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References: <002d01d8c79f$dc5fe830$951fb890$@nextmovesoftware.com> <4855eb8c-22fd-17f5-fa97-eacd33d015dc@inria.fr>
In-Reply-To: <4855eb8c-22fd-17f5-fa97-eacd33d015dc@inria.fr>
From: Richard Biener <richard.guenther@gmail.com>
Date: Thu, 15 Sep 2022 08:39:16 +0200
Message-ID: <CAFiYyc1dAZSZLk06S6830j+N4NabOSpvnfqMB8_tC9FAztFPLg@mail.gmail.com>
Subject: Re: [PATCH] Optimize (X<<C)+(Y<<C) as (X+Y)<<C for signed addition.
To: Marc Glisse <marc.glisse@inria.fr>
Cc: Roger Sayle <roger@nextmovesoftware.com>, GCC Patches <gcc-patches@gcc.gnu.org>
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On Wed, Sep 14, 2022 at 11:42 PM Marc Glisse via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Tue, 13 Sep 2022, Roger Sayle wrote:
>
> > This patch tweaks the match.pd transformation previously added to fold
> > (X<<C)+(Y<<C) as (X+Y)<<C that was previously restricted to unsigned
> > (wrapping) types, to also allow signed integer types provided that they
> > don't trap and the overflow needn't be preserved for sanitization.
> > i.e. this should now apply (by default) for "int x,y;", but is disabled
> > with -ftrapv.
>
> In https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html , I
> read:
>
> "Bitwise operators act on the representation of the value including both
> the sign and value bits, where the sign bit is considered immediately
> above the highest-value value bit. Signed =E2=80=98>>=E2=80=99 acts on ne=
gative numbers by
> sign extension.
>
> As an extension to the C language, GCC does not use the latitude given in
> C99 and C11 only to treat certain aspects of signed =E2=80=98<<=E2=80=99 =
as undefined."

Oh, I wasn't aware of that.

> To me, this means that for instance INT_MIN<<1 is well defined and
> evaluates to 0. But with this patch we turn (INT_MIN<<1)+(INT_MIN<<1) int=
o
> (INT_MIN+INT_MIN)<<1, which is UB.
>
> If we decide not to support this extension anymore, I think we need to
> change the documentation first.

Or perform the (X+Y) operation in an unsigned type.

Richard.

> --
> Marc Glisse