Subject: [PATCH] tools include: Fix strict-aliasing rules breakage "tools/include/linux/compiler.h:66: error: dereferencing pointer ‘res.41’ does break strict-aliasing rules"

From: Jiri Olsa <[email-masked]>
Date: Tue, 13 Oct 2015 10:52:14 +0200

On Mon, Oct 12, 2015 at 05:48:17PM -0700, Vinson Lee wrote:

SNIP

> >                 __builtin_memcpy((void *)res, (const void *)p, size);
> > @@ -59,11 +64,14 @@ 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)
> >  {
> > +       u64_alias_t *u64_p = (u64_alias_t*) p;
> > +       u64_alias_t *u64_res = (u64_alias_t*) res;
> > +
> >         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 8: *u64_p = *u64_res; break;
> >         default:
> >                 barrier();
> >                 __builtin_memcpy((void *)p, (const void *)res, size);
> 
> 
> This patch fixes my perf build error with GCC 4.4.
> 
> Vinson

posting complete patch

thanks,
jirka

---
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-by: Vinson Lee <[email-masked]>
Link: http://lkml.kernel.org/n/[email-masked]
Signed-off-by: Jiri Olsa <[email-masked]>
---
 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 9098083869c8..fa7208a32d76 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);
-- 
2.4.3