[clang] [llvm] [HLSL][SPIRV][DXIL] Implement `WaveActiveMax` intrinsic (PR #123428)

[Steven Perron via llvm-commits]

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@@ -2132,6 +2135,33 @@ bool SPIRVInstructionSelector::selectWaveActiveCountBits(
   return Result;
 }
 
+bool SPIRVInstructionSelector::selectWaveReduceMax(Register ResVReg,
+                                                   const SPIRVType *ResType,
+                                                   MachineInstr &I,
+                                                   bool IsUnsigned) const {
+  assert(I.getNumOperands() == 3);
+  assert(I.getOperand(2).isReg());
+  MachineBasicBlock &BB = *I.getParent();
+  Register InputRegister = I.getOperand(2).getReg();
+  SPIRVType *InputType = GR.getSPIRVTypeForVReg(InputRegister);
+
+  if (!InputType)
+    report_fatal_error("Input Type could not be determined.");
+
+  SPIRVType *IntTy = GR.getOrCreateSPIRVIntegerType(32, I, TII);
+  // Retreive the operation to use based on input type
+  bool IsFloatTy = GR.isScalarOrVectorOfType(InputRegister, SPIRV::OpTypeFloat);
+  auto IntegerOpcodeType =
+      IsUnsigned ? SPIRV::OpGroupNonUniformUMax : SPIRV::OpGroupNonUniformSMax;
+  auto Opcode = IsFloatTy ? SPIRV::OpGroupNonUniformFMax : IntegerOpcodeType;
+  return BuildMI(BB, I, I.getDebugLoc(), TII.get(Opcode))
+      .addDef(ResVReg)
+      .addUse(GR.getSPIRVTypeID(ResType))
+      .addUse(GR.getOrCreateConstInt(SPIRV::Scope::Subgroup, I, IntTy, TII))
+      .addImm(SPIRV::GroupOperation::Reduce)
+      .addUse(I.getOperand(2).getReg());
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s-perron wrote:

The typical pattern is to call `constrainAllUses` and return the result of that.

https://github.com/llvm/llvm-project/pull/123428