Expanding instructions with condition codes inter-deps

Paulo J. Matos paulo@matos-sorge.com
Mon Oct 24 12:12:00 GMT 2011

On 23/10/11 22:21, Richard Henderson wrote:
> On 10/21/2011 05:49 PM, Paul_Koning@Dell.com wrote:
>>> There are lots of parts of the compiler that don't optimize well when an insn has more than one output.  For the normal insn, just clobber the flags; don't include a second SET.
>> Yes, but... isn't the whole point of CC modeling that you can take advantage of the CC left around by an instruction?  Typically in machines with condition codes, you can eliminate test instructions (compare with zero) if the previous instruction has that variable as its output.  But if we're discouraged from writing insns with CC outputs as normal practice, and if the compiler doesn't handle such constructs well in optimization, what then?
> The solution is to have *two* insn patterns, one with a set of the flags
> and one with only a clobber.  Have a look through i386.md and
> how the flags register is handled there.

In version 4.6.1, i386.md,  I see things like:

(define_insn "addqi3_cc"
   [(set (reg:CC FLAGS_REG)
           [(match_operand:QI 1 "nonimmediate_operand" "%0,0")
            (match_operand:QI 2 "general_operand" "qn,qm")]
    (set (match_operand:QI 0 "nonimmediate_operand" "=qm,q")
         (plus:QI (match_dup 1) (match_dup 2)))]
   "ix86_binary_operator_ok (PLUS, QImode, operands)"
   "add{b}\t{%2, %0|%0, %2}"
   [(set_attr "type" "alu")
    (set_attr "mode" "QI")])

This seems to be exactly what we are doing. I can't see where there are 
separate rules for the flags register.


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