add unit tests for rounding scale down to nearest power of 2

Author: danielvegamyhre

test/float8/test_utils.py

@@ -1,35 +1,68 @@
 import pytest
 import torch
 
-from torchao.float8.float8_utils import _round_down_to_power_of_2
+from torchao.float8.float8_utils import _round_scale_down_to_power_of_2
 
 
+# source for notable single-precision cases:
+# https://en.wikipedia.org/wiki/Single-precision_floating-point_format
+#
+# TODO(danielvegamyhre):
+# 1. add case for largest normal fp32 value: 2**127 * (2 - 2**-23).
+#    need to investigate why exp2(floor(log2(x)))=inf, but bitshift returns real value.
+# 2. add case for "nan"
+#    need to investigate why exp2(floor(log2(nan)))=nan, but bitshift returns inf.
+# 3. adjust cases for subnormal values so we aren't clamping the expected results
+#    into the normal range.
+#    preliminary investigation shows it may not be possible to support all subnormals
+#    with bitshifting, so we will need to debug/improve performance of exp2(floor(log2(x)))
+#    approach.
 @pytest.mark.parametrize(
-    "input_shape",
-    [
-        (1,),
-        (2, 3),
-        (8, 2048, 4, 1024),
-    ],
-)
-@pytest.mark.parametrize(
-    "multiplier",
+    "input",
     [
         1.0,
-        2.5,
-        10.0,
+        float("inf"),
+        # smallest positive subnormal number
+        2**-126 * 2**-23,
+        # largest subnormal number
+        2**-126 * (1 - 2**-23),
+        # smallest positive normal number
+        2**-126,
+        # largest number less than one
+        1.0 - 2**-24,
+        # smallest number larger than one
+        1.0 + 2**-23,
     ],
 )
-def test_round_down_to_power_of_2(input_shape: tuple[int], multiplier: int):
-    input_tensor = torch.rand(*input_shape, dtype=torch.float32) * multiplier
-    expected_output = torch.exp2(torch.floor(torch.log2(input_tensor)))
-    result = _round_down_to_power_of_2(input_tensor)
+def test_round_scale_down_to_power_of_2_valid_inputs(input: float):
+    input_tensor = torch.tensor(input, dtype=torch.float32)
+    result = _round_scale_down_to_power_of_2(input_tensor)
+
+    # get expected value for comparison
+    # TODO(danielvegamyhre): support subnormal values
+    expected_result = torch.exp2(torch.floor(torch.log2(input_tensor)))
+    smallest_normal_fp32_value = torch.tensor(2**-126, dtype=torch.float32)
+    expected_result = torch.max(expected_result, smallest_normal_fp32_value)
+
     assert torch.equal(
-        result, expected_output
-    ), f"expected {expected_output}, but got {result}"
+        result, expected_result
+    ), f"input: {input_tensor}, expected {expected_result}, but got {result}"
 
 
-def test_non_float32_input():
-    non_float32_tensor = torch.tensor([3.0], dtype=torch.float64)
-    with pytest.raises(AssertionError, match="input must be float32 tensor"):
-        _round_down_to_power_of_2(non_float32_tensor)
+@pytest.mark.parametrize(
+    "invalid_dtype",
+    [
+        torch.bfloat16,
+        torch.float16,
+        torch.float64,
+        torch.int8,
+        torch.uint8,
+        torch.int32,
+        torch.uint32,
+        torch.int64,
+    ],
+)
+def test_non_float32_input(invalid_dtype: torch.dtype):
+    non_float32_tensor = torch.tensor([3.0], dtype=invalid_dtype)
+    with pytest.raises(AssertionError, match="scale must be float32 tensor"):
+        _round_scale_down_to_power_of_2(non_float32_tensor)

torchao/float8/float8_utils.py

@@ -49,7 +49,7 @@ def amax_to_scale(
     else:
         raise ValueError(f"Unsupported float8_dtype: {float8_dtype}")
     if round_scales_to_power_of_2:
-        res = _round_down_to_power_of_2(res)
+        res = _round_scale_down_to_power_of_2(res)
     return res
 
 
@@ -285,10 +285,23 @@ def config_has_stateful_scaling(config: Float8LinearConfig) -> bool:
     )
 
 
-def _round_down_to_power_of_2(x: torch.Tensor) -> torch.Tensor:
-    assert x.dtype == torch.float32, "input must be float32 tensor"
-    # rounds down to the nearest power of 2
-    x = x.view(torch.int32)
-    x = (x >> 23) << 23
-    x = x.view(torch.float32)
-    return x
+def _round_scale_down_to_power_of_2(x: torch.Tensor):
+    assert x.dtype == torch.float32, "scale must be float32 tensor"
+
+    # eps = smallest normal fp32 value
+    # TODO(danielvegamyhre): support subnormal values
+    eps = 2**-126
+    x = torch.clamp(
+        x,
+        min=eps,
+    )
+
+    # view as int32 to allow bitshifting
+    x_int = x.view(torch.int32)
+
+    # clear mantissa bits (rightmost 23 bits)
+    x_int = (x_int >> 23) << 23
+
+    # return result as fp32
+    result = x_int.view(torch.float32)
+    return result