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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/104438] Combine optimization opportunity
 exposed after pro_and_epilogue
Date: Tue, 08 Feb 2022 14:42:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104438

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Also combine could work that late in principle: it can deal with hard
registers, after all.  But it would be a terrible idea.  A single combine
pass is expensive enough, we don't want to run it N times.  Also, if you
run combine more than once, you get odd effects, mostly because the results
of splitters are combined back again.

After *logue insertion all the simpler (more local!) optimisations are still
run (DSE, DCE, if conversion, const prop, peep).

It is unclear why the CFG wasn't straightened out here.  Is the bb commented
as "asm" actually asm?  Then GCC will not see it is very cheap/small, yeah.=