Re: [PATCH v2] In the pipeline, USE or CLOBBER should delay execution if it starts a new live range.

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/12/23 10:41, Xi Ruoyao wrote:
> On Sat, 2023-11-11 at 13:12 -0700, Jeff Law wrote:
>>
>>
>> On 8/14/23 05:22, Jin Ma wrote:
>>> CLOBBER and USE does not represent real instructions, but in the
>>> process of pipeline optimization, they will wait for transmission
>>> in ready list like other insns, without considering resource
>>> conflicts and cycles. This results in a multi-issue CPU architecture
>>> that can be issued at any time if other regular insns have resource
>>> conflicts or cannot be launched for other reasons. As a result,
>>> its position is advanced in the generated insns sequence, which
>>> will affect register allocation and often lead to more redundant
>>> mov instructions.
>>>
>>> A simple example:
>>> https://github.com/majin2020/gcc-test/blob/master/test.c
>>> This is a function in the dhrystone benchmark.
>>>
>>> https://github.com/majin2020/gcc-test/blob/0b08c1a13de9663d7d9aba7539b960ec0607ca24/test.c.299r.sched1
>>> This is a log of the pass 'sched1' When -mtune=rocket but issue_rate
>>> == 2.
>>>
>>> The pipeline is:
>>> ;; | insn | prio |
>>> ;; |  17  |  3   | r142=a0 alu
>>> ;; |  14  |  0   | clobber r136 nothing
>>> ;; |  13  |  0   | clobber a0 nothing
>>> ;; |  18  |  2   | r143=a1 alu
>>> ...
>>> ;; |  12  |  0   | a0=r136 alu
>>> ;; |  15  |  0   | use a0 nothing
>>>
>>> In this log, insn 13 and 14 are much ahead of schedule, which risks
>>> generating
>>> redundant mov instructions, which seems unreasonable.
>>>
>>> Therefore, I submit patch again on the basis of the last review
>>> opinions to try to solve this problem.
>>>
>>> https://github.com/majin2020/gcc-test/commit/efcb43e3369e771bde702955048bfe3f501263dd#diff-805031b1be5092a2322852a248d0b0f92eef7cad5784a8209f4dfc6221407457L189
>>> This is the diff log of shed1 after patch is added.
>>>
>>> The new pipeline is:
>>> ;; | insn | prio |
>>> ;; |  17  |  3   | r142=a0 alu
>>> ...
>>> ;; |  10  |  0   | [r144]=r141 alu
>>> ;; |  13  |  0   | clobber a0 nothing
>>> ;; |  14  |  0   | clobber r136 nothing
>>> ;; |  12  |  0   | a0=r136 alu
>>> ;; |  15  |  0   | use a0 nothing
>>>
>>> gcc/ChangeLog:
>>> 	* haifa-sched.cc (use_or_clobber_starts_range_p): New.
>>> 	(prune_ready_list): USE or CLOBBER should delay execution
>>> 	if it starts a new live range.
>> OK for the trunk.  It doesn't look like you have write access and I
>> don't see anything about what testing was done.  Standard practice is
>> to
>> do a bootstrap and regression test on a primary platform such as x86,
>> aarch64, ppc64.
>>
>> I went ahead and did a bootstrap and regression test on x86_64, then
>> pushed this to the trunk.
> 
> Unfortunately this patch has triggered a bootstrap comparison failure on
> loongarch64-linux-gnu: https://gcc.gnu.org/PR112497.
It's also causing simple build failures on other targets.  For example 
c6x-elf aborts when compiling gcc.c-torture/execute/pr82210 (and others) 
with -O2 with that patch applied.

I've reverted it for now.  I'm not going to have time to investigate 
this week.

Jeff
>