[GlobalOpt] Replace aliasee if it is not in use

Author: dianqk

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -2215,6 +2215,17 @@ static bool hasUseOtherThanLLVMUsed(GlobalAlias &GA, const LLVMUsed &U) {
   return !U.usedCount(&GA) && !U.compilerUsedCount(&GA);
 }
 
+static bool hasMoreThanOneUseOtherThanLLVMUsed(GlobalValue &V,
+                                               const LLVMUsed &U) {
+  unsigned N = 2;
+  assert((!U.usedCount(&V) || !U.compilerUsedCount(&V)) &&
+         "We should have removed the duplicated "
+         "element from llvm.compiler.used");
+  if (U.usedCount(&V) || U.compilerUsedCount(&V))
+    ++N;
+  return V.hasNUsesOrMore(N);
+}
+
 static bool mayHaveOtherReferences(GlobalValue &GV, const LLVMUsed &U) {
   if (!GV.hasLocalLinkage())
     return true;
@@ -2224,8 +2235,6 @@ static bool mayHaveOtherReferences(GlobalValue &GV, const LLVMUsed &U) {
 
 static bool hasUsesToReplace(GlobalAlias &GA, const LLVMUsed &U,
                              bool &RenameTarget) {
-  if (GA.isWeakForLinker())
-    return false;
 
   RenameTarget = false;
   bool Ret = false;
@@ -2245,6 +2254,13 @@ static bool hasUsesToReplace(GlobalAlias &GA, const LLVMUsed &U,
   //   define ... @a(...)
   Constant *Aliasee = GA.getAliasee();
   GlobalValue *Target = cast<GlobalValue>(Aliasee->stripPointerCasts());
+  // Do not perform the transform if alias may be replaced at link time while
+  // someone is using the aliasee (e.g., multiple aliases potentially target it
+  // or someone calls it).
+  if (GA.isWeakForLinker())
+    Ret = false;
+  if (hasMoreThanOneUseOtherThanLLVMUsed(*Target, U))
+    return Ret;
   if (mayHaveOtherReferences(*Target, U))
     return Ret;
 

llvm/test/Transforms/GlobalOpt/alias-weak.ll

@@ -25,13 +25,12 @@ define internal void @f1() {
   ret void
 }
 
-; FIXME: We can use `f2_alias` to replace `f2` because `b2` is not in use.
+; We can use `f2_alias` to replace `f2` because `b2` is not in use.
 define internal void @f2() {
   ret void
 }
 ;.
 ; CHECK: @f1_alias = linkonce_odr hidden alias void (), ptr @f1
-; CHECK: @f2_alias = linkonce_odr hidden alias void (), ptr @f2
 ;.
 ; CHECK-LABEL: define void @foo() local_unnamed_addr {
 ; CHECK-NEXT:    call void @f1_alias()
@@ -52,6 +51,6 @@ define internal void @f2() {
 ; CHECK-NEXT:    ret void
 ;
 ;
-; CHECK-LABEL: define internal void @f2() {
+; CHECK-LABEL: define linkonce_odr hidden void @f2_alias() local_unnamed_addr {
 ; CHECK-NEXT:    ret void
 ;