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

[Xi Ruoyao <xry111@xry111.site>]

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.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University