Qing Zhao <qing.z...@oracle.com> writes:
>> On Sep 11, 2020, at 12:32 PM, Richard Sandiford <richard.sandif...@arm.com>
>> >> If we go for (2), then I think it would be better to do
>> it at the start of pass_late_compilation instead. (Some targets wouldn't
>> cope with doing it later.) The reason for doing it so late is that the
>> set of used “volatile”/caller-saved registers is not fixed at prologue
>> and epilogue generation: later optimisation passes can introduce uses
>> of volatile registers that weren't used previously. (Sorry if this
>> has already been suggested.)
>
> Yes, I agree.
>
> I thought that it might be better to move this task at the very late of the
> RTL stage, i.e, before “final” phase.
>
> Another solution is (discussed with Hongjiu):
>
> 1. Define a new target hook:
>
> targetm.return_with_zeroing(bool simple_return_p, HARD_REG_SET
> need_zeroed_hardregs, bool gpr_only)
>
> 2. Add the following routine in middle end:
>
> rtx_insn *
> generate_return_rtx (bool simple_return_p)
> {
> if (targetm.return_with_zeroing)
> {
> Compute the hardregs set for clearing into “need_zeroed_hardregs”;
> return targetm.return_with_zeroing (simple_return_p,
> need_zeroed_hardregs, gpr_only);
> }
> else
> {
> if (simple_return_p)
> return targetm.gen_simple_return ( );
> else
> return targetm.gen_return ();
> }
> }
>
> Then replace all call to “targetm.gen_simple_return” and “targetm.gen_return”
> to “generate_return_rtx()”.
>
> 3. In the target,
> Implement “return_with_zeroing”.
>
>
> Let me know your comments on this.
I think having a separate pass is better. We don't normally know
at the point of generating the return which registers will need
to be cleared. So IMO the pass should just search for all the
returns in a function and insert the zeroing instructions before
each one.
Having a target hook sounds good, but I think it should have a
default definition that just uses the move patterns to zero each
selected register. I expect the default will be good enough for
most targets.
Thanks,
Richard
