[llvm] [llvm][transforms] Add a new algorithm to SplitModule (PR #95941)
Ilia Sergachev via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 2 12:41:43 PDT 2024
https://github.com/sergachev updated https://github.com/llvm/llvm-project/pull/95941
>From b9c21bad61344e7beb13294e2cae3e06c8fc8a8c Mon Sep 17 00:00:00 2001
From: Ilia Sergachev <isergachev at nvidia.com>
Date: Wed, 5 Jun 2024 16:24:28 +0200
Subject: [PATCH 1/3] [Transforms] Add a new algorithm to SplitModule
The new algorithm augments the hash-based distribution of functions to
modules by pairing unmapped functions to empty modules before using the
hash-based distribution. It allows getting less empty modules when the
number of functions is close to the number of requested modules.
It's not in use by default and is available under a new flag.
---
.../llvm/Transforms/Utils/SplitModule.h | 2 +-
llvm/lib/Transforms/Utils/SplitModule.cpp | 34 ++++++++++++++++++-
.../tools/llvm-split/avoid-empty-modules.ll | 23 +++++++++++++
.../name-hash-based-distribution.ll | 17 ++++++++++
llvm/tools/llvm-split/llvm-split.cpp | 12 ++++++-
5 files changed, 85 insertions(+), 3 deletions(-)
create mode 100644 llvm/test/tools/llvm-split/avoid-empty-modules.ll
create mode 100644 llvm/test/tools/llvm-split/name-hash-based-distribution.ll
diff --git a/llvm/include/llvm/Transforms/Utils/SplitModule.h b/llvm/include/llvm/Transforms/Utils/SplitModule.h
index a5450738060a8..7b303172d1bb0 100644
--- a/llvm/include/llvm/Transforms/Utils/SplitModule.h
+++ b/llvm/include/llvm/Transforms/Utils/SplitModule.h
@@ -35,7 +35,7 @@ class Module;
void SplitModule(
Module &M, unsigned N,
function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback,
- bool PreserveLocals = false);
+ bool PreserveLocals = false, bool TryToAvoidEmptyModules = false);
} // end namespace llvm
diff --git a/llvm/lib/Transforms/Utils/SplitModule.cpp b/llvm/lib/Transforms/Utils/SplitModule.cpp
index 9c39c26d8b7af..2f0ada5ef0019 100644
--- a/llvm/lib/Transforms/Utils/SplitModule.cpp
+++ b/llvm/lib/Transforms/Utils/SplitModule.cpp
@@ -251,7 +251,7 @@ static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
void llvm::SplitModule(
Module &M, unsigned N,
function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback,
- bool PreserveLocals) {
+ bool PreserveLocals, bool TryToAvoidEmptyModules) {
if (!PreserveLocals) {
for (Function &F : M)
externalize(&F);
@@ -268,6 +268,38 @@ void llvm::SplitModule(
ClusterIDMapType ClusterIDMap;
findPartitions(M, ClusterIDMap, N);
+ // Find empty modules and functions not mapped to modules in ClusterIDMap.
+ // Map these functions to the empty modules so that they skip being
+ // distributed by isInPartition() based on function name hashes below.
+ // This provides better uniformity of distribution of functions to modules
+ // in some cases - for example when the number of functions equals to N.
+ if (TryToAvoidEmptyModules) {
+ DenseSet<unsigned> NonEmptyModules;
+ SmallVector<const GlobalValue *> UnmappedFunctions;
+ for (const auto &F : M.functions()) {
+ if (F.isDeclaration() ||
+ F.getLinkage() != GlobalValue::LinkageTypes::ExternalLinkage)
+ continue;
+ auto It = ClusterIDMap.find(&F);
+ if (It == ClusterIDMap.end())
+ UnmappedFunctions.push_back(&F);
+ else
+ NonEmptyModules.insert(It->second);
+ }
+ SmallVector<unsigned> EmptyModules;
+ for (unsigned I = 0; I < N; ++I) {
+ if (!NonEmptyModules.contains(I))
+ EmptyModules.push_back(I);
+ }
+ auto NextEmptyModuleIt = EmptyModules.begin();
+ for (const auto F : UnmappedFunctions) {
+ if (NextEmptyModuleIt == EmptyModules.end())
+ break;
+ ClusterIDMap.insert({F, *NextEmptyModuleIt});
+ ++NextEmptyModuleIt;
+ }
+ }
+
// FIXME: We should be able to reuse M as the last partition instead of
// cloning it. Note that the callers at the moment expect the module to
// be preserved, so will need some adjustments as well.
