LTO: Don't internalize available_externally globals.

[pcc]

Summary

This breaks C and C++ semantics because it can cause the address
of the global inside the module to differ from the address outside
of the module.

[pirama]

llvm/lib/LTO/LTO.cpp

363

Use !GLobalValue::isAvailableExternallyLinkage(S->linkage()) instead?

Also, we should update the earlier comment.

[pcc]

Add comment

llvm/lib/LTO/LTO.cpp

363

I'd prefer not to since it boils down to a comparison and it's slightly clearer to be explicit. Added comment and changed the other condition to be explicit as well.

[tejohnson]

Lgtm

llvm/lib/LTO/LTO.cpp

362

This change seems unnecessary?

[pcc]

llvm/lib/LTO/LTO.cpp

362

Just making it consistent with the code I'm adding below.

llvm/lib/LTO/LTO.cpp

static void thinLTOInternalizeAndPromoteGUID(
   for (auto &S : GVSummaryList) {
     if (isExported(S->modulePath(), GUID)) {
       if (GlobalValue::isLocalLinkage(S->linkage()))
         S->setLinkage(GlobalValue::ExternalLinkage);
     } else if (EnableLTOInternalization &&
                // Ignore local and appending linkage values since the linker
                // doesn't resolve them.
                !GlobalValue::isLocalLinkage(S->linkage()) &&
-               !GlobalValue::isAppendingLinkage(S->linkage()))
+               S->linkage() != GlobalValue::AppendingLinkage &&
+               // We can't internalize available_externally globals because this
+               // can break function pointer equality.
+               S->linkage() != GlobalValue::AvailableExternallyLinkage)
       S->setLinkage(GlobalValue::InternalLinkage);
   }
 }
 
 // Update the linkages in the given \p Index to mark exported values
 // as external and non-exported values as internal.
 void llvm::thinLTOInternalizeAndPromoteInIndex(
     ModuleSummaryIndex &Index,

llvm/test/LTO/Resolution/X86/available-externally.ll

+; RUN: opt -module-summary -o %t.bc %s
+; RUN: llvm-lto2 run %t.bc -r %t.bc,foo,px -r %t.bc,bar, -o %t2
+; RUN: llvm-nm %t2.1 | FileCheck %s
+
+; CHECK: U bar
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void ()* @foo() {
+  ret void ()* @bar
+}
+
+define available_externally void @bar() {
+  ret void
+}