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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/30484] INT_MIN % -1 is well defined for -fwrapv
Date: Tue, 24 Aug 2021 08:47:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D30484

--- Comment #18 from Vincent Lef=C3=A8vre <vincent-gcc at vinc17 dot net> -=
--
(In reply to Vincent Lef=C3=A8vre from comment #16)
> int f (int a, int b, int c)
> {
>   if (b < 0)
>     return a + b + c;
>   else
>     return a + c + b;
> }
>=20
> The generated code with -O3 has 6 instructions:
>=20
>         leal    (%rdi,%rdx), %eax
>         addl    %esi, %edi
>         addl    %edx, %edi
>         addl    %esi, %eax
>         testl   %esi, %esi
>         cmovs   %edi, %eax
>=20
> In theory, the compiler could normally optimize to produce the same code =
as
> with the source that assumes -fwrapv (here, a + b + c and a + c + b are
> obviously equivalent on a typical processor), but in practice, this is of=
ten
> not the case as shown above.

Surprisingly, GCC can optimize this second test to 2 instructions with -fwr=
apv.
I've reported PR102032 about the missed optimization without -fwrapv.=