ThinLTO doesn't apply interprocedural ConstMerge optimization

[llvmbot]

Bugzilla Link	33630
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor

Extended Description

LTO will apply the Constant Merging pass, but ThinLTO can't apply this optimization right now.

Test case:

// a.c const char *x = "blah";

// b.c

include

extern const char x; const char b = "blah";

int main(void) { puts(x); puts(b); }

//////////////////////

== THINLTO ==

$ ../build/bin/clang a.c b.c --for-linker=-mllvm --for-linker=-stats -flto=thin \ -fuse-ld=lld -o out &>&1 | grep "constmerge"

nothing

== LTO ==

$ ../build/bin/clang a.c b.c --for-linker=-mllvm --for-linker=-stats -flto \ -fuse-ld=lld -o out &>&1 | grep "constmerge"

1 constmerge - Number of global constants merged

[efriedma-quic]

Is this actually important? The strings will get merged by the linker.