[PATCH] Fix for PR55492 : __atomic_load doesn't match ACQUIRE memory model

[PATCH] Fix for PR55492 : __atomic_load doesn't match ACQUIRE memory model

[Yvan Roux]

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Hi,

on ARMv7, the code generated for the __atomic_load builtins in the
__ATOMIC_ACQUIRE memory model, puts a memory barrier before the load, whereas
the semantic of the acquire memory model implies a barrier after.

The issue seems to be in expand_atomic_load which puts a memory fence before
the load in any memory model. The attached patch fixes the problem.

Thanks,
Yvan

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2012-11-29  Yvan Roux  <yvan.roux@st.com>

	PR target/55942
	* optabs.c (expand_atomic_load): Condition the memory fence to the
	the right memory model.

diff --git a/gcc/optabs.c b/gcc/optabs.c
index 353727f..14956be 100644
--- a/gcc/optabs.c
+++ b/gcc/optabs.c
@@ -7478,12 +7478,13 @@ expand_atomic_load (rtx target, rtx mem, enum memmodel model)
     target = gen_reg_rtx (mode);
 
   /* Emit the appropriate barrier before the load.  */
-  expand_mem_thread_fence (model);
+  if (model == MEMMODEL_SEQ_CST)
+    expand_mem_thread_fence (model);
 
   emit_move_insn (target, mem);
 
-  /* For SEQ_CST, also emit a barrier after the load.  */
-  if (model == MEMMODEL_SEQ_CST)
+  /* For SEQ_CST or ACQUIRE, also emit a barrier after the load.  */
+  if (model == MEMMODEL_SEQ_CST || model == MEMMODEL_ACQUIRE)
     expand_mem_thread_fence (model);
 
   return target;
-- 
1.7.9.5

Re: [PATCH] Fix for PR55492 : __atomic_load doesn't match ACQUIRE memory model

[Andrew MacLeod]

On 11/29/2012 10:42 AM, Yvan Roux wrote:
> Hi,
>
> on ARMv7, the code generated for the __atomic_load builtins in the
> __ATOMIC_ACQUIRE memory model, puts a memory barrier before the load, whereas
> the semantic of the acquire memory model implies a barrier after.
>
> The issue seems to be in expand_atomic_load which puts a memory fence before
> the load in any memory model. The attached patch fixes the problem.
>
Patch is fine.

If you can confirm you have a copyright assignment on file I can take 
care of checking it in.

Andrew

Re: [PATCH] Fix for PR55492 : __atomic_load doesn't match ACQUIRE memory model

[Yvan Roux]

> If you can confirm you have a copyright assignment on file I can take care
> of checking it in.

Yes, I am covered by the copyright assignement RT 211150 between
STMicroelectronics and FSF.

Thanks,
Yvan

Re: [PATCH] Fix for PR55492 : __atomic_load doesn't match ACQUIRE memory model

[Marcus Shawcroft]

On 29/11/12 15:42, Yvan Roux wrote:
> Hi,
>
> on ARMv7, the code generated for the __atomic_load builtins in the
> __ATOMIC_ACQUIRE memory model, puts a memory barrier before the load, whereas
> the semantic of the acquire memory model implies a barrier after.
>
> The issue seems to be in expand_atomic_load which puts a memory fence before
> the load in any memory model. The attached patch fixes the problem.
>
> Thanks,
> Yvan
>

The same issue exists in the 4.7 branch.

Can the fix be back ported?

/Marcus

Re: [PATCH] Fix for PR55492 : __atomic_load doesn't match ACQUIRE memory model

[Yvan Roux]

> Can the fix be back ported?

Yes, the back port on 4.7 is straightforward, it just needs to be commited.

Yvan