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 Sun, 2023-11-12 at 11:02 -0700, Jeff Law wrote:
> 
> 
> 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.

So I'm marking the PR fixed.  Please CC me when iterating this patch for
another round so I can test it on loongarch64-linux-gnu.

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