diff --git a/llvm/test/tools/llvm-split/avoid-empty-modules.ll b/llvm/test/tools/llvm-split/avoid-empty-modules.ll
new file mode 100644
index 0000000000000..654b8410a704f
--- /dev/null
+++ b/llvm/test/tools/llvm-split/avoid-empty-modules.ll
@@ -0,0 +1,23 @@
+; RUN: llvm-split -o %t %s -j 2 -avoid-empty-modules
+; RUN: llvm-dis -o - %t0 | FileCheck --check-prefix=CHECK0 %s
+; RUN: llvm-dis -o - %t1 | FileCheck --check-prefix=CHECK1 %s
+
+; CHECK0-NOT: define
+; CHECK0: declare extern_weak void @e
+; CHECK0: define void @A
+; CHECK0-NOT: define
+
+; CHECK1-NOT: define
+; CHECK1: declare extern_weak void @e
+; CHECK1: define void @C
+; CHECK1-NOT: define
+
+declare extern_weak void @e(...)
+
+define void @A() {
+ ret void
+}
+
+define void @C() {
+ ret void
+}
diff --git a/llvm/test/tools/llvm-split/name-hash-based-distribution.ll b/llvm/test/tools/llvm-split/name-hash-based-distribution.ll
new file mode 100644
index 0000000000000..d9c4fc1eae0ad
--- /dev/null
+++ b/llvm/test/tools/llvm-split/name-hash-based-distribution.ll
@@ -0,0 +1,17 @@
+; RUN: llvm-split -o %t %s -j 2
+; RUN: llvm-dis -o - %t0 | FileCheck --check-prefix=CHECK0 %s
+; RUN: llvm-dis -o - %t1 | FileCheck --check-prefix=CHECK1 %s
+
+; CHECK0-NOT: define
+
+; CHECK1-NOT: declare
+; CHECK1: define void @A
+; CHECK1: define void @C
+
+define void @A() {
+ ret void
+}
+
+define void @C() {
+ ret void
+}
diff --git a/llvm/tools/llvm-split/llvm-split.cpp b/llvm/tools/llvm-split/llvm-split.cpp
index 39a89cb1d2e75..786e92dd75084 100644
--- a/llvm/tools/llvm-split/llvm-split.cpp
+++ b/llvm/tools/llvm-split/llvm-split.cpp
@@ -53,6 +53,12 @@ static cl::opt<bool>
cl::desc("Split without externalizing locals"),
cl::cat(SplitCategory));
+static cl::opt<bool>
+ TryToAvoidEmptyModules("avoid-empty-modules", cl::Prefix, cl::init(false),
+ cl::desc("Try to avoid generating empty modules by "
+ "modifying the distribution of functions"),
+ cl::cat(SplitCategory));
+
static cl::opt<std::string>
MTriple("mtriple",
cl::desc("Target triple. When present, a TargetMachine is created "
@@ -122,6 +128,9 @@ int main(int argc, char **argv) {
errs() << "warning: -preserve-locals has no effect when using "
"TargetMachine::splitModule\n";
}
+ if (TryToAvoidEmptyModules)
+ errs() << "warning: -avoid-empty-modules has no effect when using "
+ "TargetMachine::splitModule\n";
if (TM->splitModule(*M, NumOutputs, HandleModulePart))
return 0;
@@ -131,6 +140,7 @@ int main(int argc, char **argv) {
"splitModule implementation\n";
}
- SplitModule(*M, NumOutputs, HandleModulePart, PreserveLocals);
+ SplitModule(*M, NumOutputs, HandleModulePart, PreserveLocals,
+ TryToAvoidEmptyModules);
return 0;
}
>From 615e93523df94f1347fa70f73c90d45f05993ca7 Mon Sep 17 00:00:00 2001
From: Ilia Sergachev <isergachev at nvidia.com>
