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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 24 Feb 2020 14:05:38 -0800
Message-ID: <CAKwvOdko+Qb=h_Pm=oFUnsiBg7Am9u=Z83g8cNyY1QZQS=Mh7g@mail.gmail.com>
Subject: Re: [PATCH] perf: fix -Wstring-compare
To:     Ian Rogers <irogers@google.com>,
        David Laight <David.Laight@aculab.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
Cc:     Nick Desaulniers <nick.desaulniers@gmail.com>,
        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>
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On Mon, Feb 24, 2020 at 10:20 AM 'Ian Rogers' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> 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 (==).

I wouldn't define a static string in a header.  Though I could:
1. forward declare it in the header with extern linkage.
2. define it in *one* .c source file.

Thoughts on that vs the current approach?
-- 
Thanks,
~Nick Desaulniers