https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751
rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> changed:
What |Removed |Added
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CC|richard.sandiford at arm dot com |
--- Comment #39 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org>
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(In reply to Richard Biener from comment #34)
> On RTL there are "special" RTXen used for this kind of stuff, like
> (use:mode ..) or (clobber const0_rtx), but I'm the wrong person to
> ask which one would be most appropriate for a general operand in
> an otherwise generic instruction. Maybe Richard has a guess.
I think the best bet with existing RTL is (scratch:<MODE>). It's not an exact
fit for current usage (or for the documentation), but it's similar in spirit to
the cratch in (mem:BLK (scratch:P)) (which also isn't an exact match for the
documentation).
I don't expect this to work out of the box. Some changes to target-independent
code will be needed. But if we restrict this use to expanders for now, the
changes should be relatively small. I think the main thing would be to make
maybe_legitimize_operand turn a scratch rtx into a fresh pseudo if the
predicate doesn't accept a scratch. We'd then be restoring the semantics of an
uninitialised SSA_NAME.
If we did that, I think we could convert uninitialised SSA_NAMEs into SCRATCHes
for everything that goes through expand_fn_using_insn. There should be no need
to restrict it to COND_* functions.