Date: Tue, 2 Jul 2024 21:30:21 +0200
Subject: [PATCH 2/3] Skip redundant work if EmptyModules is empty
address review comment
---
llvm/lib/Transforms/Utils/SplitModule.cpp | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SplitModule.cpp b/llvm/lib/Transforms/Utils/SplitModule.cpp
index 2f0ada5ef0019..e8376acb79b8c 100644
--- a/llvm/lib/Transforms/Utils/SplitModule.cpp
+++ b/llvm/lib/Transforms/Utils/SplitModule.cpp
@@ -177,11 +177,13 @@ static void findPartitions(Module &M, ClusterIDMapType &ClusterIDMap,
// To guarantee determinism, we have to sort SCC according to size.
// When size is the same, use leader's name.
for (ClusterMapType::iterator I = GVtoClusterMap.begin(),
- E = GVtoClusterMap.end(); I != E; ++I)
+ E = GVtoClusterMap.end();
+ I != E; ++I)
if (I->isLeader())
Sets.push_back(
std::make_pair(std::distance(GVtoClusterMap.member_begin(I),
- GVtoClusterMap.member_end()), I));
+ GVtoClusterMap.member_end()),
+ I));
llvm::sort(Sets, [](const SortType &a, const SortType &b) {
if (a.first == b.first)
@@ -291,12 +293,14 @@ void llvm::SplitModule(
if (!NonEmptyModules.contains(I))
EmptyModules.push_back(I);
}
- auto NextEmptyModuleIt = EmptyModules.begin();
- for (const auto F : UnmappedFunctions) {
- if (NextEmptyModuleIt == EmptyModules.end())
- break;
- ClusterIDMap.insert({F, *NextEmptyModuleIt});
- ++NextEmptyModuleIt;
+ if (!EmptyModules.empty()) {
+ auto NextEmptyModuleIt = EmptyModules.begin();
+ for (const auto F : UnmappedFunctions) {
+ if (NextEmptyModuleIt == EmptyModules.end())
+ break;
+ ClusterIDMap.insert({F, *NextEmptyModuleIt});
+ ++NextEmptyModuleIt;
+ }
}
}
>From 9a1d9ecae68c2762e69b5ca8fceb03f9bcefb73b Mon Sep 17 00:00:00 2001
From: Ilia Sergachev <isergachev at nvidia.com>
Date: Tue, 2 Jul 2024 21:41:28 +0200
Subject: [PATCH 3/3] Document flags of SplitModule()
address review comment
---
llvm/include/llvm/Transforms/Utils/SplitModule.h | 3 +++
1 file changed, 3 insertions(+)
diff --git a/llvm/include/llvm/Transforms/Utils/SplitModule.h b/llvm/include/llvm/Transforms/Utils/SplitModule.h
index 7b303172d1bb0..3f4ffaf2d6b7e 100644
--- a/llvm/include/llvm/Transforms/Utils/SplitModule.h
+++ b/llvm/include/llvm/Transforms/Utils/SplitModule.h
@@ -24,6 +24,9 @@ class Module;
/// Splits the module M into N linkable partitions. The function ModuleCallback
/// is called N times passing each individual partition as the MPart argument.
+/// PreserveLocals: Split without externalizing locals.
+/// TryToAvoidEmptyModules: Try to avoid generating empty modules by
+/// modifying the distribution of functions.
///
/// FIXME: This function does not deal with the somewhat subtle symbol
/// visibility issues around module splitting, including (but not limited to):
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