[PowerPC] Mark zext of a small scalar load as free

This initial implementation of PPCTargetLowering::isZExtFree marks as free
zexts of small scalar loads (that are not sign-extending). This callback is
used by SelectionDAGBuilder's RegsForValue::getCopyToRegs, and thus to
determine whether a zext or an anyext is used to lower illegally-typed PHIs.
Because later truncates of zero-extended values are nops, this allows for the
elimination of later unnecessary truncations.

Fixes the initial complaint associated with PR22120.

llvm-svn: 225584

1 files changed, 20 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index 6b24c21c8bc..e93c7819b1a 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -9780,6 +9780,26 @@ bool PPCTargetLowering::isTruncateFree(EVT VT1, EVT VT2) const {
   return NumBits1 == 64 && NumBits2 == 32;
 }
 
+bool PPCTargetLowering::isZExtFree(SDValue Val, EVT VT2) const {
+  // Generally speaking, zexts are not free, but they are free when they can be
+  // folded with other operations.
+  if (LoadSDNode *LD = dyn_cast<LoadSDNode>(Val)) {
+    EVT MemVT = LD->getMemoryVT();
+    if ((MemVT == MVT::i1 || MemVT == MVT::i8 || MemVT == MVT::i16 ||
+         (Subtarget.isPPC64() && MemVT == MVT::i32)) &&
+        (LD->getExtensionType() == ISD::NON_EXTLOAD ||
+         LD->getExtensionType() == ISD::ZEXTLOAD))
+      return true;
+  }
+
+  // FIXME: Add other cases...
+  //  - 32-bit shifts with a zext to i64
+  //  - zext after ctlz, bswap, etc.
+  //  - zext after and by a constant mask
+
+  return TargetLowering::isZExtFree(Val, VT2);
+}
+
 bool PPCTargetLowering::isLegalICmpImmediate(int64_t Imm) const {
   return isInt<16>(Imm) || isUInt<16>(Imm);
 }