From 8ef3fd8d59ba0100bc6e83350ab1e978536aa531 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i@maskray.me>
Date: Tue, 11 Oct 2022 15:30:07 -0700
Subject: [LTO] Make local linkage GlobalValue in non-prevailing COMDAT
 available_externally

For a local linkage GlobalObject in a non-prevailing COMDAT, it remains defined while its
leader has been made available_externally. This violates the COMDAT rule that
its members must be retained or discarded as a unit.

To fix this, update the regular LTO change D34803 to track local linkage
GlobalValues, and port the code to ThinLTO (GlobalAliases are not handled.)

This fixes two problems.

(a) `__cxx_global_var_init` in a non-prevailing COMDAT group used to
linger around (unreferenced, hence benign), and is now correctly discarded.
```
int foo();
inline int v = foo();
```

(b) Fix https://github.com/llvm/llvm-project/issues/58215:
as a size optimization, we place private `__profd_` in a COMDAT with a
`__profc_` key. When FuncImport.cpp makes `__profc_` available_externally due to
a non-prevailing COMDAT, `__profd_` incorrectly remains private. This change
makes the `__profd_` available_externally.

```
cat > c.h <<'eof'
extern void bar();
inline __attribute__((noinline)) void foo() {}
eof
cat > m1.cc <<'eof'
int main() {
  bar();
  foo();
}
eof
cat > m2.cc <<'eof'
__attribute__((noinline)) void bar() {
  foo();
}
eof

clang -O2 -fprofile-generate=./t m1.cc m2.cc -flto -fuse-ld=lld -o t_gen
rm -fr t && ./t_gen && llvm-profdata show -function=foo t/default_*.profraw

clang -O2 -fprofile-generate=./t m1.cc m2.cc -flto=thin -fuse-ld=lld -o t_gen
rm -fr t && ./t_gen && llvm-profdata show -function=foo t/default_*.profraw
```

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D135427
---
 llvm/lib/LTO/LTO.cpp | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

(limited to 'llvm/lib/LTO/LTO.cpp')

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 10ca98f..0ce8519 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -696,11 +696,11 @@ handleNonPrevailingComdat(GlobalValue &GV,
   if (!NonPrevailingComdats.count(C))
     return;
 
-  // Additionally need to drop externally visible global values from the comdat
-  // to available_externally, so that there aren't multiply defined linker
-  // errors.
-  if (!GV.hasLocalLinkage())
-    GV.setLinkage(GlobalValue::AvailableExternallyLinkage);
+  // Additionally need to drop all global values from the comdat to
+  // available_externally, to satisfy the COMDAT requirement that all members
+  // are discarded as a unit. The non-local linkage global values avoid
+  // duplicate definition linker errors.
+  GV.setLinkage(GlobalValue::AvailableExternallyLinkage);
 
   if (auto GO = dyn_cast<GlobalObject>(&GV))
     GO->setComdat(nullptr);
-- 
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