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Annotation of 43BSDReno/libexec/pcc/c2.tahoe/NOTES, revision 1.1.1.1

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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