[PowerPC][NFC] Do not enter prefix selection if it cannot do better.

[stefanp]

Summary

Do not try to materialize a constant using prefix instructions if the selection
using non prefix instructions was able to do it using a single non prefix
instruction.

[NeHuang]

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

1174

Please correct me if I my understand is wrong here. It seems to me InstCntDirect=1 case has been covered in the check below.

 // If the prefix and non-prefix cases use the same number of instructions
// we will prefer the non-prefix case.
    if (ResultP && (!Result || InstCntDirectP < InstCntDirect))

Then intention of adding an early exit for InstCntDirect == 1 case is to save the trial of calling selectI64ImmDirectPrefix and further checks below.

[amyk]

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

1172

nit: Spelling.

[nemanjai]

LGTM.

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

1173

I think you should simplify this comment to something like:

// If we have prefixed instructions and there is a chance we can
// materialize the constant with fewer prefixed instructions than
// non-prefixed, try that.

This clearly communicates that there is no point doing any further checks if we can materialize this with just a single non-prefixed instruction (i.e. we won't be able to materialize it with zero instructions).

[stefanp]

Updated comment.

[stefanp]

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

1174

Yes that is correct. There is no need to call selectI64ImmDirectPrefix if we already know that it will fail the check InstCntDirectP < InstCntDirect and that the result it produces won't be used.

[NeHuang]

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

1174

Thanks. LGTM.

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

static SDNode *selectI64Imm(SelectionDAG *CurDAG, const SDLoc &dl, uint64_t Imm,
   unsigned InstCntDirect = 0;
   // No more than 3 instructions is used if we can select the i64 immediate
   // directly.
   SDNode *Result = selectI64ImmDirect(CurDAG, dl, Imm, InstCntDirect);
 
   const PPCSubtarget &Subtarget =
       CurDAG->getMachineFunction().getSubtarget<PPCSubtarget>();
 
-  if (Subtarget.hasPrefixInstrs()) {
+  // If we have prefixed instructions and there is a chance we can
+  // materialize the constant with fewer prefixed instructions than
+  // non-prefixed, try that.
+  if (Subtarget.hasPrefixInstrs() && InstCntDirect != 1) {
     unsigned InstCntDirectP = 0;
     SDNode *ResultP = selectI64ImmDirectPrefix(CurDAG, dl, Imm, InstCntDirectP);
     // Use the prefix case in either of two cases:
     // 1) We have no result from the non-prefix case to use.
     // 2) The non-prefix case uses more instructions than the prefix case.
     // If the prefix and non-prefix cases use the same number of instructions
     // we will prefer the non-prefix case.
     if (ResultP && (!Result || InstCntDirectP < InstCntDirect)) {
       if (InstCnt)