[PATCH 2/2] perf tools: Fix strict alias issue for find_first_bit

[Arnaldo Carvalho de Melo]

From: Jiri Olsa <jolsa@xxxxxxxxxx>

When compiling perf tool code with gcc 4.4.7 I'm getting
following error:

    CC       util/session.o
  cc1: warnings being treated as errors
  util/session.c: In function âperf_session_deliver_eventâ:
  tools/perf/util/include/linux/bitops.h:109: error: dereferencing pointer âpâ does break strict-aliasing rules
  tools/perf/util/include/linux/bitops.h:101: error: dereferencing pointer âpâ does break strict-aliasing rules
  util/session.c:697: note: initialized from here
  tools/perf/util/include/linux/bitops.h:101: note: initialized from here
  make[1]: *** [util/session.o] Error 1
  make: *** [util/session.o] Error 2

The aliased types here are u64 and unsigned long pointers, which is safe
for the find_first_bit processing.

This error shows up for me only for gcc 4.4 on 32bit x86, even for
-Wstrict-aliasing=3, while newer gcc are quiet and scream here for
-Wstrict-aliasing={2,1}. Looks like newer gcc changed the rules for
strict alias warnings.

The gcc documentation offers workaround for valid aliasing by using
__may_alias__ attribute:

  http://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Type-Attributes.html

Using this workaround for the find_first_bit function.

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Corey Ashford <cjashfor@xxxxxxxxxxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1393434867-20271-1-git-send-email-jolsa@xxxxxxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 tools/perf/util/include/linux/bitops.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/include/linux/bitops.h b/tools/perf/util/include/linux/bitops.h
index 45cf10a562bd..dadfa7e54287 100644
--- a/tools/perf/util/include/linux/bitops.h
+++ b/tools/perf/util/include/linux/bitops.h
@@ -87,13 +87,15 @@ static __always_inline unsigned long __ffs(unsigned long word)
 	return num;
 }
 
+typedef const unsigned long __attribute__((__may_alias__)) long_alias_t;
+
 /*
  * Find the first set bit in a memory region.
  */
 static inline unsigned long
 find_first_bit(const unsigned long *addr, unsigned long size)
 {
-	const unsigned long *p = addr;
+	long_alias_t *p = (long_alias_t *) addr;
 	unsigned long result = 0;
 	unsigned long tmp;
 
-- 
1.8.1.4

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