https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112935
--- Comment #5 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #4)
> /* Default RTX cost initializer. */
> ...
> int_mult_si (COSTS_N_INSNS (1)),
> int_mult_di (COSTS_N_INSNS (1)),
>
>
> That seems wrong.
> I suspect you will get other improvements when you touch this.
>
> E.g.
> ```
> int f(int t)
> {
> return t * 17;
> }
> ```
> Should really be:
> shift followed by an add.
> But currently is just a mult.
>
> What is interesting is I think -Os cost is the opposite from the -O2 cost ...
>
> That is -Os produces the better code generation due to the cost for mult
> being set to 4:
> /* RTX costs to use when optimizing for size. */
> ...
> .int_mult_si_ (4)
> .int_mult_di_ (4)
4 is just COSTS_N_INSNS(1), so in -Os we are making all instructions cost the
same. This should be correct because in -Os we should minimize the number of
the instructions. In loongarch_rtx_costs though we have:
case MULT:
if (float_mode_p)
*total = loongarch_fp_mult_cost (mode);
else if (mode == DImode && !TARGET_64BIT)
*total = (speed
? loongarch_cost->int_mult_si * 3 + 6
: COSTS_N_INSNS (7));
else if (!speed)
*total = COSTS_N_INSNS (1) + 1;
else if (mode == DImode)
*total = loongarch_cost->int_mult_di;
else
*total = loongarch_cost->int_mult_si;
return false;
so we still slightly penalty multiplication. To me we should code
COSTS_N_INSNS (1) + 1 into loongarch_rtx_cost_optimize_size instead of special
casing it in loongarch_rtx_costs.
For the default value (used when -O2) I'll do some micro-benchmark...
