Need advice: x86 redudant compare to zero

[Richard Kenner]

    My question is: where and how would you suggest we do this  
    optimization. With peephole2? Or in combine? In i386.md, I see  
    pattern *subsi_2 looks like what I'd like to combine these two insn  
    into:

    (define_insn "*subsi_2"
       [(set (reg FLAGS_REG)
         (compare
           (minus:SI (match_operand:SI 1 "nonimmediate_operand" "0,0")
                 (match_operand:SI 2 "general_operand" "ri,rm"))
           (const_int 0)))
        (set (match_operand:SI 0 "nonimmediate_operand" "=rm,r")
         (minus:SI (match_dup 1) (match_dup 2)))]
       "ix86_match_ccmode (insn, CCGOCmode)
        && ix86_binary_operator_ok (MINUS, SImode, operands)"
       "sub{l}\t{%2, %0|%0, %2}"
       [(set_attr "type" "alu")
        (set_attr "mode" "SI")])

    But I do not see a peephole2 that would generate this insn. Does  
    anyone know how this pattern is used?

This is the sort of thing you'd expect combine.c to make when it
combines an scc insn with arithmetic when the arithmetic result
is also used.