[PATCH, AArch64 v2 05/11] aarch64: Emit LSE st<op> instructions

[Richard Henderson]

On 10/30/18 8:32 PM, James Greenhalgh wrote:
> On Tue, Oct 02, 2018 at 11:19:09AM -0500, Richard Henderson wrote:
>> When the result of an operation is not used, we can ignore the
>> result by storing to XZR.  For two of the memory models, using
>> XZR with LD<op> has a preferred assembler alias, ST<op>.
> 
> ST<op> has different semantics to LD<op>, in particular, ST<op> is not
> ordered by a DMB LD; so this could weaken the LDADD and break C11 semantics.
> 
> The relevant Arm Arm text is:
> 
>   If the destination register is not one of WZR or XZR, LDADDA and
>   LDADDAL load from memory with acquire semantics

You're quite right.  I must have glossed over that clause when looking at this
before.  I'll make sure there's a temp register to clobber for v2.


r~