Re: [patch] MIPS/gcc: Revert removal of DImode shifts for 32-bit targets

[Richard Sandiford <rsandifo@redhat.com>]

Nigel Stephens <nigel@mips.com> writes:
>>If we handle it in a target-independent way, with each insn exposed
>>separately, we will be able to optimize special cases better.
>>We'll also get the usual scheduling benefits.
>
> I agree that we should open-code it for the obvious reasons, but does it
> have to be target independent, or could/should we prototype it with
> define_expand?

I think we should only use define_expands if there's a truly
MIPS-specific feature in the expansion (as there is in the block
move stuff, for example, where we use left/right loads and stores).

Now obviously I'm only guessing what insn sequence you're using,
but I suspect it doesn't involve anything that the middle-end
couldn't work out from stock optabs.  If there are different
trade-offs to be made during the expansion, they should probably
be predicated on rtx_costs.

Richard