[PATCH] D73230: [X86][ELF] Prefer to lower MC_GlobalAddress operands to .Lfoo$local

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

In D73230#2207477 <https://reviews.llvm.org/D73230#2207477>, @bd1976llvm wrote:

> So, clang's behavior has changed so that --wrap no longer wraps symbol definitions for default symbols + -fpic + -fno-semantic-interposition (-fno-semantic-interposition is the default); however, you can restore the old behavior via -fsemantic-interposition. For hidden symbols + -fpic --wrap no longer wraps symbol definitions and there is no way to restore the old definition wrapping behavior.

I think the summary is correct. -fvisibility=hidden nullifies -fsemantic-interposition when the definition is available in the same translation unit. Given how GCC and GNU ld handle/document it, I'd say the previous `hidden` clang behavior working with -Wl,--wrap=foo is accidental rather than intentional. If you don't pass explicit -fsemantic-interposition, in -fPIC mode clang can freely inline foo into call sites, which will also defeat the intended -Wl,--wrap=foo behavior.

> Any undefined reference to symbol will be resolved to "__wrap_symbol".

I think the reasonably portable approach making the wrapping scheme work is `__attribute__((weak))`. An alternative is to move the definitions to a separate translation unit (it does not work with -r or GCC LTO, though).


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