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Subject[PATCH 2/2] perf tools: Fix strict alias issue for find_first_bit
From: Jiri Olsa <>

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 ‘perf_session_deliver_event’:
tools/perf/util/include/linux/bitops.h:109: error: dereferencing pointer ‘p’ does break strict-aliasing rules
tools/perf/util/include/linux/bitops.h:101: error: dereferencing pointer ‘p’ 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:

Using this workaround for the find_first_bit function.

Signed-off-by: Jiri Olsa <>
Cc: Corey Ashford <>
Cc: David Ahern <>
Cc: Frederic Weisbecker <>
Cc: Ingo Molnar <>
Cc: Namhyung Kim <>
Cc: Paul Mackerras <>
Cc: Peter Zijlstra <>
Signed-off-by: Arnaldo Carvalho de Melo <>
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;

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