On Mon, Apr 10, 2023 at 1:13 PM Hongtao Liu <crazy...@gmail.com> wrote: > > 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 I mean SUBREG_PROMOTED_UNSIGNED_SET. > (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
-- BR, Hongtao
