Not familiar with combine inerts, I'd like to know if
it's low hanging fruit to teach insn combine to perform
optimizations like the following.
Suppose following C code, int = HI
int y15;
int x15;
void qmul8_xy (char c, int x, int y)
{
y15 = y * c;
x15 = x * c;
}
and that the target has an insn like
(set (reg:HI)
(mult:HI (sign_extend:HI (reg:QI))
(reg:HI)))
If there was just one multiplication, combine would
combine the sign_extend into the special mul insn.
(Provided costs are advantageous, of course)
However, with the code example from above, combine
won't insert the sign_extend even if it was an
improvement, i.e. the sign_extend does not disappear
but the special mult is cheaper (or same cost) as the
ordinary mult.
The original RTL reads:
== Stage 0 ==
(set (reg:HI 42)
(sign_extend:HI (reg:QI 24)))
(set (reg:HI 48)
(mult:HI (reg:HI 42)
(reg:HI 47)))
(set (reg:HI 49)
(mult:HI (reg:HI 42)
(reg:HI 46)))
== Stage 1 ==
Cheaper than Stage 0 but not performed
(set (reg:HI 42)
(sign_extend:HI (reg:QI 24)))
(set (reg:HI 48)
(mult:HI (sign_extend:HI (reg:QI 24))
(reg:HI 47)))
(set (reg:HI 49)
(mult:HI (reg:HI 42)
(reg:HI 46)))
== Stage 2 ==
Cheaper than Stage 0 and Stage 1, would be performed
if we had Stage 1.
(set (reg:HI 48)
(mult:HI (sign_extend:HI (reg:QI 24))
(reg:HI 47)))
(set (reg:HI 49)
(mult:HI (sign_extend:HI (reg:QI 24))
(reg:HI 46)))
I also observe this for other examples where insns could
be consumed in other insns without disappearing but
with decreasing cost.
Thanks for comments.
Johann
