Aurelien Buhrig <aurelien.buhrig.gcc@xxxxxxxxx> writes: > I have a question about combining compare with arithmetic operations. > The doc says about (compare:m x y): > > "Represents the result of subtracting y from x for purposes of > comparison. The result is computed without overflow, as if with infinite > precision. > Of course, machines can’t really subtract with infinite precision. > However, they can pretend to do so when only the sign of the result will > be used, which is the case when the result is stored in the condition > code. And that is the only way this kind of expression may validly be > used: as a value to be stored in the condition codes, either (cc0) or a > register." > > So is it valid to combine a subm pattern (which sets the flags as a > compare does) and a cbranchm with a signed comparison operator? > > If it is, what is the expected behavior when an overflow occurs? > Should we branch if (compare x y) is true, or if (compare result 0) is true? I'm finding it hard to answer this question in the abstract. Can you show us the RTL? Perhaps you are asking whether you may generate (set (pc) (if_then_else (sub r0 r1) (label_ref) (pc))) That is not valid RTL; the first operand of if_then_else must be a comparison operation. This would be valid: (set (pc) (if_then_else (eq (sub r0 r1) (const_int 0)) (label_ref) (pc))) And I think that answers your question about overflow. Ian
