[llvm] 3adb89b - [ThinLTO] Make cloneUsedGlobalVariables deterministic
Fangrui Song via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 23 16:09:13 PST 2021
Author: Fangrui Song
Date: 2021-02-23T16:09:05-08:00
New Revision: 3adb89bb9f8e73c82787babb2f877fece7394770
URL: https://github.com/llvm/llvm-project/commit/3adb89bb9f8e73c82787babb2f877fece7394770
DIFF: https://github.com/llvm/llvm-project/commit/3adb89bb9f8e73c82787babb2f877fece7394770.diff
LOG: [ThinLTO] Make cloneUsedGlobalVariables deterministic
Iterating on `SmallPtrSet<GlobalValue *, 8>` with more than 8 elements
is not deterministic. Use a SmallVector instead because `Used` is guaranteed to contain unique elements.
While here, decrease inline element counts from 8 to 4. The number of
`llvm.used`/`llvm.compiler.used` elements is usually 0 or 1. For full
LTO/hybrid LTO, the number may be large, so we need to be careful.
According to tejohnson's analysis https://reviews.llvm.org/D97128#2582399 , 4 is
good for a large project with WholeProgramDevirt, when available_externally
vtables are placed in the llvm.compiler.used set.
Differential Revision: https://reviews.llvm.org/D97128
Added:
Modified:
llvm/include/llvm/IR/Module.h
llvm/lib/IR/Module.cpp
llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/IR/Module.h b/llvm/include/llvm/IR/Module.h
index 3ac57b7e5750..63d66c5fd63a 100644
--- a/llvm/include/llvm/IR/Module.h
+++ b/llvm/include/llvm/IR/Module.h
@@ -893,6 +893,12 @@ class Module {
GlobalVariable *collectUsedGlobalVariables(const Module &M,
SmallPtrSetImpl<GlobalValue *> &Set,
bool CompilerUsed);
+/// Given "llvm.used" or "llvm.compiler.used" as a global name, collect the
+/// initializer elements of that global in a SmallVector and return the global
+/// itself.
+GlobalVariable *collectUsedGlobalVariables(const Module &M,
+ SmallVectorImpl<GlobalValue *> &Vec,
+ bool CompilerUsed);
/// An raw_ostream inserter for modules.
inline raw_ostream &operator<<(raw_ostream &O, const Module &M) {
diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 9395b2bb849c..4c244611fbb3 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -658,6 +658,21 @@ VersionTuple Module::getSDKVersion() const {
return Result;
}
+GlobalVariable *llvm::collectUsedGlobalVariables(
+ const Module &M, SmallVectorImpl<GlobalValue *> &Vec, bool CompilerUsed) {
+ const char *Name = CompilerUsed ? "llvm.compiler.used" : "llvm.used";
+ GlobalVariable *GV = M.getGlobalVariable(Name);
+ if (!GV || !GV->hasInitializer())
+ return GV;
+
+ const ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
+ for (Value *Op : Init->operands()) {
+ GlobalValue *G = cast<GlobalValue>(Op->stripPointerCasts());
+ Vec.push_back(G);
+ }
+ return GV;
+}
+
GlobalVariable *llvm::collectUsedGlobalVariables(
const Module &M, SmallPtrSetImpl<GlobalValue *> &Set, bool CompilerUsed) {
const char *Name = CompilerUsed ? "llvm.compiler.used" : "llvm.used";
diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index 2ab9dcd09910..797416f5e13a 100644
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -199,23 +199,20 @@ void forEachVirtualFunction(Constant *C, function_ref<void(Function *)> Fn) {
// values whose defs were cloned into that module.
static void cloneUsedGlobalVariables(const Module &SrcM, Module &DestM,
bool CompilerUsed) {
- SmallPtrSet<GlobalValue *, 8> Used;
- SmallPtrSet<GlobalValue *, 8> NewUsed;
+ SmallVector<GlobalValue *, 4> Used, NewUsed;
// First collect those in the llvm[.compiler].used set.
collectUsedGlobalVariables(SrcM, Used, CompilerUsed);
// Next build a set of the equivalent values defined in DestM.
for (auto *V : Used) {
auto *GV = DestM.getNamedValue(V->getName());
if (GV && !GV->isDeclaration())
- NewUsed.insert(GV);
+ NewUsed.push_back(GV);
}
// Finally, add them to a llvm[.compiler].used variable in DestM.
if (CompilerUsed)
- appendToCompilerUsed(
- DestM, std::vector<GlobalValue *>(NewUsed.begin(), NewUsed.end()));
+ appendToCompilerUsed(DestM, NewUsed);
else
- appendToUsed(DestM,
- std::vector<GlobalValue *>(NewUsed.begin(), NewUsed.end()));
+ appendToUsed(DestM, NewUsed);
}
// If it's possible to split M into regular and thin LTO parts, do so and write
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