Re: [PATCH 2/2 v3][IRA,LRA] Fix PR86939, IRA incorrectly creates an interference between a pseudo register and a hard register

[Peter Bergner <bergner at linux dot ibm dot com>]

On 10/10/18 7:57 PM, Peter Bergner wrote:
> The problem is, that hard reg %r26 is defined in insn 32, to be used in
> insn 33, so using %r26 as the reload reg is wrong, because it will clobber
> the value we set in insn 32.  Looking thru LRA, it looks like LRA assumes
> that for a reload, if one of the input pseudos dies in the insn, then the
> hard reg it was assigned to is available to use.  That assumption is (now)
> wrong, because another pseudo may be using that hard reg or in this case,
> the hard reg itself is still live.
> 
> For this example, pseudo 109 also dies in insn 49 and since it's hard reg
> %r28 isn't live thru the insn, we could have used that instead.  However,
> we cannot just look at REG_DEAD notes for free hard regs to use for reload
> regs.  We need to make sure that that hard reg isn't also assigned to another
> pseudo that is live at that insn or even that the hard reg itself is live.
> 
> Vlad, you know the LRA code better than anyone.  Will it be easy to find
> all the places where we create reload regs and fix them up so that we
> look at more than just REG_DEAD notes?  Even though looking at REG_DEAD
> notes isn't enough, I still think the majority of the time those regs
> probably will be free to use, we just have to check for the special
> cases like above where they are not.

Ok, after working in gdb, I see that the PA-RISC port still uses reload
and not LRA, but it too seems to have the same issue of reusing input
regs that have REG_DEAD notes, so the question still stands.  It's just
that whatever fix we come up with will have to be to both LRA and reload.

Peter