Ignore i64 ConstantInt indices from ExtractElement and InsertElement instructions that fit within 32 bits

Author: Icohedron

llvm/lib/Target/DirectX/DXILShaderFlags.cpp

@@ -118,6 +118,24 @@ void ModuleShaderFlags::updateFunctionFlags(ComputedShaderFlags &CSF,
   if (!CSF.Int64Ops) {
     for (const Value *Op : I.operands()) {
       if (Op->getType()->isIntegerTy(64)) {
+
+        // Special-case handling for ConstantInt indices of ExtractElement and
+        // InsertElement instructions (likely inserted by the Scalarization
+        // pass). As long as the constant indices fit within 32 bits, we do not
+        // consider them to count as 64-bit integer usage.
+        switch (I.getOpcode()) {
+        case Instruction::ExtractElement:
+        case Instruction::InsertElement:
+          if (Op == I.getOperand(I.getNumOperands() - 1)) { // Index operand
+            if (isa<ConstantInt>(Op)) {
+              const ConstantInt *CI = cast<ConstantInt>(Op);
+              // The constant index must fit within 32 bits
+              if (CI->getZExtValue() <= UINT32_MAX)
+                continue;
+            }
+          }
+        }
+
         CSF.Int64Ops = true;
         break;
       }