https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97784
--- Comment #6 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> There is targetm.sched.reassociation_width which specifies how re-assocation
> should make such sequence "wide".
Ah cool, thank you :-)
> Andrew is correct that we don't do this
> for any types that are TYPE_OVERFLOW_UNDEFINED.
Yes; but I see the sub-optimal behaviour for unsigned, too.
> And powerpc has
>
> static int
> rs6000_reassociation_width (unsigned int opc ATTRIBUTE_UNUSED,
> machine_mode mode)
> {
> switch (rs6000_tune)
> {
> case PROCESSOR_POWER8:
> case PROCESSOR_POWER9:
> case PROCESSOR_POWER10:
> if (DECIMAL_FLOAT_MODE_P (mode))
> return 1;
> if (VECTOR_MODE_P (mode))
> return 4;
> if (INTEGRAL_MODE_P (mode))
> return 1;
Yeah this last 1 is the problem :-)
> thus you get width 1 which means a linear chain (even if the user wrote
> a tree).
Yup.
> Note RTL doesn't do any such thing like re-assocation (I guess in principle
> scheduling could, and that's the only place where it would make sense
> on RTL).
RTL unrolling can, actually! "Variable expansion" is its horrible name
(and it makes a lot of sense there: it allows breaking a bit linear chain
into pieces).
