linux-kernel - Re: [PATCH v2] perf tools: avoid sample_reg

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Message-ID: <20190930103909.GA9622@kernel.org>
Date:   Mon, 30 Sep 2019 07:39:09 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Ian Rogers <irogers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2] perf tools: avoid sample_reg_masks being const + weak

Em Sun, Sep 29, 2019 at 11:05:14PM +0200, Jiri Olsa escreveu:
> On Fri, Sep 27, 2019 at 02:43:41PM -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.
> > 
> > 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.
> > 
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> 
> Acked-by: Jiri Olsa <jolsa@...nel.org>

Thanks, applied.

- Arnaldo