[Bug tree-optimization/63464] compare one character to many: faster

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63464

--- Comment #17 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Or try to deal with this in combine.  We have:
(insn 91 90 92 18 (parallel [
            (set (reg:DI 149 [ D.1943 ])
                (and:DI (reg:DI 147 [ D.1943 ])
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) vrp66-1.c:32 379 {*anddi_1}
     (expr_list:REG_DEAD (reg:DI 147 [ D.1943 ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))
(insn 92 91 94 18 (parallel [
            (set (reg:DI 150 [ D.1943 ])
                (xor:DI (reg:DI 149 [ D.1943 ])
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) vrp66-1.c:32 401 {*xordi_1}
     (expr_list:REG_DEAD (reg:DI 149 [ D.1943 ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))
(insn 94 92 95 18 (parallel [
            (set (reg:QI 93 [ D.1944 ])
                (and:QI (subreg:QI (reg:DI 150 [ D.1943 ]) 0)
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) vrp66-1.c:32 383 {*andqi_1}
     (expr_list:REG_DEAD (reg:DI 150 [ D.1943 ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

and I believe nonzero_bits is smart enough to say that nonzero_bits of
(subreg:QI (reg:DI 150 [ D.1943 ]) 0) is 1.  But the problem is that
simplify-rtl attempts to match all kinds of weird patterns, like:
(parallel [
        (set (reg:QI 93 [ D.1944 ])
            (eq:QI (zero_extract:DI (reg:DI 147 [ D.1943 ])
                    (const_int 1 [0x1])
                    (const_int 0 [0]))
                (const_int 0 [0])))
        (clobber (reg:CC 17 flags))
    ])
etc.  Perhaps if everything fails, if for and with constant it would just try
to call nonzero_bits on the non-constant operand and if only bits set in the
constant mask are set, it would try to match the stmt into a plain assignment?