Ian Rogers 42466b9f29 perf tools: Avoid 'sample_reg_masks' being const + weak ... 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: f49573d1ee 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. 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. Further notes: Jiri asked: "Is this just a precaution or you actualy saw some breakage?" Ian answered: "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." Signed-off-by: Ian Rogers <irogers@google.com> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Acked-by: Jiri Olsa <jolsa@kernel.org> Cc: Albert Ou <aou@eecs.berkeley.edu> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: Alexey Budankov <alexey.budankov@linux.intel.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: clang-built-linux@googlegroups.com Cc: Guo Ren <guoren@kernel.org> Cc: Kan Liang <kan.liang@linux.intel.com> Cc: linux-riscv@lists.infradead.org Cc: Mao Han <han_mao@c-sky.com> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Namhyung Kim <namhyung@kernel.org> Cc: Palmer Dabbelt <palmer@sifive.com> Cc: Paul Walmsley <paul.walmsley@sifive.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Stephane Eranian <eranian@google.com> Link: http://lore.kernel.org/lkml/20191001003623.255186-1-irogers@google.com Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>		2019-10-10 09:29:33 -03:00
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