[all-commits] [llvm/llvm-project] 8ef3fd: [LTO] Make local linkage GlobalValue in non-prevai...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 8ef3fd8d59ba0100bc6e83350ab1e978536aa531
      https://github.com/llvm/llvm-project/commit/8ef3fd8d59ba0100bc6e83350ab1e978536aa531
  Author: Fangrui Song <i at maskray.me>
  Date:   2022-10-11 (Tue, 11 Oct 2022)

  Changed paths:
    M llvm/lib/LTO/LTO.cpp
    M llvm/lib/Transforms/IPO/FunctionImport.cpp
    M llvm/test/LTO/Resolution/X86/comdat-mixed-lto.ll
    M llvm/test/ThinLTO/X86/Inputs/linkonce_resolution_comdat.ll
    M llvm/test/ThinLTO/X86/linkonce_resolution_comdat.ll

  Log Message:
  -----------
  [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