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Date: Wed, 14 Sep 2022 23:42:02 +0200 (CEST)
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.
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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