On Sun, Apr 9, 2023 at 9:15 AM Jeff Law via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > > > On 4/6/23 05:37, Jakub Jelinek wrote: > > On Thu, Apr 06, 2023 at 12:51:20PM +0200, Eric Botcazou wrote: > >>> If we want to fix it in the combiner, I think the fix would be following. > >>> The optimization is about > >>> (and:SI (subreg:SI (reg:HI xxx) 0) (const_int 0x84c)) > >>> and IMHO we can only optimize it into > >>> (subreg:SI (and:HI (reg:HI xxx) (const_int 0x84c)) 0) > >>> if we know that the upper bits of the REG are zeros. > >> > >> The reasoning is that, for WORD_REGISTER_OPERATIONS, the subword AND > >> operation > >> is done on the full word register, in other words that it's in effect: > >> > >> (subreg:SI (and:SI (reg:SI xxx) (const_int 0x84c)) 0) > >> > >> that is equivalent to the initial RTL so correct for > >> WORD_REGISTER_OPERATIONS. > > > > If the > > (and:SI (subreg:SI (reg:HI xxx) 0) (const_int 0x84c)) > > to > > (subreg:SI (and:HI (reg:HI xxx) (const_int 0x84c)) 0) > I think it is. In both cases the AND wipes the upper 16 bits. > > > > not really sure what for WORD_REGISTER_OPERATIONS > > means AND with a constant which has the most significant bit set for the > > upper bits. > That's a very good question. I'm not sure either. Obviously in the > non-constant case all the bits up to word_mode get used. The same thing > is going to happen in the constant case. > > THe fact that constants are sign extended from the mode bit is a gcc-ism > though and not necessarily indicative of what hardware is going to to. > > > > >> > >> What happens if you disable the step I mentioned (patchlet attached)? > > > > That patch doesn't change anything at all on the testcase, it is still > > miscompiled. > That may be an artifact of later code in combine coming along and > mucking things up in a manner similar. That what I saw after twiddling > simplify_binary_operation_1. See simplify_and_const_int_1 and its calls > to nonzero_bits Li pan and I tried to set SUBREG_PROMOTED_UNSIGNED_P for the (subreg:SI (reg:HI xxx)) after the AND was optimized off. But it looks RA doesn't handle it as expected,not sure it I understand the semantics of SUBREG_PROMOTED_UNSIGNED_P correctly. > > jeff
-- BR, Hongtao
