[MLIR] Fix type incompatibility in BitcastOp fold (llvm#125193)

This commit fixes a bug in the `arith::BitcastOp::fold` function where a
poisoned constant value was incorrectly cast to `IntegerAttr`, causing a
`cast<Ty>() argument of incompatible type!` error.

Author: VadimCurca

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -1761,6 +1761,10 @@ OpFoldResult arith::BitcastOp::fold(FoldAdaptor adaptor) {
   if (llvm::isa<ShapedType>(resType))
     return {};
 
+  /// Bitcast poison.
+  if (llvm::isa<ub::PoisonAttr>(operand))
+    return ub::PoisonAttr::get(getContext());
+
   /// Bitcast integer or float to integer or float.
   APInt bits = llvm::isa<FloatAttr>(operand)
                    ? llvm::cast<FloatAttr>(operand).getValue().bitcastToAPInt()

mlir/test/Dialect/Arith/canonicalize.mlir

@@ -1843,6 +1843,28 @@ func.func @bitcastOfBitcast(%arg : i16) -> i16 {
 
 // -----
 
+// CHECK-LABEL: @bitcastPoisonItoFP(
+func.func @bitcastPoisonItoFP() -> f32 {
+  // CHECK: %[[P:.+]] = ub.poison : f32
+  // CHECK: return %[[P]] : f32
+  %p = ub.poison : i32
+  %res = arith.bitcast %p : i32 to f32
+  return %res : f32
+}
+
+// -----
+
+// CHECK-LABEL: @bitcastPoisonFPtoI(
+func.func @bitcastPoisonFPtoI() -> i32 {
+  // CHECK: %[[P:.+]] = ub.poison : i32
+  // CHECK: return %[[P]] : i32
+  %p = ub.poison : f32
+  %res = arith.bitcast %p : f32 to i32
+  return %res : i32
+}
+
+// -----
+
 // CHECK-LABEL: test_maxsi
 // CHECK-DAG: %[[C0:.+]] = arith.constant 42
 // CHECK-DAG: %[[MAX_INT_CST:.+]] = arith.constant 127