, 13 Nov 2023 17:16:59 -0800
Subject: [PATCH 6/6] [BOLT] CDSplit Main Logic Part 1/3
The first diff in a series of 3 that implements the main logic of
CDSplit. Under X86, function splitting can lead to block size increase.
This is because conditional and unconditional branch instructions whose
offset is under 8 bits can be encoded with 2 bytes. If the offset is
greater than 8 bits, then they need 6 and 5 bytes respectively.
Splitting a short conditional / unconditional branch will thus increase
the size of the src basic block by 4 and 3 bytes respectively. CDSplit
takes into account the potential block size increase when it makes
splitting decisions. This diff implements a function
estimatePostSplitBBAddress in CDSplit that approximates the block level
size increase at the given split index of the given function.
---
bolt/include/bolt/Passes/CDSplit.h | 18 ++++++
bolt/lib/Passes/CDSplit.cpp | 90 ++++++++++++++++++++++++++++++
2 files changed, 108 insertions(+)
diff --git a/bolt/include/bolt/Passes/CDSplit.h b/bolt/include/bolt/Passes/CDSplit.h
index a8d162d22b56469..20145e7656407c9 100644
--- a/bolt/include/bolt/Passes/CDSplit.h
+++ b/bolt/include/bolt/Passes/CDSplit.h
@@ -48,9 +48,27 @@ class CDSplit : public BinaryFunctionPass {
// Conditional and unconditional successors of each BB.
DenseMap<const BinaryBasicBlock *, JumpInfo> JumpInfos;
+ /// Sizes of branch instructions used to approximate block size increase
+ /// due to hot-warm splitting. Initialized to be 0. These values are updated
+ /// if the architecture is X86.
+ uint8_t BRANCH_SIZE = 0;
+ uint8_t LONG_UNCOND_BRANCH_SIZE_DELTA = 0;
+ uint8_t LONG_COND_BRANCH_SIZE_DELTA = 0;
+
/// Helper functions to initialize global variables.
void initialize(BinaryContext &BC);
+ /// Populate BinaryBasicBlock::OutputAddressRange with estimated basic block
+ /// start and end addresses for hot and warm basic blocks, assuming hot-warm
+ /// splitting happens at \p SplitIndex. Also return estimated end addresses
+ /// of the hot fragment before and after splitting.
+ /// The estimations take into account the potential addition of branch
+ /// instructions due to split fall through branches as well as the need to
+ /// use longer branch instructions for split (un)conditional branches.
+ std::pair<size_t, size_t>
+ estimatePostSplitBBAddress(const BasicBlockOrder &BlockOrder,
+ const size_t SplitIndex);
+
/// Split function body into 3 fragments: hot / warm / cold.
void runOnFunction(BinaryFunction &BF);
diff --git a/bolt/lib/Passes/CDSplit.cpp b/bolt/lib/Passes/CDSplit.cpp
index 6b6f1454512aa02..854bcd52d3eafb7 100644
--- a/bolt/lib/Passes/CDSplit.cpp
+++ b/bolt/lib/Passes/CDSplit.cpp
@@ -149,6 +149,96 @@ void CDSplit::initialize(BinaryContext &BC) {
}
}
}
+
+ // If X86, long branch instructions take more bytes than short branches.
+ // Adjust sizes of branch instructions used to approximate block size
+ // increase due to hot-warm splitting.
+ if (BC.isX86()) {
+ // a short branch takes 2 bytes.
+ BRANCH_SIZE = 2;
+ // a long uncond branch takes BRANCH_SIZE + 3 bytes.
+ LONG_UNCOND_BRANCH_SIZE_DELTA = 3;
+ // a long cond branch takes BRANCH_SIZE + 4 bytes.
+ LONG_COND_BRANCH_SIZE_DELTA = 4;
+ }
+}
+
+std::pair<size_t, size_t>
+CDSplit::estimatePostSplitBBAddress(const BasicBlockOrder &BlockOrder,
+ const size_t SplitIndex) {
+ assert(SplitIndex < BlockOrder.size() && "Invalid split index");
+ // Helper function estimating if a branch needs a longer branch instruction.
