Em Wed, Oct 09, 2019 at 04:07:37PM -0700, Ian Rogers escreveu: > On Tue, Oct 8, 2019 at 5:31 AM Jiri Olsa <[email protected]> wrote: > > On Mon, Sep 30, 2019 at 05:36:23PM -0700, Ian Rogers wrote: > > > Being const + weak breaks with some compilers that constant-propagate > > > from the weak symbol. This behavior is outside of the specification, but > > > in LLVM is chosen to match GCC's behavior. > > > > > > LLVM's implementation was set in this patch: > > > https://github.com/llvm/llvm-project/commit/f49573d1eedcf1e44893d5a062ac1b72c8419646 > > > A const + weak symbol is set to be weak_odr: > > > https://llvm.org/docs/LangRef.html > > > ODR is one definition rule, and given there is one constant definition > > > constant-propagation is possible. It is possible to get this code to > > > miscompile with LLVM when applying link time optimization. As compilers > > > become more aggressive, this is likely to break in more instances.
> > is this just aprecaution or you actualy saw some breakage? > We saw a breakage with clang with thinlto enabled for linking. Our > compiler team had recently seen, and were surprised by, a similar > issue and were able to dig out the weak ODR issue. This is useful info, I'll add it to the commit log message. > > > Move the definition of sample_reg_masks to the conditional part of > > > perf_regs.h and guard usage with HAVE_PERF_REGS_SUPPORT. This avoids the > > > weak symbol. > > > Fix an issue when HAVE_PERF_REGS_SUPPORT isn't defined from patch v1. > > > In v3, add perf_regs.c for architectures that HAVE_PERF_REGS_SUPPORT but > > > don't declare sample_regs_masks. > > looks good to me (again ;-)), let's see if it passes Arnaldo's farm It passed a few of the usual places where things like this break, I'll submit it to a full set of build environments soon, together with what is sitting in acme/perf/core. Thanks, - Arnaldo
