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From: "user202729 at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/115576] [14/15 regression] Worse code generated for
 simple struct conversion since r14-2386-gbdf2737cda53a8
Date: Wed, 26 Jun 2024 07:10:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115576

--- Comment #5 from user202729 <user202729 at protonmail dot com> ---
I think this can be resolved by implementing some code in combine.cc to
replace:

(set:TI (reg:TI 101) (zero_extend:TI (...:DI ...)))
(set:DI (reg:DI ...) (subreg:DI (reg:TI 101) 8))

with

(set:TI (reg:TI 101) (zero_extend:TI (...:DI ...)))
(set:DI (reg:DI ...) (const_int 0))

This certainly is always an improvement, because 0 is simpler than extracti=
ng
the subregister.

After that, a few other passes of combine rescanning should be able to cons=
tant
fold the 0 forward.

Unfortunately, I don't know how to modify combine.cc or some other files to
handle this pattern. Can anyone give a suggestion? (maybe add a
define_peephole2 in common.md ? )=