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# Copyright (c) 2021, NVIDIA CORPORATION. All rights reserved.
#
# NVIDIA CORPORATION and its licensors retain all intellectual property
# and proprietary rights in and to this software, related documentation
# and any modifications thereto. Any use, reproduction, disclosure or
# distribution of this software and related documentation without an express
# license agreement from NVIDIA CORPORATION is strictly prohibited.
"""Fused multiply-add, with slightly faster gradients than `torch.addcmul()`."""
import torch
#----------------------------------------------------------------------------
def fma(a, b, c): # => a * b + c
return _FusedMultiplyAdd.apply(a, b, c)
#----------------------------------------------------------------------------
class _FusedMultiplyAdd(torch.autograd.Function): # a * b + c
@staticmethod
def forward(ctx, a, b, c): # pylint: disable=arguments-differ
out = torch.addcmul(c, a, b)
ctx.save_for_backward(a, b)
ctx.c_shape = c.shape
return out
@staticmethod
def backward(ctx, dout): # pylint: disable=arguments-differ
a, b = ctx.saved_tensors
c_shape = ctx.c_shape
da = None
db = None
dc = None
if ctx.needs_input_grad[0]:
da = _unbroadcast(dout * b, a.shape)
if ctx.needs_input_grad[1]:
db = _unbroadcast(dout * a, b.shape)
if ctx.needs_input_grad[2]:
dc = _unbroadcast(dout, c_shape)
return da, db, dc
#----------------------------------------------------------------------------
def _unbroadcast(x, shape):
extra_dims = x.ndim - len(shape)
assert extra_dims >= 0
dim = [i for i in range(x.ndim) if x.shape[i] > 1 and (i < extra_dims or shape[i - extra_dims] == 1)]
if len(dim):
x = x.sum(dim=dim, keepdim=True)
if extra_dims:
x = x.reshape(-1, *x.shape[extra_dims+1:])
assert x.shape == shape
return x
#----------------------------------------------------------------------------
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