Re: [PATCH v2, middle-end]: Introduce memory_blockage named insn pattern

[Jeff Law <law at redhat dot com>]

On 10/13/2017 12:27 PM, Uros Bizjak wrote:
> On Fri, Oct 13, 2017 at 7:30 PM, Jeff Law <law@redhat.com> wrote:
>> On 09/05/2017 07:50 AM, Uros Bizjak wrote:
>>> Revised patch, incorporates fixes from Alexander's review comments.
>>>
>>> I removed some implementation details from Alexander's description of
>>> memory_blockage named pattern.
>>>
>>>
>>> 2017-09-05  Uros Bizjak  <ubizjak@gmail.com>
>>>
>>>     * target-insns.def: Add memory_blockage.
>>>     * optabs.c (expand_memory_blockage): New function.
>>>     (expand_asm_memory_barrier): Rename ...
>>>     (expand_asm_memory_blockage): ... to this.
>>>     (expand_mem_thread_fence): Call expand_memory_blockage
>>>     instead of expand_asm_memory_barrier.
>>>     (expand_mem_singnal_fence): Ditto.
>>>     (expand_atomic_load): Ditto.
>>>     (expand_atomic_store): Ditto.
>>>     * doc/md.texi (Standard Pattern Names For Generation):
>>>     Document memory_blockage instruction pattern.
>>>
>>> Bootstrapped and regression tested together with a followup x86 patch
>>> on x86_64-linux-gnu {,-m32}.
>>>
>>> OK for mainline?
>> SO I don't see anything technically wrong here.  But what I don't
>> understand is the value in providing another way to do the same thing.
>>
>> I see a patch that introduces calls to gen_memory_blockage in the x86
>> backend.  But what I don't see is why those couldn't just be
>> expand_asm_memory_barrier?
>>
>> Were you planning a x86 specific expander?  If so what advantages does
>> it have over the asm-style variant?
>>
>> I feel like I'm missing something here.
> 
> Yes: Alexander's patch generates asm-style blockage instruction in the
> generic expansion part. If we want to include this instruction in the
> peephole2 sequence, we need a different (simple and deterministic)
> instruction with known (simple) RTL pattern. Proposed middle end patch
> allows us to emit target-dependant UNSPEC pattern instead of a generic
> asm-style pattern for the blockage instruction. This way, we can
> include target-dependent UNSPEC in peephole2 sequence, as shown in a
> follow-up target patch.
Ah, you want to use it as a sequence within a peep2 match.  That makes
sense now.

OK for the trunk.

jeff