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


On 12/20/18 2:26 PM, Segher Boessenkool wrote:
> On Thu, Dec 20, 2018 at 09:07:41PM +0100, Eric Botcazou wrote:
>>> It's not a terrible workaround, no.  It looks like it will make some asm
>>> once again fail though?  If argument registers are forwarded to in the asm.
>>
>> The combiner change looks like a big hammer for such a corner case though.
> 
> That isn't the primary goal, no; the primary goal is to get better RA (by
> not having the combiner do RA's work, and doing a mediocre job of it).  It
> does solve these problems though.

My recent RA patches improved conflict information by noticing when we
don't need a conflict for a copy insn between a hard reg and a pseudo
when the source operand is live past the copy.  However, conflict info
for copies between two pseudos wasn't affected because IRA/LRA computes
liveness differently for pseudos than for hard regs.  I have seen copies
between two pseudos still get to RA and we're not able to do anything
about them at the moment because we still think they conflict.

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.

Peter




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