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

[Sarah Spall via llvm-commits]

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@@ -3166,109 +3171,228 @@ bool SPIRVInstructionSelector::selectFirstBitHigh32(Register ResVReg,
       .constrainAllUses(TII, TRI, RBI);
 }
 
-bool SPIRVInstructionSelector::selectFirstBitHigh64(Register ResVReg,
-                                                    const SPIRVType *ResType,
-                                                    MachineInstr &I,
-                                                    bool IsSigned) const {
-  Register OpReg = I.getOperand(2).getReg();
-  // 1. split our int64 into 2 pieces using a bitcast
-  unsigned count = GR.getScalarOrVectorComponentCount(ResType);
-  SPIRVType *baseType = GR.retrieveScalarOrVectorIntType(ResType);
+bool SPIRVInstructionSelector::selectFirstBitSet64(
+    Register ResVReg, const SPIRVType *ResType, MachineInstr &I,
+    Register SrcReg, unsigned BitSetOpcode, bool SwapPrimarySide) const {
+  unsigned ComponentCount = GR.getScalarOrVectorComponentCount(ResType);
+  SPIRVType *BaseType = GR.retrieveScalarOrVectorIntType(ResType);
+  bool ZeroAsNull = STI.isOpenCLEnv();
+  Register ConstIntZero =
+      GR.getOrCreateConstInt(0, I, BaseType, TII, ZeroAsNull);
+  Register ConstIntOne =
+      GR.getOrCreateConstInt(1, I, BaseType, TII, ZeroAsNull);
+
+  // SPIRV doesn't support vectors with more than 4 components. Since the
+  // algoritm below converts i64 -> i32x2 and i64x4 -> i32x8 it can only
+  // operate on vectors with 2 or less components. When largers vectors are
+  // seen. Split them, recurse, then recombine them.
+  if (ComponentCount > 2) {
+    unsigned LeftComponentCount = ComponentCount / 2;
----------------
spall wrote:

> I tried to write it to handle that case. It should just keep recursing and splitting the vectors in half until its under 2 components.
> 
> I do think we are strictly limited by SPIRV here though. Say hlsl supported u64x8, we still have to accept the vec8 in as a parameter and return a vec8 out. Both of which would require invalid SPIRV

I don't think this code can validly handle vectors that are > vec4 because the splitting action will create vectors which are too large in some cases.


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