[llvm] [GlobalISel] Support saturated truncate (PR #150219)

[Matt Arsenault via llvm-commits]

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@@ -5844,6 +5844,123 @@ void CombinerHelper::applyUMulHToLShr(MachineInstr &MI) const {
   MI.eraseFromParent();
 }
 
+bool CombinerHelper::matchTruncSSatS(MachineInstr &MI,
+                                     Register &MatchInfo) const {
+  Register Dst = MI.getOperand(0).getReg();
+  Register Src = MI.getOperand(1).getReg();
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Src);
+  unsigned NumDstBits = DstTy.getScalarSizeInBits();
+  unsigned NumSrcBits = SrcTy.getScalarSizeInBits();
+  assert(NumSrcBits > NumDstBits && "Unexpected types for truncate operation");
+
+  APInt SignedMax = APInt::getSignedMaxValue(NumDstBits).sext(NumSrcBits);
+  APInt SignedMin = APInt::getSignedMinValue(NumDstBits).sext(NumSrcBits);
+  if (isLegal({TargetOpcode::G_TRUNC_SSAT_S, {DstTy, SrcTy}})) {
+    if (mi_match(
+            Src, MRI,
+            m_GSMin(m_GSMax(m_Reg(MatchInfo), m_SpecificICstOrSplat(SignedMin)),
+                    m_SpecificICstOrSplat(SignedMax))))
+      return true;
+    if (mi_match(
+            Src, MRI,
+            m_GSMax(m_GSMin(m_Reg(MatchInfo), m_SpecificICstOrSplat(SignedMax)),
+                    m_SpecificICstOrSplat(SignedMin))))
+      return true;
+  }
+  return false;
+}
+
+void CombinerHelper::applyTruncSSatS(MachineInstr &MI,
+                                     Register &MatchInfo) const {
+  Register Dst = MI.getOperand(0).getReg();
+  Builder.buildTruncSSatS(Dst, MatchInfo);
+  MI.eraseFromParent();
+}
+
+bool CombinerHelper::matchTruncSSatU(MachineInstr &MI,
+                                     Register &MatchInfo) const {
+  Register Dst = MI.getOperand(0).getReg();
+  Register Src = MI.getOperand(1).getReg();
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Src);
+  unsigned NumDstBits = DstTy.getScalarSizeInBits();
+  unsigned NumSrcBits = SrcTy.getScalarSizeInBits();
+  assert(NumSrcBits > NumDstBits && "Unexpected types for truncate operation");
+
+  APInt UnsignedMax = APInt::getMaxValue(NumDstBits).zext(NumSrcBits);
+  if (isLegal({TargetOpcode::G_TRUNC_SSAT_U, {DstTy, SrcTy}})) {
----------------
arsenm wrote:

How is this guarding against running without LegalizerInfo? This will assert without it. Should this combine trigger in the absence of legality information? 

https://github.com/llvm/llvm-project/pull/150219