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 ‘>>’ 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."
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) 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. Or perform the (X+Y) operation in an unsigned type. Richard. > -- > Marc Glisse
