[clang] [llvm] [HLSL] implement elementwise firstbithigh hlsl builtin (PR #111082)

[Farzon Lotfi via cfe-commits]

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@@ -2626,6 +2671,151 @@ Register SPIRVInstructionSelector::buildPointerToResource(
                                                  MIRBuilder);
 }
 
+bool SPIRVInstructionSelector::selectFirstBitHigh16(Register ResVReg,
+                                                    const SPIRVType *ResType,
+                                                    MachineInstr &I,
+                                                    bool IsSigned) const {
+  unsigned Opcode = IsSigned ? SPIRV::OpSConvert : SPIRV::OpUConvert;
+  // zero or sign extend
+  Register ExtReg = MRI->createVirtualRegister(GR.getRegClass(ResType));
+  bool Result =
+      selectUnOpWithSrc(ExtReg, ResType, I, I.getOperand(2).getReg(), Opcode);
+  return Result & selectFirstBitHigh32(ResVReg, ResType, I, ExtReg, IsSigned);
+}
+
+bool SPIRVInstructionSelector::selectFirstBitHigh32(Register ResVReg,
+                                                    const SPIRVType *ResType,
+                                                    MachineInstr &I,
+                                                    Register SrcReg,
+                                                    bool IsSigned) const {
+  unsigned Opcode = IsSigned ? GL::FindSMsb : GL::FindUMsb;
+  return BuildMI(*I.getParent(), I, I.getDebugLoc(), TII.get(SPIRV::OpExtInst))
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farzonl wrote:

Nvmd upon looking only llvm IR has a `setOperand` I don't see one in MIR so this is fine. 

https://github.com/llvm/llvm-project/pull/111082