[PATCH] D17288: [CodeGenPrepare] Do select to branch transform when cmp's operand is expensive.

[Junmo Park via llvm-commits]

flyingforyou added a comment.

Sanjay,

> As an example, I don't think this transform or the load transform would help an in-order CPU of any architecture. But we're still doing the transform for those subtargets.


We already consider that we apply "select to branch" opt to for which one.

  // Do we have efficient codegen support for this kind of 'selects' ?
  if (TLI->isSelectSupported(SelectKind)) {
    // We have efficient codegen support for the select instruction.
    // Check if it is profitable to keep this 'select'.
    if (!TLI->isPredictableSelectExpensive() ||
        !isFormingBranchFromSelectProfitable(TTI, SI))
      return false;
  }

We already check through `TLI->isPredictableSelectExpensive()`. This can be set in X86ISelLowering.cpp.

  // A predictable cmov does not hurt on an in-order CPU.
  // FIXME: Use a CPU attribute to trigger this, not a CPU model.
  PredictableSelectIsExpensive = !Subtarget.isAtom();

If `PredictableSelectIsExpensive ` is not true, we don't transform "select to branch" opt.

And I think we can use `getSchedModel().isOutOfOrder()` for setting the flag `PredictableSelectIsExpensive`.

Junmo.


http://reviews.llvm.org/D17288