+ // The function returns true if the following two conditions are satisfied:
+ // condition 1. One of SrcBB and DstBB is in hot, the other is in warm.
+ // condition 2. The pre-split branch distance is within 8 bytes.
+ auto needNewLongBranch = [&](const BinaryBasicBlock *SrcBB,
+ const BinaryBasicBlock *DstBB) {
+ if (!SrcBB || !DstBB)
+ return false;
+ // The following checks for condition 1.
+ if (SrcBB->isSplit() || DstBB->isSplit())
+ return false;
+ if ((SrcBB->getLayoutIndex() <= SplitIndex) ==
+ (DstBB->getLayoutIndex() <= SplitIndex))
+ return false;
+ // The following checks for condition 2.
+ return (AbsoluteDifference(BBOffsets[DstBB],
+ BBOffsets[SrcBB] + BBSizes[SrcBB]) <=
+ std::numeric_limits<int8_t>::max());
+ };
+
+ // Populate BB.OutputAddressRange with estimated new start and end addresses
+ // and compute the old end address of the hot section and the new end address
+ // of the hot section.
+ size_t OldHotEndAddr;
+ size_t NewHotEndAddr;
+ size_t CurrentAddr = BBOffsets[BlockOrder[0]];
+ for (BinaryBasicBlock *BB : BlockOrder) {
+ // We only care about new addresses of blocks in hot/warm.
+ if (BB->isSplit())
+ break;
+ size_t NewSize = BBSizes[BB];
+ // Need to add a new branch instruction if a fall-through branch is split.
+ bool NeedNewUncondBranch =
+ (JumpInfos[BB].UncondSuccessor && !JumpInfos[BB].HasUncondBranch &&
+ BB->getLayoutIndex() == SplitIndex);
+
+ NewSize += BRANCH_SIZE * NeedNewUncondBranch +
+ LONG_UNCOND_BRANCH_SIZE_DELTA *
+ needNewLongBranch(BB, JumpInfos[BB].UncondSuccessor) +
+ LONG_COND_BRANCH_SIZE_DELTA *
+ needNewLongBranch(BB, JumpInfos[BB].CondSuccessor);
+ BB->setOutputStartAddress(CurrentAddr);
+ CurrentAddr += NewSize;
+ BB->setOutputEndAddress(CurrentAddr);
+ // Temporarily set the start address of the warm fragment of the current
+ // function to be 0. We will update it later when we can get a better
+ // estimate.
+ if (BB->getLayoutIndex() == SplitIndex) {
+ NewHotEndAddr = CurrentAddr;
+ CurrentAddr = 0;
+ }
+ OldHotEndAddr = BBOffsets[BB] + BBSizes[BB];
+ }
+
+ // Update the start and end addresses of blocks in the warm fragment.
+ // First get a better estimate of the start address of the warm fragment.
+ assert(OrigHotSectionSize + NewHotEndAddr >= OldHotEndAddr);
+ size_t WarmSectionStartAddr =
+ OrigHotSectionSize + NewHotEndAddr - OldHotEndAddr;
+ // Do the correction.
+ for (size_t Index = SplitIndex + 1; Index < BlockOrder.size(); Index++) {
+ BinaryBasicBlock *BB = BlockOrder[Index];
+ // We only care about new addresses of blocks in warm.
+ if (BB->isSplit())
+ break;
+ size_t StartAddrOffset = BB->getOutputAddressRange().first;
+ size_t EndAddrOffset = BB->getOutputAddressRange().second;
+ BB->setOutputStartAddress(WarmSectionStartAddr + StartAddrOffset);
+ BB->setOutputEndAddress(WarmSectionStartAddr + EndAddrOffset);
+ }
+
+ return std::make_pair(OldHotEndAddr, NewHotEndAddr);
}
/// Find the best index for splitting. The returned value is the index of the
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