Re: [PATCH] perf: fix -Wstring-compare

From: Ian Rogers <irogers@google.com>
To: David Laight <David.Laight@aculab.com>
Cc: Nick Desaulniers <nick.desaulniers@gmail.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Jin Yao <yao.jin@linux.intel.com>,
	Changbin Du <changbin.du@intel.com>,
	John Keeping <john@metanate.com>,
	Song Liu <songliubraving@fb.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] perf: fix -Wstring-compare
Date: Mon, 24 Feb 2020 10:19:33 -0800	[thread overview]
Message-ID: <CAP-5=fXO+YMO9asspqYdvXATZONCbBYMGbdVNU_3+W3BdeunGg@mail.gmail.com> (raw)
In-Reply-To: <dad75d5a7aa443e39dc20972d80ee83c@AcuMS.aculab.com>

On Mon, Feb 24, 2020 at 8:03 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Ian Rogers
> > Sent: 24 February 2020 05:56
> > On Sun, Feb 23, 2020 at 11:35 AM Nick Desaulniers
> > <nick.desaulniers@gmail.com> wrote:
> > >
> > > Clang warns:
> > >
> > > util/block-info.c:298:18: error: result of comparison against a string
> > > literal is unspecified (use an explicit string comparison function
> > > instead) [-Werror,-Wstring-compare]
> > >         if ((start_line != SRCLINE_UNKNOWN) && (end_line != SRCLINE_UNKNOWN)) {
> > >                         ^  ~~~~~~~~~~~~~~~
> > > util/block-info.c:298:51: error: result of comparison against a string
> > > literal is unspecified (use an explicit string comparison function
> > > instead) [-Werror,-Wstring-compare]
> > >         if ((start_line != SRCLINE_UNKNOWN) && (end_line != SRCLINE_UNKNOWN)) {
> > >                                                          ^  ~~~~~~~~~~~~~~~
> > > util/block-info.c:298:18: error: result of comparison against a string
> > > literal is unspecified (use an explicit string
> > > comparison function instead) [-Werror,-Wstring-compare]
> > >         if ((start_line != SRCLINE_UNKNOWN) && (end_line != SRCLINE_UNKNOWN)) {
> > >                         ^  ~~~~~~~~~~~~~~~
> > > util/block-info.c:298:51: error: result of comparison against a string
> > > literal is unspecified (use an explicit string comparison function
> > > instead) [-Werror,-Wstring-compare]
> > >         if ((start_line != SRCLINE_UNKNOWN) && (end_line != SRCLINE_UNKNOWN)) {
> > >                                                          ^  ~~~~~~~~~~~~~~~
> > > util/map.c:434:15: error: result of comparison against a string literal
> > > is unspecified (use an explicit string comparison function instead)
> > > [-Werror,-Wstring-compare]
> > >                 if (srcline != SRCLINE_UNKNOWN)
> > >                             ^  ~~~~~~~~~~~~~~~
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/900
> > > Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
> > > ---
> > > Note: was generated off of mainline; can rebase on -next if it doesn't
> > > apply cleanly.

Reviewed-by: Ian Rogers <irogers@google.com>

> > Looks good to me. Some more context:
> > https://clang.llvm.org/docs/DiagnosticsReference.html#wstring-compare
> > The spec says:
> > J.1 Unspecified behavior
> > The following are unspecified:
> > .. Whether two string literals result in distinct arrays (6.4.5).
>
> Just change the (probable):
> #define SRCLINE_UNKNOWN "unknown"
> with
> static const char SRC_LINE_UNKNOWN[] = "unk";
>
>         David

The SRCLINE_UNKNOWN is used to convey information. Having multiple
distinct pointers (static) would mean the compiler could likely remove
all comparisons as the compiler could prove that pointer is never
returned by a function - ie comparisons are either known to be true
(!=) or false (==).

Thanks,
Ian

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