From: Jiri Olsa <jolsa@redhat.com>
To: linux-kernel@vger.kernel.org
Cc: Jiri Olsa <jolsa@redhat.com>,
Corey Ashford <cjashfor@linux.vnet.ibm.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
Ingo Molnar <mingo@elte.hu>, Namhyung Kim <namhyung@kernel.org>,
Paul Mackerras <paulus@samba.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
David Ahern <dsahern@gmail.com>
Subject: [RFC] perf tools: Fix strict alias issue for find_first_bit
Date: Wed, 26 Feb 2014 18:14:26 +0100
Message-ID: <1393434867-20271-1-git-send-email-jolsa@redhat.com> (raw)
hi,
got issue below when compiling perf tool on i686 with gcc 4.4,
but not sure the patch is correct workaround here.
thanks for comments,
jirka
---
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’:
/root/linux/tools/perf/util/include/linux/bitops.h:109: error: dereferencing pointer ‘p’ does break strict-aliasing rules
/root/linux/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
/root/linux/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@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: David Ahern <dsahern@gmail.com>
---
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;
--
1.8.3.1
next reply index
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-26 17:14 Jiri Olsa [this message]
2014-02-26 17:18 ` Peter Zijlstra
2014-02-26 17:47 ` [PATCH] perf tools: Do not compile with -fstrict-aliasing Jiri Olsa
2014-03-02 8:57 ` [tip:perf/urgent] perf tools: Fix strict alias issue for find_first_bit tip-bot for Jiri Olsa
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