Use exp2 for mx scaling

stack-info: PR: #1530, branch: drisspg/stack/26

Author: drisspg

torchao/prototype/mx_formats/mx_tensor.py

@@ -127,10 +127,7 @@ def to_mx(
 
     # For now, calculate the scale in floating point.
     # TODO(future) audit if there is a need to bit shift exponents instead.
-    scale_fp = torch.pow(
-        torch.full(max_abs.size(), 2.0, device=scale_e8m0_biased.device),
-        scale_e8m0_unbiased,
-    )
+    scale_fp = torch.exp2(scale_e8m0_unbiased).to(torch.float32)
 
     # Today, 2**-127 returns 0 in compile+inductor+triton because it is in the
     # float32 denormal range. For now, manually adjust the fp scale. This is
@@ -177,13 +174,10 @@ def to_mx(
 
 def get_fp_scale(scale_e8m0):
     s_offset = scale_e8m0.to(torch.int16) - E8M0_EXPONENT_BIAS
-    # TODO(later): it would be nice if there was a way to do the 2^x operation
-    # in PyTorch without creating a tensor of twos
-    two = torch.full(s_offset.size(), 2.0, device=scale_e8m0.device)
-    # pow(two, s_offset) can be out of range of floating point formats.
+
     # TODO(later): handle this for float16 if we decide to support float16
     # scales.
-    s_fp = torch.pow(two, s_offset)
+    s_fp = torch.exp2(s_offset)
 
     # If a block exponent was 255, set values of that block to NaN
     s_fp = torch.where(scale_e8m0 != E8M0_EXPONENT_NAN_VAL, s_fp, float("nan"))