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From:   Ian Rogers <irogers@google.com>
Date:   Sun, 23 Feb 2020 21:55:38 -0800
Message-ID: <CAP-5=fU=+uYZDb2uSFO8CTJ-Ange4Nxh4mmsOC1MS=Tedois9g@mail.gmail.com>
Subject: Re: [PATCH] perf: fix -Wstring-compare
To:     Nick Desaulniers <nick.desaulniers@gmail.com>
Cc:     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>
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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.

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).

>  tools/perf/builtin-diff.c    | 3 ++-
>  tools/perf/util/block-info.c | 3 ++-
>  tools/perf/util/map.c        | 2 +-
>  3 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/builtin-diff.c b/tools/perf/builtin-diff.c
> index f8b6ae557d8b..c03c36fde7e2 100644
> --- a/tools/perf/builtin-diff.c
> +++ b/tools/perf/builtin-diff.c
> @@ -1312,7 +1312,8 @@ static int cycles_printf(struct hist_entry *he, struct hist_entry *pair,
>         end_line = map__srcline(he->ms.map, bi->sym->start + bi->end,
>                                 he->ms.sym);
>
> -       if ((start_line != SRCLINE_UNKNOWN) && (end_line != SRCLINE_UNKNOWN)) {
> +       if ((strncmp(start_line, SRCLINE_UNKNOWN, strlen(SRCLINE_UNKNOWN)) != 0) &&
> +           (strncmp(end_line, SRCLINE_UNKNOWN, strlen(SRCLINE_UNKNOWN)) != 0)) {
>                 scnprintf(buf, sizeof(buf), "[%s -> %s] %4ld",
>                           start_line, end_line, block_he->diff.cycles);
>         } else {
> diff --git a/tools/perf/util/block-info.c b/tools/perf/util/block-info.c
> index c4b030bf6ec2..fbbb6d640dad 100644
> --- a/tools/perf/util/block-info.c
> +++ b/tools/perf/util/block-info.c
> @@ -295,7 +295,8 @@ static int block_range_entry(struct perf_hpp_fmt *fmt, struct perf_hpp *hpp,
>         end_line = map__srcline(he->ms.map, bi->sym->start + bi->end,
>                                 he->ms.sym);
>
> -       if ((start_line != SRCLINE_UNKNOWN) && (end_line != SRCLINE_UNKNOWN)) {
> +       if ((strncmp(start_line, SRCLINE_UNKNOWN, strlen(SRCLINE_UNKNOWN)) != 0) &&
> +           (strncmp(end_line, SRCLINE_UNKNOWN, strlen(SRCLINE_UNKNOWN)) != 0)) {
>                 scnprintf(buf, sizeof(buf), "[%s -> %s]",
>                           start_line, end_line);
>         } else {
> diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
> index a08ca276098e..95428511300d 100644
> --- a/tools/perf/util/map.c
> +++ b/tools/perf/util/map.c
> @@ -431,7 +431,7 @@ int map__fprintf_srcline(struct map *map, u64 addr, const char *prefix,
>
>         if (map && map->dso) {
>                 char *srcline = map__srcline(map, addr, NULL);
> -               if (srcline != SRCLINE_UNKNOWN)
> +               if (strncmp(srcline, SRCLINE_UNKNOWN, strlen(SRCLINE_UNKNOWN)) != 0)
>                         ret = fprintf(fp, "%s%s", prefix, srcline);
>                 free_srcline(srcline);
>         }
> --
> 2.17.1
>
> --
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