Re: [PATCH v3] perf tools: avoid sample_reg_masks being const + weak

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Andi Kleen <ak@linux.intel.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Alexey Budankov <alexey.budankov@linux.intel.com>,
	Palmer Dabbelt <palmer@sifive.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Stephane Eranian <eranian@google.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>, Guo Ren <guoren@kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Namhyung Kim <namhyung@kernel.org>,
	linux-riscv@lists.infradead.org, Jiri Olsa <jolsa@redhat.com>,
	Mao Han <han_mao@c-sky.com>,
	Kan Liang <kan.liang@linux.intel.com>
Subject: Re: [PATCH v3] perf tools: avoid sample_reg_masks being const + weak
Date: Thu, 10 Oct 2019 09:29:08 -0300	[thread overview]
Message-ID: <20191010122908.GA19434@kernel.org> (raw)
In-Reply-To: <CAP-5=fUSgjyLkZJaHTvdFbzZijy6Gzmx5UZHK_brxVEhFpMG8g@mail.gmail.com>

Em Wed, Oct 09, 2019 at 04:07:37PM -0700, Ian Rogers escreveu:
> On Tue, Oct 8, 2019 at 5:31 AM Jiri Olsa <jolsa@redhat.com> 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

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