https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114741
Tamar Christina <tnfchris at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |tnfchris at gcc dot gnu.org,
| |vmakarov at gcc dot gnu.org
--- Comment #5 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #4)
> (In reply to Wilco from comment #2)
> > It looks like the underlying bug is '^' being incorrectly treated like '?'
> > in record_reg_classes (which is never used during reload). Fixing that
> > results in the expected code being generated in all cases. It looks this
> > issue was introduced in the original commit
> > d1457701461d5a49ca6b5d8a6d1c83a37a6dc771
>
> static const struct cpu_regmove_cost generic_armv9_a_regmove_cost =
> {
> 1, /* GP2GP */
> /* Spilling to int<->fp instead of memory is recommended so set
> realistic costs compared to memmov_cost. */
> 3, /* GP2FP */
> 2, /* FP2GP */
> 2 /* FP2FP */
> };
>
>
> Note these costs are broken.
> TARGET_REGISTER_MOVE_COST has this to say about the special value 2:
> ```
> If reload sees an insn consisting of a single set between two hard
> registers, and if TARGET_REGISTER_MOVE_COST applied to their classes returns
> a value of 2, reload does not check to ensure that the constraints of the
> insn are met. Setting a cost of other than 2 will allow reload to verify
> that the constraints are met. You should do this if the ‘movm’ pattern’s
> constraints do not allow such copying.
> ```
>
> The way I implemented this for thunderx was have GP2GP being cost of 2 and
> then be relative from there. That gave better code generation in general and
> even less spilling. I know I wrote about this before too.
I don't think this is related to this at all
the old generic costs, which armv8 was taken from doesn't use 2, and is broken
https://github.com/gcc-mirror/gcc/blob/master/gcc/config/aarch64/tuning_models/generic.h#L42
generic-armv8-a doesn't use 2 and is broken
https://github.com/gcc-mirror/gcc/blob/master/gcc/config/aarch64/tuning_models/generic_armv8_a.h#L43
neoverse-n2 uses 2 and isn't broken
https://github.com/gcc-mirror/gcc/blob/master/gcc/config/aarch64/tuning_models/neoversen2.h
I think the issue is what Wilco mentioned before.
The GCC docs claim that ^ shouldn't affect initial costing/IRA, but it clearly
does.
it's penalizing the r->r alternative during initial costing and not just during
lra if a reload is needed.
so the question to Vlad is it correct that ^ is treated the same way as ?
during initial costing? i.e. it's penalizing the alternative.
