[llvm] [GISel] Combine shift + trunc + shift pattern (PR #155583)

[David Green via llvm-commits]

================
@@ -2094,6 +2094,70 @@ bool CombinerHelper::matchCommuteShift(MachineInstr &MI,
   return true;
 }
 
+bool CombinerHelper::matchLshrOfTruncOfLshr(MachineInstr &MI,
+                                            LshrOfTruncOfLshr &MatchInfo,
+                                            MachineInstr &ShiftMI) const {
+  unsigned ShiftOpcode = MI.getOpcode();
+  assert(ShiftOpcode == TargetOpcode::G_LSHR);
+
+  Register N0 = MI.getOperand(1).getReg();
+  Register N1 = MI.getOperand(2).getReg();
+  unsigned OpSizeInBits = MRI.getType(N0).getScalarSizeInBits();
+
+  APInt N1C, N001C;
+  if (!mi_match(N1, MRI, m_ICstOrSplat(N1C)))
+    return false;
+  auto N001 = ShiftMI.getOperand(2).getReg();
+  if (!mi_match(N001, MRI, m_ICstOrSplat(N001C)))
+    return false;
+
+  if (N001C.getBitWidth() > N1C.getBitWidth())
+    N1C = N1C.zext(N001C.getBitWidth());
+  else
+    N001C = N001C.zext(N1C.getBitWidth());
+
+  Register InnerShift = ShiftMI.getOperand(0).getReg();
+  LLT InnerShiftTy = MRI.getType(InnerShift);
+  uint64_t InnerShiftSize = InnerShiftTy.getScalarSizeInBits();
+  if ((N1C + N001C).ult(InnerShiftSize)) {
+    MatchInfo.Src = ShiftMI.getOperand(1).getReg();
+    MatchInfo.ShiftAmt = N1C + N001C;
+    MatchInfo.ShiftAmtTy = MRI.getType(N001);
+    MatchInfo.InnerShiftTy = InnerShiftTy;
+
+    if ((N001C + OpSizeInBits) == InnerShiftSize)
+      return true;
+    if (MRI.hasOneUse(N0) && MRI.hasOneUse(InnerShift)) {
+      MatchInfo.Mask = true;
+      MatchInfo.MaskVal = APInt(N1C.getBitWidth(), OpSizeInBits) - N1C;
+      return true;
+    }
+  }
+  return false;
+}
+
+void CombinerHelper::applyLshrOfTruncOfLshr(
+    MachineInstr &MI, LshrOfTruncOfLshr &MatchInfo) const {
+  unsigned ShiftOpcode = MI.getOpcode();
+  assert(ShiftOpcode == TargetOpcode::G_LSHR);
+
+  Register Dst = MI.getOperand(0).getReg();
+  auto ShiftAmt =
+      Builder.buildConstant(MatchInfo.ShiftAmtTy, MatchInfo.ShiftAmt);
+  auto Shift =
+      Builder.buildLShr(MatchInfo.InnerShiftTy, MatchInfo.Src, ShiftAmt);
+  if (MatchInfo.Mask == true) {
+    APInt MaskVal =
+        APInt::getLowBitsSet(MatchInfo.InnerShiftTy.getScalarSizeInBits(),
+                             MatchInfo.MaskVal.getZExtValue());
+    auto Mask = Builder.buildConstant(MatchInfo.ShiftAmtTy, MaskVal);
----------------
davemgreen wrote:

Should ShiftAmtTy be InnerShiftTy to match the And?

https://github.com/llvm/llvm-project/pull/155583