perf srcline: Optimize comparision against SRCLINE_UNKNOWN

This is a string constant that gets returned and then strcmp() around,
we can instead just do a pointer comparision.

That requires a new global variable to comply with these warnings from
some versions of clang and gcc:

  41    68.95 fedora:rawhide                : FAIL clang version 16.0.4 (Fedora 16.0.4-1.fc39)
    result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Werror,-Wstring-compare]
            if (start_line != SRCLINE_UNKNOWN &&
                           ^  ~~~~~~~~~~~~~~~  41

Ack comments:

Agreed, the strcmps make me nervous as they won't distinguish heap from
a global meaning we could end up with things like pointers to freed
memory. The comparison with the global is always going to be same imo.

Acked-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ali Saidi <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Athira Rajeev <[email protected]>
Cc: Brian Robbins <[email protected]>
Cc: Changbin Du <[email protected]>
Cc: Dmitrii Dolgov <[email protected]>
Cc: Fangrui Song <[email protected]>
Cc: German Gomez <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Ivan Babrou <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jing Zhang <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: K Prateek Nayak <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Naveen N. Rao <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Steinar H. Gunderson <[email protected]>
Cc: Suzuki Poulouse <[email protected]>
Cc: Wenyu Liu <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Yang Jihong <[email protected]>
Cc: Ye Xingchen <[email protected]>
Cc: Yuan Can <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

1 files changed, 2 insertions, 2 deletions

diff --git a/tools/perf/builtin-diff.c b/tools/perf/builtin-diff.c
index eec89567ae48..e8a1b16aa5f8 100644
--- a/tools/perf/builtin-diff.c
+++ b/tools/perf/builtin-diff.c
@@ -1378,8 +1378,8 @@ 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 ((strncmp(start_line, SRCLINE_UNKNOWN, strlen(SRCLINE_UNKNOWN)) != 0) &&
-	    (strncmp(end_line, SRCLINE_UNKNOWN, strlen(SRCLINE_UNKNOWN)) != 0)) {
+	if (start_line != SRCLINE_UNKNOWN &&
+	    end_line != SRCLINE_UNKNOWN) {
 		scnprintf(buf, sizeof(buf), "[%s -> %s] %4ld",
 			  start_line, end_line, block_he->diff.cycles);
 	} else {