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Annotation of 43BSDReno/libexec/pcc/c2.tahoe/NOTES, revision 1.1

1.1     ! root        1: The pattern looks like this:
        !             2: 
        !             3:        movl    P,rA
        !             4:        xxxl3   Q,rA,rB
        !             5:        yyyl2   R,rB
        !             6: 
        !             7: xxx and yyy may be any of several binary operators like 'add',
        !             8: 'mul', 'and'.  rA and rB must both be temporary registers.
        !             9: 
        !            10: Here's how it happens...  We work backwards from the last instruction:
        !            11: 
        !            12:        movl    P,rA
        !            13:        xxxl3   Q,rA,rB
        !            14: =>     yyyl2   R,rB
        !            15: 
        !            16: Here we note that rB is a 'modify' operand and mark rB alive.
        !            17: 
        !            18:        movl    P,rA
        !            19: =>     xxxl3   Q,rA,rB
        !            20:        yyyl2   R,rB
        !            21: 
        !            22: We see that this instruction writes into rB and kills it, but since
        !            23: rB is previously alive, the store is not redundant and we leave it
        !            24: alone.
        !            25: 
        !            26: =>     movl    P,rA
        !            27:        xxxl3   Q,rA,rB
        !            28:        yyyl2   R,rB
        !            29: 
        !            30: We notice that this is a useless store into rA, since we can replace
        !            31: rA with P in the following instruction.  We delete this instruction
        !            32: and modify the following one.
        !            33: 
        !            34: =>     xxxl3   Q,P,rB
        !            35:        yyyl2   R,rB
        !            36: 
        !            37: Unfortunately rB is no longer alive because we killed it evaluating
        !            38: the earlier form of this very instruction.  The optimizer assumes that
        !            39: we have a redundant store and deletes the instruction.  Bleah.

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