From: Marc Glisse <marc.glisse@inria.fr>
To: Roger Sayle <roger@nextmovesoftware.com>
Cc: 'GCC Patches' <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] Optimize (X<<C)+(Y<<C) as (X+Y)<<C for signed addition.
Date: Wed, 14 Sep 2022 23:42:02 +0200 (CEST) [thread overview]
Message-ID: <4855eb8c-22fd-17f5-fa97-eacd33d015dc@inria.fr> (raw)
In-Reply-To: <002d01d8c79f$dc5fe830$951fb890$@nextmovesoftware.com>
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 ‘>>’ acts on negative 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 ‘<<’ as undefined."
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) into
(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.
--
Marc Glisse
next prev parent reply other threads:[~2022-09-14 21:42 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-13 18:37 Roger Sayle
2022-09-14 8:28 ` Richard Biener
2022-09-14 21:42 ` Marc Glisse [this message]
2022-09-15 6:39 ` Richard Biener
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