Re: Committed: fix PR49285, breakage building libgcc2 for MMIX

[Richard Sandiford <richard.sandiford@linaro.org>]

Hans-Peter Nilsson <hp@bitrange.com> writes:
> I needed a baseline for PR48542 (problems with setjmp/longjmp), but
> building MMIX was broken on trunk.  It was "bitten" by the fixups to
> predicate matching done recentlish.  The comment above the MMIX
> truncdfsf2 expander complains about having to use an expander to
> force an operand to memory - and then goes on and uses memory_operand
> for that operand anyway!  That accidentally worked before the fixup,
> because after copying the failing operand to a new pseudo, the
> expander was called without the optabs machinery doublechecking the
> new pseudo against the predicate.  While I stand by the comment and
> still think the optabs machinery could automatically fix up operands
> as needed to force an operand into memory when the predicate says so,
> it hasn't worked like that before, and I guess it's no use adding
> something like that just for one port, where an expander will do.

FWIW, having a target-independent feature sounds like a good idea to me.
It's similar in spirit to the new code that tries to force addresses
into registers if a memory predicate rejects a mem.  That too is something
that was often handled by expanders, but which is nicer without.

The implementation could be along the same lines.  Have an allows_reg
field in recog_data that says whether the predicate allows registers.
If it doesn't, try forcing to a stack slot instead of to registers.

Richard