From b39c2a57a00a841f057a75b41df4c26173288b66 Mon Sep 17 00:00:00 2001 From: Jiri Olsa Date: Wed, 26 Feb 2014 18:14:26 +0100 Subject: [PATCH] perf tools: Fix strict alias issue for find_first_bit MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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: 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 Cc: Corey Ashford Cc: David Ahern Cc: Frederic Weisbecker Cc: Ingo Molnar Cc: Namhyung Kim Cc: Paul Mackerras Cc: Peter Zijlstra Link: http://lkml.kernel.org/r/1393434867-20271-1-git-send-email-jolsa@redhat.com 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 45cf10a..dadfa7e 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; -- 2.7.4