MaskRay added a comment. In D101873#2738643 <https://reviews.llvm.org/D101873#2738643>, @peter.smith wrote:
> I've no comments on the code in D101872 <https://reviews.llvm.org/D101872> , > and D10873 <https://reviews.llvm.org/D10873> they look reasonable to me. I > guess it is down to whether this is the right thing to do or not. > > Just to check my understanding: > > - Clang defaults to -fno-semantic-interposition (GCC I believe has the > opposite default fsemantic-interposition) Clang defaults to a state between -fno-semantic-interposition and -fsemantic-interposition (cc1 -fhalf-no-semantic-interposition): interprocedural optimizations are not disabled for default visibility definitions, but dso_local is not added. This has been the traditional behavior for years. Adding dso_local (-fno-semantic-interposition) will use `.Lfoo$local`, which can break a small amount of applications, so I don't want to change the default. > - With this change code in the same translation unit that defines the global > will use a local alias for the global rather than accessing via the GOT, but > the global will still be defined with default visibility. Only when -fno-semantic-interpoistion is explicitly specified, i.e. -fpic -fno-semantic-interpoistion > - Symbol interposing is still permitted at link time so the global can be > interposed, but as the code is using a local alias it will still use the > original value. > - Without this chang clang would use fhalf-no-semantic-interposition which I > believe permits some assumptions about symbol interpositioning such as > resolving some short range assembly pc-relative references to a local alias. > These would be out of range if the symbol were interposed anyway. > > If I've got this right, particularly the default then this makes me nervous > about the default behaviour as it could silently break some existing code. If > a user had to opt in explicitly with -fno-semantic-interposition then fair > enough. > > Can you let me know, if I'm being overly cautious here? For example are > programs that would be affected by this already broken by the > half-no-semantic-interpositioning anyway? Is symbol interpositioning so rare > that the X86 version of this didn't break anything? Have I got the default of > -fno-semantic-interpositioning wrong? Yes, the first step:) -fno-semantic-interposition is not the default. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D101873/new/ https://reviews.llvm.org/D101873 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
