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'
#include "c.h"
int main() {
bar();
foo();
}
eof
cat > m2.cc <<'eof'
#include "c.h"
__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
```
If a GlobalAlias references a GlobalValue which is just changed to
available_externally, change the GlobalAlias as well (e.g. C5/D5 comdats due to
cc1 -mconstructor-aliases). The GlobalAlias may be referenced by other
available_externally functions, so it cannot easily be removed.
Depends on D137441: we use available_externally to mark a GlobalAlias in a
non-prevailing COMDAT, similar to how we handle GlobalVariable/Function.
GlobalAlias may refer to a ConstantExpr, not changing GlobalAlias to
GlobalVariable gives flexibility for future extensions (the use case is niche.
For simplicity we don't handle it yet). In addition, available_externally
GlobalAlias is the most straightforward implementation and retains the aliasee
information to help optimizers.
See windows-vftable.ll: Windows vftable uses an alias pointing to a
private constant where the alias is the COMDAT leader. The COMDAT use case
is skeptical and ThinLTO does not discard the alias in the non-prevailing COMDAT.
This patch retains the behavior.
See new tests ctor-dtor-alias2.ll: depending on whether the complete object
destructor emitted, when ctor/dtor aliases are used, we may see D0/D2 COMDATs in
one TU and D0/D1/D2 in a D5 COMDAT in another TU.
Allow such a mix-and-match with `if (GO->getComdat()->getName() == GO->getName()) NonPrevailingComdats.insert(GO->getComdat());`
GlobalAlias handling in ThinLTO is still weird, but this patch should hopefully
improve the situation for at least all cases I can think of.
Reviewed By: tejohnson
Differential Revision: https://reviews.llvm.org/D135427
12050a3fb7