[llvm] [GlobalISel][AArch64] Add G_FPTOSI_SAT/G_FPTOUI_SAT (PR #96297)

[Matt Arsenault via llvm-commits]

================
@@ -7285,6 +7335,106 @@ LegalizerHelper::LegalizeResult LegalizerHelper::lowerFPTOSI(MachineInstr &MI) {
   return Legalized;
 }
 
+LegalizerHelper::LegalizeResult
+LegalizerHelper::lowerFPTOINT_SAT(MachineInstr &MI) {
+  auto [Dst, DstTy, Src, SrcTy] = MI.getFirst2RegLLTs();
+
+  bool IsSigned = MI.getOpcode() == TargetOpcode::G_FPTOSI_SAT;
+  unsigned SatWidth = DstTy.getScalarSizeInBits();
+
+  // Determine minimum and maximum integer values and their corresponding
+  // floating-point values.
+  APInt MinInt, MaxInt;
+  if (IsSigned) {
+    MinInt = APInt::getSignedMinValue(SatWidth);
+    MaxInt = APInt::getSignedMaxValue(SatWidth);
+  } else {
+    MinInt = APInt::getMinValue(SatWidth);
+    MaxInt = APInt::getMaxValue(SatWidth);
+  }
+
+  const fltSemantics &Semantics = getFltSemanticForLLT(SrcTy.getScalarType());
+  APFloat MinFloat(Semantics);
+  APFloat MaxFloat(Semantics);
+
+  APFloat::opStatus MinStatus =
+      MinFloat.convertFromAPInt(MinInt, IsSigned, APFloat::rmTowardZero);
+  APFloat::opStatus MaxStatus =
+      MaxFloat.convertFromAPInt(MaxInt, IsSigned, APFloat::rmTowardZero);
+  bool AreExactFloatBounds = !(MinStatus & APFloat::opStatus::opInexact) &&
+                             !(MaxStatus & APFloat::opStatus::opInexact);
+
+  // If the integer bounds are exactly representable as floats, emit a
+  // min+max+fptoi sequence. Otherwise we have to use a sequence of comparisons
+  // and selects.
+  if (AreExactFloatBounds) {
+    // Clamp Src by MinFloat from below. If Src is NaN the result is MinFloat.
+    auto MaxC = MIRBuilder.buildFConstant(SrcTy, MinFloat);
+    auto MaxP = MIRBuilder.buildFCmp(CmpInst::FCMP_ULT,
+                                     SrcTy.changeElementSize(1), Src, MaxC);
+    auto Max = MIRBuilder.buildSelect(SrcTy, MaxP, Src, MaxC);
+    // Clamp by MaxFloat from above. NaN cannot occur.
+    auto MinC = MIRBuilder.buildFConstant(SrcTy, MaxFloat);
+    auto MinP =
+        MIRBuilder.buildFCmp(CmpInst::FCMP_OGT, SrcTy.changeElementSize(1), Max,
+                             MinC, MachineInstr::FmNoNans);
+    auto Min =
+        MIRBuilder.buildSelect(SrcTy, MinP, Max, MinC, MachineInstr::FmNoNans);
+    // Convert clamped value to integer. In the unsigned case we're done,
+    // because we mapped NaN to MinFloat, which will cast to zero.
+    if (!IsSigned) {
+      MIRBuilder.buildFPTOUI(Dst, Min);
+      MI.eraseFromParent();
+      return Legalized;
+    }
+
+    // Otherwise, select 0 if Src is NaN.
+    auto FpToInt = MIRBuilder.buildFPTOSI(DstTy, Min);
+    auto IsZero = MIRBuilder.buildFCmp(CmpInst::FCMP_UNO,
+                                       DstTy.changeElementSize(1), Src, Src);
+    MIRBuilder.buildSelect(Dst, IsZero, MIRBuilder.buildConstant(DstTy, 0),
+                           FpToInt);
+    MI.eraseFromParent();
+    return Legalized;
+  }
+
+  // Result of direct conversion. The assumption here is that the operation is
+  // non-trapping and it's fine to apply it to an out-of-range value if we
+  // select it away later.
+  auto FpToInt = IsSigned ? MIRBuilder.buildFPTOSI(DstTy, Src)
+                          : MIRBuilder.buildFPTOUI(DstTy, Src);
+
+  // If Src ULT MinFloat, select MinInt. In particular, this also selects
+  // MinInt if Src is NaN.
+  auto ULT =
+      MIRBuilder.buildFCmp(CmpInst::FCMP_ULT, SrcTy.changeElementSize(1), Src,
+                           MIRBuilder.buildFConstant(SrcTy, MinFloat));
+  auto Max = MIRBuilder.buildSelect(
+      DstTy, ULT, MIRBuilder.buildConstant(DstTy, MinInt), FpToInt);
+  // If Src OGT MaxFloat, select MaxInt.
+  auto OGT =
+      MIRBuilder.buildFCmp(CmpInst::FCMP_OGT, SrcTy.changeElementSize(1), Src,
+                           MIRBuilder.buildFConstant(SrcTy, MaxFloat));
+
+  // In the unsigned case we are done, because we mapped NaN to MinInt, which
+  // is already zero.
+  if (!IsSigned) {
+    MIRBuilder.buildSelect(Dst, OGT, MIRBuilder.buildConstant(DstTy, MaxInt),
+                           Max, MachineInstr::FmNoNans);
----------------
arsenm wrote:

Should the fcmp also get the flag? 

https://github.com/llvm/llvm-project/pull/96297