[llvm] [HLSL][SPIR-V] Add SV_DispatchThreadID semantic support (PR #82536)

[Nathan Gauër via llvm-commits]

================
@@ -1660,6 +1667,68 @@ bool SPIRVInstructionSelector::selectLog10(Register ResVReg,
   return Result;
 }
 
+bool SPIRVInstructionSelector::selectDXThreadId(Register ResVReg,
+                                                const SPIRVType *ResType,
+                                                MachineInstr &I) const {
+  // DX intrinsic: @llvm.dx.thread.id(i32)
+  // ID  Name      Description
+  // 93  ThreadId  reads the thread ID
+
+  MachineIRBuilder MIRBuilder(I);
+  const SPIRVType *U32Type = GR.getOrCreateSPIRVIntegerType(32, MIRBuilder);
+  const SPIRVType *Vec3Ty =
+      GR.getOrCreateSPIRVVectorType(U32Type, 3, MIRBuilder);
+  const SPIRVType *PtrType = GR.getOrCreateSPIRVPointerType(
+      Vec3Ty, MIRBuilder, SPIRV::StorageClass::Input);
+
+  // Create new register for GlobalInvocationID builtin variable.
+  Register NewRegister =
+      MIRBuilder.getMRI()->createVirtualRegister(&SPIRV::IDRegClass);
+  MIRBuilder.getMRI()->setType(NewRegister, LLT::pointer(0, 32));
+  GR.assignSPIRVTypeToVReg(PtrType, NewRegister, MIRBuilder.getMF());
+
+  // Build GlobalInvocationID global variable with the necessary decorations.
+  Register Variable = GR.buildGlobalVariable(
+      NewRegister, PtrType,
+      getLinkStringForBuiltIn(SPIRV::BuiltIn::GlobalInvocationId), nullptr,
+      SPIRV::StorageClass::Input, nullptr, true, true,
+      SPIRV::LinkageType::Import, MIRBuilder, false);
+
+  // Create new register for loading value.
+  MachineRegisterInfo *MRI = MIRBuilder.getMRI();
+  Register LoadedRegister = MRI->createVirtualRegister(&SPIRV::IDRegClass);
+  MIRBuilder.getMRI()->setType(LoadedRegister, LLT::pointer(0, 32));
+  GR.assignSPIRVTypeToVReg(Vec3Ty, LoadedRegister, MIRBuilder.getMF());
+
+  // Load v3uint value from the global variable.
+  BuildMI(*I.getParent(), I, I.getDebugLoc(), TII.get(SPIRV::OpLoad))
+      .addDef(LoadedRegister)
+      .addUse(GR.getSPIRVTypeID(Vec3Ty))
+      .addUse(Variable);
----------------
Keenuts wrote:

Shall this be moved inside a function to be reused for other intrinsics what are fetched from an Input variable in SPIR-V?

Maybe a function like:
```cpp
MachineInstruction* getSPIRVBuiltInValue(SPIRVType *valueType, SPIRV::BuiltIn builtIn);

[...]

MI = getSPIRVBuiltInValue(Vec3Ty, SPIRV::BuiltIn::GlobalInvocationId);
```



https://github.com/llvm/llvm-project/pull/82536