[llvm-commits] [PATCH] add-carray/sub-borrow optimization (CodeGen, X86)

[Shuxin Yang]

Hi, Manman:

    I read "coding guide" the other day. The document also suggests 
"return early" to make the logic easier.
But it doesn't say on what condition the "return early" is appropriate. 
In this case, we have to duplicate
non-trivial code in order to return early.  For sloppy engineers like 
me, maintaining duplication is difficult.
I'd like to improve maintenance even at the cost of sacrificed readability.

Thanks
Shuxin
>>> +      CC = X86::COND_B;
>>> +      SDValue NewSub = DAG.getNode(X86ISD::SUB, EFLAGS.getDebugLoc(),
>>> +                                   EFLAGS.getNode()->getVTList(),
>>> +                                   EFLAGS.getOperand(1), EFLAGS.getOperand(0));
>>> +      EFLAGS = SDValue(NewSub.getNode(), EFLAGS.getResNo());
>>> +      SDValue NewVal = DAG.getNode(X86ISD::SETCC, DL, N->getVTList(),
>>> +                                   DAG.getConstant(CC, MVT::i8), EFLAGS);
>>> +      N = NewVal.getNode ();
>
> I suggested to return instead of falling to the "if" mainly due to readability.
>    if (CC == X86::COND_B)
>      return DAG.getNode(ISD::AND, DL, MVT::i8,
>                         DAG.getNode(X86ISD::SETCC_CARRY, DL, MVT::i8,
>                                     DAG.getConstant(CC, MVT::i8), EFLAGS),
>                         DAG.getConstant(1, MVT::i8));
>
> If we return SETCC_CARRY directly, there is no need to create NewVal.
> I guess we have different opinions on readability :)
>
> Thanks,
> Manman
>
>