[llvm] r290016 - [BPI] Use a safer constructor to calculate branch probabilities
Vedant Kumar via llvm-commits
llvm-commits at lists.llvm.org
Fri Dec 16 16:09:51 PST 2016
Author: vedantk
Date: Fri Dec 16 18:09:51 2016
New Revision: 290016
URL: http://llvm.org/viewvc/llvm-project?rev=290016&view=rev
Log:
[BPI] Use a safer constructor to calculate branch probabilities
BPI may trigger signed overflow UB while computing branch probabilities
for cold calls or to unreachables. For example, with our current choice
of weights, we'll crash if there are >= 2^12 branches to an unreachable.
Use a safer BranchProbability constructor which is better at handling
fractions with large denominators.
rdar://problem/29368161
Differential Revision: https://reviews.llvm.org/D27862
Added:
llvm/trunk/unittests/Analysis/BranchProbabilityInfoTest.cpp
Modified:
llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp
llvm/trunk/unittests/Analysis/CMakeLists.txt
Modified: llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp?rev=290016&r1=290015&r2=290016&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp (original)
+++ llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp Fri Dec 16 18:09:51 2016
@@ -162,12 +162,12 @@ bool BranchProbabilityInfo::calcUnreacha
return true;
}
- BranchProbability UnreachableProb(UR_TAKEN_WEIGHT,
- (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
- UnreachableEdges.size());
- BranchProbability ReachableProb(UR_NONTAKEN_WEIGHT,
- (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
- ReachableEdges.size());
+ auto UnreachableProb = BranchProbability::getBranchProbability(
+ UR_TAKEN_WEIGHT,
+ (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) * UnreachableEdges.size());
+ auto ReachableProb = BranchProbability::getBranchProbability(
+ UR_NONTAKEN_WEIGHT,
+ (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) * ReachableEdges.size());
for (unsigned SuccIdx : UnreachableEdges)
setEdgeProbability(BB, SuccIdx, UnreachableProb);
@@ -300,12 +300,12 @@ bool BranchProbabilityInfo::calcColdCall
return true;
}
- BranchProbability ColdProb(CC_TAKEN_WEIGHT,
- (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
- ColdEdges.size());
- BranchProbability NormalProb(CC_NONTAKEN_WEIGHT,
- (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
- NormalEdges.size());
+ auto ColdProb = BranchProbability::getBranchProbability(
+ CC_TAKEN_WEIGHT,
+ (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) * ColdEdges.size());
+ auto NormalProb = BranchProbability::getBranchProbability(
+ CC_NONTAKEN_WEIGHT,
+ (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) * NormalEdges.size());
for (unsigned SuccIdx : ColdEdges)
setEdgeProbability(BB, SuccIdx, ColdProb);
Added: llvm/trunk/unittests/Analysis/BranchProbabilityInfoTest.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Analysis/BranchProbabilityInfoTest.cpp?rev=290016&view=auto
==============================================================================
--- llvm/trunk/unittests/Analysis/BranchProbabilityInfoTest.cpp (added)
+++ llvm/trunk/unittests/Analysis/BranchProbabilityInfoTest.cpp Fri Dec 16 18:09:51 2016
@@ -0,0 +1,88 @@
+//===- BranchProbabilityInfoTest.cpp - BranchProbabilityInfo unit tests ---===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/BranchProbabilityInfo.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/DataTypes.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gtest/gtest.h"
+
+namespace llvm {
+namespace {
+
+struct BranchProbabilityInfoTest : public testing::Test {
+ std::unique_ptr<BranchProbabilityInfo> BPI;
+ std::unique_ptr<DominatorTree> DT;
+ std::unique_ptr<LoopInfo> LI;
+ LLVMContext C;
+
+ BranchProbabilityInfo &buildBPI(Function &F) {
+ DT.reset(new DominatorTree(F));
+ LI.reset(new LoopInfo(*DT));
+ BPI.reset(new BranchProbabilityInfo(F, *LI));
+ return *BPI;
+ }
+
+ std::unique_ptr<Module> makeLLVMModule() {
+ const char *ModuleString = "define void @f() { exit: ret void }\n";
+ SMDiagnostic Err;
+ return parseAssemblyString(ModuleString, Err, C);
+ }
+};
+
+TEST_F(BranchProbabilityInfoTest, StressUnreachableHeuristic) {
+ auto M = makeLLVMModule();
+ Function *F = M->getFunction("f");
+
+ // define void @f() {
+ // entry:
+ // switch i32 undef, label %exit, [
+ // i32 0, label %preexit
+ // ... ;;< Add lots of cases to stress the heuristic.
+ // ]
+ // preexit:
+ // unreachable
+ // exit:
+ // ret void
+ // }
+
+ auto *ExitBB = &F->back();
+ auto *EntryBB = BasicBlock::Create(C, "entry", F, /*insertBefore=*/ExitBB);
+
+ auto *PreExitBB =
+ BasicBlock::Create(C, "preexit", F, /*insertBefore=*/ExitBB);
+ new UnreachableInst(C, PreExitBB);
+
+ unsigned NumCases = 4096;
+ auto *I32 = IntegerType::get(C, 32);
+ auto *Undef = UndefValue::get(I32);
+ auto *Switch = SwitchInst::Create(Undef, ExitBB, NumCases, EntryBB);
+ for (unsigned I = 0; I < NumCases; ++I)
+ Switch->addCase(ConstantInt::get(I32, I), PreExitBB);
+
+ BranchProbabilityInfo &BPI = buildBPI(*F);
+
+ // FIXME: This doesn't seem optimal. Since all of the cases handled by the
+ // switch have the *same* destination block ("preexit"), shouldn't it be the
+ // hot one? I'd expect the results to be reversed here...
+ EXPECT_FALSE(BPI.isEdgeHot(EntryBB, PreExitBB));
+ EXPECT_TRUE(BPI.isEdgeHot(EntryBB, ExitBB));
+}
+
+} // end anonymous namespace
+} // end namespace llvm
Modified: llvm/trunk/unittests/Analysis/CMakeLists.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Analysis/CMakeLists.txt?rev=290016&r1=290015&r2=290016&view=diff
==============================================================================
--- llvm/trunk/unittests/Analysis/CMakeLists.txt (original)
+++ llvm/trunk/unittests/Analysis/CMakeLists.txt Fri Dec 16 18:09:51 2016
@@ -8,6 +8,7 @@ set(LLVM_LINK_COMPONENTS
add_llvm_unittest(AnalysisTests
AliasAnalysisTest.cpp
BlockFrequencyInfoTest.cpp
+ BranchProbabilityInfoTest.cpp
CallGraphTest.cpp
CFGTest.cpp
CGSCCPassManagerTest.cpp
More information about the llvm-commits
mailing list