Re: managing register spills?

Cliff Click <>
13 Jun 1997 21:59:41 -0400

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From: Cliff Click <>
Newsgroups: comp.compilers
Date: 13 Jun 1997 21:59:41 -0400
Organization: RISC Software, Motorola
References: 97-06-030
Keywords: registers, optimize

George C. Lindauer wrote:
> It seems easy enough, but suppose I have two register banks, register
> bank A and register bank B. All the registers in both banks are
> overused and have spilled. Now I want to move a value from bank A to
> bank B, allocating a new register in bank B and freeing the old one
> from bank A. Clearly, the free will cause an unspill, and the
> allocate will cause a spill. But if I free the bank a register BEFORE
> the move I've lost the value I want to move. And if I free it AFTER
> the move this is invalid because I've spilled from bank B and since I
> am using a stack to manage the spills the bank A unspill value is no
> longer available. An exchange instruction would be nice about
> now... unfortunately I don't have one.
> What do I do?

I can only guess what 'unspill' means, it's not a common term
in the register-allocation community that I've heard of before.

I've written about 4 full-blown Chaitin-style register allocators
before (at a few not-so-full-blown ones). The way I deal with this
problem is:
    1) I'm moving a value from A to B, this is a write to B and B
          is full, so...
    2) I spill something from B to the stack. Bank A is not affected.
          The spill happens just prior to the move. The spill is nothing
          more than a store to the stack (and a stack-slot allocation).
          I also insert the reloads before uses of the spilled value.
    3) I then issue the move from A to B.
    4) The register in A is now free. I do not "unspill" anything
          into at this time.
    5) A later round of Chaitin coloring will pick up on the now-free
          register in bank A and assign it some useful work.

Cliff Click, Ph.D. Compiler Researcher & Designer
RISC Software, Motorola PowerPC Compilers (512) 891-7240

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