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
