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


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?

J'


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