Re: Special Memory Constraint [was Re: Indirect memory addresses vs. lra]

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Aug 20, 2019 at 9:07 AM John Darrington
<john@darrington.wattle.id.au> wrote:
>
> On Tue, Aug 20, 2019 at 08:56:39AM +0200, Richard Biener wrote:
>
>      > Most of these suggestions involve adding some sort of virtual registers
>      > So I hacked the machine description to add two new registers Z1 and Z2
>      > with the same mode as X and Y.
>      >
>      > Obviously the assembler balks at this.  However the compiler still
>      > ICEs at the same place as before.
>      >
>      > So this suggests that our original diagnosis, viz: there are not enough
>      > address registers was not accurate, and in fact there is some other
>      > problem?
>
>      That sounds likely.  Given you have indirect addressing you could
>      simulate N virtual regs by placing them in a virtual reg table in memory
>      and accessed via a fixed address register (assuming all instructions
>      that would need an address reg also can take that indirect from memory).
>
> That was my plan.  Accordingly, extending the md to provide N additional
> regs (N currently = 2) was the first step.  Having doubled the number
> of available address registers, I had expected this would fix most of the
> ICEs (but cause a lot of assembler errors).
>
> However it hasn't eliminated any ICEs.  lra is still complaining
> "unable to find a register to spill" So the plan seems to have fallen
> over at the first hurdle.  Why can it still not spill registers despite
> having a lot more of them?

You really have to sit down and trace the LRA code with a debugger
to tell...  unfortunately the dumps aren't verbose enough to tell.
Usually after spilling the insn constraints can still not be satisfied,
the main question is usually why.

Richard.

> J'