https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92712
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Before the first revision mentioned above *.optimized dump contained just t *
v, the second one doesn't change anything in *.optimized and is a RTL costing
matter.
_4 = (unsigned int) t_1(D);
_10 = _4 + 4294967295;
_8 = (int) _10;
_13 = v_3(D) * _8;
x_5 = v_3(D) + _13;
and can be seen even on simpler:
int
foo (int t, int v)
{
t = t - 1U;
v *= t;
return v + t;
}
which we don't optimize at GIMPLE level. We don't optimize even:
int
bar (int t, int v)
{
t = t - 1;
v *= t;
return v + t;
}
Rather than hoping it is optimized during combine (the change there was that
while combining b=a-1 into c=b*d we attempted c=a*d-d we now attempt c=(a-1)*d
and similarly for the 3 insn combination with e=c+d, where we attempted and
succeeded to combine that into e=a*d while now we attempt and fail e=(a-1)*d+d:
-Successfully matched this instruction:
+Failed to match this instruction:
(parallel [
(set (reg/v:SI 91 [ <retval> ])
- (mult:SI (reg/v:SI 92 [ t ])
+ (plus:SI (mult:SI (plus:SI (reg/v:SI 92 [ t ])
+ (const_int -1 [0xffffffffffffffff]))
+ (reg/v:SI 93 [ v ]))
(reg/v:SI 93 [ v ])))
(clobber (reg:CC 17 flags))
])
), I think it would be useful to optimize this in match.pd, plus maybe teach
simplify-rtx.c to handle this.
