This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH] Fix overzealous DSE on sparc
From: Richard Sandiford <rdsandiford@googlemail.com>
Date: Thu, 03 May 2012 10:17:44 +0100
> Richard Guenther <richard.guenther@gmail.com> writes:
>> On Thu, May 3, 2012 at 10:31 AM, Richard Sandiford
>> <rdsandiford@googlemail.com> wrote:
>>> David Miller <davem@davemloft.net> writes:
>>>> From: Richard Sandiford <rdsandiford@googlemail.com>
>>>> Date: Wed, 02 May 2012 20:37:58 +0100
>>>>
>>>>> I think the DSE assuption is fair though. ?If you reuse MEMs like
>>>>> this, then they're no longer just serving the purpose described by
>>>>> MEM_EXPR.
>>>>
>>>> The following seems to work, and matches what calls.c does when it
>>>> passes a value by reference. ?Is this what you had in mind?
>>>
>>> Yeah, looks good to me.
>>
>> I don't think that will work reliably (well, maybe now by luck, so better than
>> nothing). You'd at least need to adjust the ESCAPED points-to set of the
>> function, too (yes, DSE does very very conservative use analysis right now).
>
> Ah.
>
>> Why not simply clear MEM_EXPR for the MEM?
>
> The problem is that MEM rtxes aren't shared. AIUI, the store will already
> have been emitted by this point, using a distinct (but equivalent) MEM rtx.
Again, why doesn't calls.c have to do anything about this when it
passes arguments by reference? It does exactly what I'm changing the
sparc libcall code to do.