Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 25 Feb 2020 13:28:50 -0800 | Subject | Re: [PATCH] perf: fix -Wstring-compare |
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On Tue, Feb 25, 2020 at 1:35 AM David Laight <David.Laight@aculab.com> wrote: > > From: Nick Desaulniers > > Sent: 24 February 2020 22:06 > > 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? > > The string compares are just stupid. > If the 'fake' strings are not printed you could use: > #define SRCLINE_UNKNOWN ((const char *)1) > > Otherwise defining the strings in one of the C files is better. > Relying on the linker to merge the strings from different compilation > units is so broken...
Note: it looks like free_srcline() already does strcmp, so my patch basically does a more consistent job for string comparisons. Forward declaring then defining in tools/perf/util/srcline.c involves changing the function signatures and struct members to `const char*` rather than `char*`, which is of questionable value. I wouldn't mind changing my patch to just use strcmp instead of strncmp, or convert free_srcline() to use strncmp instead, if folks felt strongly about being consistent. Otherwise I think my patch with Ian's Reviewed-by is the best approach. -- Thanks, ~Nick Desaulniers
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