[clang] [llvm] [HLSL] Implement elementwise firstbitlow builtin (PR #116858)

[Ashley Coleman via llvm-commits]

================
@@ -3178,98 +3178,74 @@ bool SPIRVInstructionSelector::selectFirstBitSet64Overflow(
     Register ResVReg, const SPIRVType *ResType, MachineInstr &I,
     Register SrcReg, unsigned BitSetOpcode, bool SwapPrimarySide) const {
 
+  // SPIR-V only allow vecs of size 2,3,4. Calling with a larger vec requires
+  // creating a param reg and return reg with an invalid vec size. If that is
+  // resolved then this function is valid for vectors of any component size.
   unsigned ComponentCount = GR.getScalarOrVectorComponentCount(ResType);
-  SPIRVType *BaseType = GR.retrieveScalarOrVectorIntType(ResType);
-  bool ZeroAsNull = STI.isOpenCLEnv();
-  Register ConstIntZero =
-      GR.getOrCreateConstInt(0, I, BaseType, TII, ZeroAsNull);
-  unsigned LeftComponentCount = ComponentCount / 2;
-  unsigned RightComponentCount = ComponentCount - LeftComponentCount;
-  bool LeftIsVector = LeftComponentCount > 1;
+  assert(ComponentCount < 5 && "Vec 5+ will generate invalid SPIR-V ops");
 
-  // Split the SrcReg in half into 2 smaller vec registers
-  // (ie i64x4 -> i64x2, i64x2)
+  bool ZeroAsNull = STI.isOpenCLEnv();
   MachineIRBuilder MIRBuilder(I);
-  SPIRVType *OpType = GR.getOrCreateSPIRVIntegerType(64, MIRBuilder);
-  SPIRVType *LeftVecOpType;
-  SPIRVType *LeftVecResType;
-  if (LeftIsVector) {
-    LeftVecOpType =
-        GR.getOrCreateSPIRVVectorType(OpType, LeftComponentCount, MIRBuilder);
-    LeftVecResType =
-        GR.getOrCreateSPIRVVectorType(BaseType, LeftComponentCount, MIRBuilder);
-  } else {
-    LeftVecOpType = OpType;
-    LeftVecResType = BaseType;
-  }
-
-  SPIRVType *RightVecOpType =
-      GR.getOrCreateSPIRVVectorType(OpType, RightComponentCount, MIRBuilder);
-  SPIRVType *RightVecResType =
-      GR.getOrCreateSPIRVVectorType(BaseType, RightComponentCount, MIRBuilder);
+  SPIRVType *BaseType = GR.retrieveScalarOrVectorIntType(ResType);
+  SPIRVType *I64Type = GR.getOrCreateSPIRVIntegerType(64, MIRBuilder);
+  SPIRVType *I64x2Type = GR.getOrCreateSPIRVVectorType(I64Type, 2, MIRBuilder);
+  SPIRVType *Vec2ResType =
+      GR.getOrCreateSPIRVVectorType(BaseType, 2, MIRBuilder);
 
-  Register LeftSideIn =
-      MRI->createVirtualRegister(GR.getRegClass(LeftVecOpType));
-  Register RightSideIn =
-      MRI->createVirtualRegister(GR.getRegClass(RightVecOpType));
+  std::vector<Register> PartialRegs;
 
-  bool Result;
+  // Loops 0, 2, 4, ... but stops one loop early when ComponentCount is odd
+  unsigned CurrentComponent = 0;
+  for (; CurrentComponent + 1 < ComponentCount; CurrentComponent += 2) {
----------------
V-FEXrt wrote:

The 2 case is avoided in `selectFirstBitSet64`

```c++
  if (ComponentCount > 2) {
    return selectFirstBitSet64Overflow(ResVReg, ResType, I, SrcReg,
                                       BitSetOpcode, SwapPrimarySide);
  }
```

I didn't want to artificially not support/special case size 2 since it makes the code more complicated and it's already handled by the caller

https://github.com/llvm/llvm-project/pull/116858