[pallas:mosaic_gpu] Add test for FragmentedArray.bitcast.

jax/experimental/mosaic/gpu/fragmented_array.py

@@ -463,8 +463,8 @@ def __init__(
 
     if (_is_signed is not None) != ir.IntegerType.isinstance(self.mlir_dtype):
       raise TypeError(
-          "is_signed must only be non-None if the MLIR type is an integer"
-          f" type, got {_is_signed=} for {self.mlir_dtype}"
+          "is_signed must be non-None if and only if the MLIR type is an"
+          f" integer type, got {_is_signed=} for {self.mlir_dtype}"
       )
 
     match self.layout:
@@ -962,6 +962,12 @@ def fast_instr(x):
     return fast_instr
 
   def bitcast(self, elt: ir.Type, *, output_is_signed: bool | None = None):
+    if (output_is_signed is not None) != ir.IntegerType.isinstance(elt):
+      raise TypeError(
+          "output_is_signed must be non-None if and only if the MLIR type is an"
+          f" integer type, got {output_is_signed=} for {elt}"
+      )
+
     if elt == self.mlir_dtype:
       return self
     reg_type = self.registers.flat[0].type

tests/mosaic/gpu_test.py

@@ -1577,6 +1577,40 @@ def kernel(ctx, _):
 
     _ = mgpu.as_gpu_kernel(kernel, (1, 1, 1), (128, 1, 1), (), (), None)()
 
+  @parameterized.parameters(
+      (jnp.float16, jnp.float16),  # Noop
+      (jnp.int16, jnp.bfloat16),
+      (jnp.int16, jnp.float16),
+      (jnp.uint16, jnp.float16),
+      (jnp.float32, jnp.int32),
+      (jnp.float32, jnp.uint32),
+      (jnp.uint32, jnp.int32),
+      (jnp.int32, jnp.uint32),
+  )
+  def test_bitcast(self, in_dtype, out_dtype):
+    out_ir_type = utils.dtype_to_ir_type(out_dtype)
+    in_is_signed = utils.is_signed(in_dtype)
+    out_is_signed = utils.is_signed(out_dtype)
+
+    def kernel(ctx, inp, out, smem):
+      del ctx, smem
+      arr = mgpu.FragmentedArray.load_strided(inp, is_signed=in_is_signed)
+      arr = arr.bitcast(out_ir_type, output_is_signed=out_is_signed)
+      arr.store_untiled(out)
+
+    x = jnp.arange(256, dtype=in_dtype)
+    reference = jax.lax.bitcast_convert_type(x, out_dtype)
+
+    result = mgpu.as_gpu_kernel(
+        kernel,
+        (1, 1, 1),
+        (128, 1, 1),
+        x,
+        reference,
+        None,
+    )(x)
+    np.testing.assert_array_equal(result, reference)
+
 
 class ProfilerTest(TestCase):