[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

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 9395b2b..4c24461 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -659,6 +659,21 @@ VersionTuple Module::getSDKVersion() const {
 }
 
 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";
   GlobalVariable *GV = M.getGlobalVariable(Name);