[ARM] isTruncateFree fix

[samparker]

Summary

Fix the isTruncateFree logic to match my comment, as the previous logic was way too general. Now the only truncates that are free are i64 -> i32. The original patch was in D37516.

[javed.absar]

lib/Target/ARM/ARMISelLowering.cpp

12336 ↗

(On Diff #116540)

maybe factor it out as a separate function to avoid code duplication?

bool ARMTargetLowering::isTruncate64To32(unsigned SrcBits, unsigned DstBits) {

return SrcBits == 64 & DstBits == 32);

}

[samparker]

lib/Target/ARM/ARMISelLowering.cpp

12336 ↗

(On Diff #116540)

I don't see the value in that here since its a very specific expression. As far as I can see, all the other backends have a similar approach to this as well.

[eastig]

lib/Target/ARM/ARMISelLowering.cpp

12336 ↗

(On Diff #116540)

I agree with Sam I don't see the big value of ARMTargetLowering::isTruncate64To32.

[SjoerdMeijer]

Looks good to me (sorry I missed this in the other/previous review; I looked at it and thought it was alright...)

llvm/trunk/lib/Target/ARM/ARMISelLowering.cpp

 }
 
 // 64-bit integers are split into their high and low parts and held in two
 // different registers, so the trunc is free since the low register can just
 // be used.
 bool ARMTargetLowering::isTruncateFree(Type *SrcTy, Type *DstTy) const {
   if (!SrcTy->isIntegerTy() || !DstTy->isIntegerTy())
     return false;
-  unsigned NumBits1 = SrcTy->getPrimitiveSizeInBits();
-  unsigned NumBits2 = DstTy->getPrimitiveSizeInBits();
-  return NumBits1 > NumBits2;
+  unsigned SrcBits = SrcTy->getPrimitiveSizeInBits();
+  unsigned DestBits = DstTy->getPrimitiveSizeInBits();
+  return (SrcBits == 64 && DestBits == 32);
 }
 
 bool ARMTargetLowering::isTruncateFree(EVT SrcVT, EVT DstVT) const {
   if (SrcVT.isVector() || DstVT.isVector() || !SrcVT.isInteger() ||
       !DstVT.isInteger())
     return false;
-  unsigned NumBits1 = SrcVT.getSizeInBits();
-  unsigned NumBits2 = DstVT.getSizeInBits();
-  return NumBits1 > NumBits2;
+  unsigned SrcBits = SrcVT.getSizeInBits();
+  unsigned DestBits = DstVT.getSizeInBits();
+  return (SrcBits == 64 && DestBits == 32);
 }
 
 bool ARMTargetLowering::isZExtFree(SDValue Val, EVT VT2) const {
   if (Val.getOpcode() != ISD::LOAD)
     return false;
 
   EVT VT1 = Val.getValueType();
   if (!VT1.isSimple() || !VT1.isInteger() ||