https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82628
--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> --- So, for the -1UL <= 1UL comparison, the cmpl instruction sets the CF, and then sbbl subtracts -1U (2nd operand) + 1U (CF) from 0U (1st operand/destination) and sets CF flag, but the RTL representation of this, i.e. comparison of (2nd operand + CF) and (1st operand), i.e. 0 and 0, doesn't set the CF. From the description of SBBL, it is unclear if it sets CF according to op1 - (SImode) (op2 + CF) or (DImode) op1 - ((DImode) op2 + CF), but from the above it looks like in order to do this we rely on the latter, i.e. it is actually 0 - (0xffffffffULL + 1) == 0 - 0x100000000ULL = 0 with CF. So, do we need to represent it differently in the instruction, as comparison in twice as wide mode? I think we'd then need different patterns for signed and unsigned comparisons, one would sign-extend, the other zero-extend, and then perhaps just use CCOmode or CCZmode. Do we need to fix also subborrow<mode>? What about the addcarry<mode>?
