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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/102840] [12 Regression]
 gcc.target/i386/pr22076.c by r12-4475
Date: Tue, 19 Oct 2021 17:42:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102840

--- Comment #3 from Roger Sayle <roger at nextmovesoftware dot com> ---
With -m64, before:
test:   movq    .LC1(%rip), %mm0
        paddb   .LC0(%rip), %mm0
        movq    %xmm0, x(%rip)
        ret

And after:
test:   movq    .LC2(%rip), %rax
        movq    %rax, x(%rip)
        ret

So we have two movq before, and two movq after, but clearly we've avoided t=
he
computation at run-time.

It's difficult (for me) to judge whether the -m32's use of immediate consta=
nts
is now better than -m64's load memory/store memory idiom in the "average ca=
se",
but worst case [data cache miss], the former is clearly better [requiring o=
nly
fewer memory transactions].=