[tip:perf/core] tools include: Fix strict-aliasing rules breakage

[tip-bot for Jiri Olsa <tipbot@zytor.com>]

Commit-ID:  c95f3432118c6b7a3bde63aa6eb95ccd163119eb
Gitweb:     http://git.kernel.org/tip/c95f3432118c6b7a3bde63aa6eb95ccd163119eb
Author:     Jiri Olsa <jolsa@redhat.com>
AuthorDate: Tue, 13 Oct 2015 10:52:14 +0200
Committer:  Arnaldo Carvalho de Melo <acme@redhat.com>
CommitDate: Tue, 13 Oct 2015 11:43:16 -0300

tools include: Fix strict-aliasing rules breakage

Vinson reported build breakage with gcc 4.4 due to strict-aliasing.

   CC       util/annotate.o
 cc1: warnings being treated as errors
 util/annotate.c: In function ‘disasm__purge’:
 linux-next/tools/include/linux/compiler.h:66: error: dereferencing
 pointer ‘res.41’ does break strict-aliasing rules

The reason is READ_ONCE/WRITE_ONCE code we took from kernel sources.  They
intentionaly break aliasing rules. While this is ok for kernel because it's
built with -fno-strict-aliasing, it breaks perf which is build with
-Wstrict-aliasing=3.

Using extra __may_alias__ type to allow aliasing in this case.

Reported-and-tested-by: Vinson Lee <vlee@twopensource.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Martin Liska <mliska@suse.cz>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Rabin Vincent <rabin@rab.in>
Cc: linux-next@vger.kernel.org
Link: http://lkml.kernel.org/r/20151013085214.GB2705@krava.brq.redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/include/linux/compiler.h | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
index 9098083..fa7208a 100644
--- a/tools/include/linux/compiler.h
+++ b/tools/include/linux/compiler.h
@@ -43,13 +43,29 @@
 
 #include <linux/types.h>
 
+/*
+ * Following functions are taken from kernel sources and
+ * break aliasing rules in their original form.
+ *
+ * While kernel is compiled with -fno-strict-aliasing,
+ * perf uses -Wstrict-aliasing=3 which makes build fail
+ * under gcc 4.4.
+ *
+ * Using extra __may_alias__ type to allow aliasing
+ * in this case.
+ */
+typedef __u8  __attribute__((__may_alias__))  __u8_alias_t;
+typedef __u16 __attribute__((__may_alias__)) __u16_alias_t;
+typedef __u32 __attribute__((__may_alias__)) __u32_alias_t;
+typedef __u64 __attribute__((__may_alias__)) __u64_alias_t;
+
 static __always_inline void __read_once_size(const volatile void *p, void *res, int size)
 {
 	switch (size) {
-	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;
-	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;
-	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;
-	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;
+	case 1: *(__u8_alias_t  *) res = *(volatile __u8_alias_t  *) p; break;
+	case 2: *(__u16_alias_t *) res = *(volatile __u16_alias_t *) p; break;
+	case 4: *(__u32_alias_t *) res = *(volatile __u32_alias_t *) p; break;
+	case 8: *(__u64_alias_t *) res = *(volatile __u64_alias_t *) p; break;
 	default:
 		barrier();
 		__builtin_memcpy((void *)res, (const void *)p, size);
@@ -60,10 +76,10 @@ static __always_inline void __read_once_size(const volatile void *p, void *res,
 static __always_inline void __write_once_size(volatile void *p, void *res, int size)
 {
 	switch (size) {
-	case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
-	case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
-	case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
-	case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
+	case 1: *(volatile  __u8_alias_t *) p = *(__u8_alias_t  *) res; break;
+	case 2: *(volatile __u16_alias_t *) p = *(__u16_alias_t *) res; break;
+	case 4: *(volatile __u32_alias_t *) p = *(__u32_alias_t *) res; break;
+	case 8: *(volatile __u64_alias_t *) p = *(__u64_alias_t *) res; break;
 	default:
 		barrier();
 		__builtin_memcpy((void *)p, (const void *)res, size);