[RFC] perf tools: Fix strict alias issue for find_first_bit

[RFC] perf tools: Fix strict alias issue for find_first_bit

[Jiri Olsa]

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

Re: [RFC] perf tools: Fix strict alias issue for find_first_bit

[Peter Zijlstra]

On Wed, Feb 26, 2014 at 06:14:26PM +0100, Jiri Olsa wrote:
> 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
> 

The kernel builds with -fno-strict-aliasing because the C aliasing rules
are a bunch of monkey poo. Perf tool should probably do the same.

[PATCH] perf tools: Do not compile with -fstrict-aliasing

[Jiri Olsa]

On Wed, Feb 26, 2014 at 06:18:05PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 26, 2014 at 06:14:26PM +0100, Jiri Olsa wrote:
> > 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
> > 
> 
> The kernel builds with -fno-strict-aliasing because the C aliasing rules
> are a bunch of monkey poo. Perf tool should probably do the same.

my pleasure ;-)

thanks,
jirka


---
Switching off the strict aliasing for perf tool compilation,
because it causes compilation issues/errors for older gcc,
and to quote Peter:
  "C aliasing rules are a bunch of monkey poo"

We don't enable strict aliasing actively, it is forced by
-Ox option. Using -fno-strict-aliasing option at the same
time we use -O.

Removing -Wstrict-aliasing=3 option as well.

Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
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/config/Makefile     | 2 +-
 tools/scripts/Makefile.include | 1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/tools/perf/config/Makefile b/tools/perf/config/Makefile
index f7c81d3..e7786c7 100644
--- a/tools/perf/config/Makefile
+++ b/tools/perf/config/Makefile
@@ -94,7 +94,7 @@ ifndef DEBUG
 endif
 
 ifeq ($(DEBUG),0)
-  CFLAGS += -O6
+  CFLAGS += -O6 -fno-strict-aliasing
 endif
 
 ifdef PARSER_DEBUG
diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index 8abbef1..3667b5b 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -32,7 +32,6 @@ EXTRA_WARNINGS += -Wold-style-definition
 EXTRA_WARNINGS += -Wpacked
 EXTRA_WARNINGS += -Wredundant-decls
 EXTRA_WARNINGS += -Wshadow
-EXTRA_WARNINGS += -Wstrict-aliasing=3
 EXTRA_WARNINGS += -Wstrict-prototypes
 EXTRA_WARNINGS += -Wswitch-default
 EXTRA_WARNINGS += -Wswitch-enum
-- 
1.8.3.1

[tip:perf/urgent] perf tools: Fix strict alias issue for find_first_bit

[tip-bot for Jiri Olsa]

Commit-ID:  b39c2a57a00a841f057a75b41df4c26173288b66
Gitweb:     http://git.kernel.org/tip/b39c2a57a00a841f057a75b41df4c26173288b66
Author:     Jiri Olsa <jolsa@redhat.com>
AuthorDate: Wed, 26 Feb 2014 18:14:26 +0100
Committer:  Arnaldo Carvalho de Melo <acme@redhat.com>
CommitDate: Fri, 28 Feb 2014 10:39:40 -0300

perf tools: Fix strict alias issue for find_first_bit

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 <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.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>
Link: http://lkml.kernel.org/r/1393434867-20271-1-git-send-email-jolsa@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.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;