[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

Author: MaskRay

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) {

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";

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