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Re: [patch] Fix PR rtl-optimization/87727


On 12/20/18 4:41 PM, Jeff Law wrote:
> On 12/20/18 2:30 PM, Peter Bergner wrote:
>> For stage1, I'd like to fix that conflict wart if I can.  I have also
>> wondered about adding a copy coalesce phase just before we enter RA,
>> which would ensure the copies are removed, instead of hoping RA assigns
>> the same reg to the source and destination of the copy making it a nop
>> that can be removed.
> The difficulty with coalescing is that if you get too aggressive then
> you end up removing degrees of freedom from the allocator and you can
> easily make the final results worse.

I agree, but being too aggressive leading to bad decisions/code is
true for a lot of optimizations. :-)   I do plan on first attacking
the conservative conflict info for pseudos first and seeing what
that buys us before attempting any coalescing.

As for removing degrees of freedom for the allocator, sometimes that can
be a good thing, if it can makes the allocator simpler.  For example, I
think we have forced the allocator to do too much by not only being an RA,
but being an instruction selector as well.  Doing both RA and instruction
selection at the same time makes everything very complicated and I think
we probably don't compute allocation costs correctly, since we seem to
calculate costs on a per alternative per insn basis and I don't think we
ever see what the ramifications of using an alternative in one insn
has on the costs of another alternative in another insn.  Sometimes using
the cheapest alternative in one insn and the cheapest alternative in
another insn can lead us into a situation that requires spilling to
resolve the conflicting choices.

I've wondered if running something like lra_constraints() (but using
pseudos for fixups rather than hard regs) early in the rtl passes as
a pseudo instruction selection pass wouldn't make things easier for
the following passes like RA, etc?

Peter


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