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Date: Tue, 25 Aug 2020 23:58:54 -0400 (EDT)
From: Hans-Peter Nilsson <hp@bitrange.com>
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To: law@redhat.com
cc: Senthil Kumar Selvaraj <senthil.thecoder@gmail.com>, 
 Pip Cet <pipcet@gmail.com>, gcc@gcc.gnu.org, ebotcazou@adacore.com, 
 Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: Clobber REG_CC only for some constraint alternatives?
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On Mon, 24 Aug 2020, Jeff Law via Gcc wrote:
> On Thu, 2020-08-20 at 21:36 +0530, Senthil Kumar Selvaraj via Gcc wrote:
> > The post-reload splitter introduces the clobber. The wiki
> > suggests that approach if most insns clobber REG_CC, perhaps because of
> > the missed optimizations you describe below?
> If most patterns set/clobber the flags, then yes, it's slightly better to only
> expose them after reload.   Various passes that directly grub through RTL rather
> than using helpers like single_set will optimize things better.

The "slightly" being omissions like the one fixed in combine.c
last month. ;-)  There's one in reload too ("Discard obvious
no-ops, even without -O"), but it seems to be acting the other
way(!)

If, for a machine where most insns clobber REG_CC, you instead
emit with the clobber initially, you can have a tidier port: one
define_subst:ed pattern instead of...  How many is it with the
only-expose-after-reload method (counting define_insn,
define_split, possibly a define_expand too that wasn't there for
cc0)?  Two or three, per (original) insn with cc0?

brgds, H-P