Re: Native Code Generation from Abstract Stack Machine

Peter Seibel <peter@javamonkey.com> writes:


> I assume there are standard techniques for turning stack machine
> code into abstract register code. Are there any good references
> where I can read about them?


> [The simplest way is to treat the registers as a stack and run
> through each basic block assigning values to registers. -John]


Okay. If I understand you and what you mean by abstract register code
this means I take each basic block and for each value pushed on the
stack create a new abstract register (i.e. temporary variable). So if
one of my basic blocks is


        push 1
        push 2
        goto C


That gets translated to:


        t1 = 1
        t2 = 2
        goto C


Assuming that's right, now suppose I have three basic blocks A, B, and
C. A is as above. B is similar in stack code and gets translated to:


        t3 = 10
        t4 = 20
        goto C


And C is simply an ADD instruction which pops two values. Presumably
we need to generate abstract registers to hold the "inputs" to a basic
block. So it gets translated to this:


      t5 = t6 + t7


Since both A and B are predecessors of C but they are storing those
values into different abstract registers I need to somehow arrange to
get the correct values in t6 and t7. So do I generate A and B like this:


A:
        t1 = 1
        t2 = 2
        t6 = t1
        t7 = t2
        goto C


B:


        t3 = 10
        t4 = 20
        t6 = t3
        t7 = t2
        goto C


and then count on copy propagation and dead-code elimination to get
rid of some of the temporaries? Or am I making this too complicated?


-Peter


--
Peter Seibel peter@javamonkey.com

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