rewrite_gelu.cpp

/*
 * The MIT License (MIT)
 *
 * Copyright (c) 2015-2022 Advanced Micro Devices, Inc. All rights reserved.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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 * THE SOFTWARE.
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#include <migraphx/rewrite_gelu.hpp>
#include <migraphx/make_op.hpp>
#include <migraphx/matcher.hpp>
#include <migraphx/match/gelu_erf.hpp>
#include <migraphx/match/gelu_tanh.hpp>
#include <migraphx/common.hpp>

namespace migraphx {
inline namespace MIGRAPHX_INLINE_NS {

/**
 * Finds GELU blocks using the Gaussian distribution and replaces them with the sigmoid approximation
 * if the data type is fp16.
 */
struct find_gelu_erf
{
    auto matcher() const { return match::gelu_erf(); }

    void apply(module& m, const match::matcher_result& r) const
    {
        auto ins = r.result;
        auto x   = r.instructions["x"];
        if(x->get_shape().type() != migraphx::shape::half_type)
            return;

        auto lit = m.add_literal(literal{shape{x->get_shape().type()}, {1.702f}});
        auto mul = insert_common_op(m, ins, make_op("mul"), {x, lit});
        auto sig = m.insert_instruction(ins, make_op("neg"), mul);
        sig      = m.insert_instruction(ins, make_op("exp"), sig);
        auto one = m.add_literal(literal{shape{x->get_shape().type()}, {1.0f}});
        sig      = insert_common_op(m, ins, make_op("add"), {sig, one});
        sig      = m.insert_instruction(ins, make_op("div"), x, sig);
        m.replace_instruction(ins, sig);
    }
};

/**
 * Find fastGELU blocks (where the graph already does a GELU approximation) and replace them
 * with an alternative approximation that is less likely to overflow.
 * The replacement approximation is equivalent to:
 * GELU(x) ~= 0.5 * x * ( 1 + tanh( sqrt(2/M_PI) * (x + 0.044715 * x^3)))
 */
struct find_tanh_fast_gelu
{
    auto matcher() const { return match::gelu_tanh(); }

    void apply(module& m, const match::matcher_result& r) const
    {
        auto ins = r.result;
        auto x   = r.instructions["x"];
        auto sqrt_2_rpi = m.add_literal(literal{shape{x->get_shape().type()}, {0.7978845608028653558798921198687637369517172623298693153318516593}});
        auto fit_const = m.add_literal(literal{shape{x->get_shape().type()}, {0.044715f}});
        auto one = m.add_literal(literal{shape{x->get_shape().type()}, {1.0f}});
        auto xb = insert_common_op(m, ins, make_op("mul"), {x, sqrt_2_rpi});
        auto a = insert_common_op(m, ins, make_op("mul"), {xb, fit_const});
        auto b = m.insert_instruction(ins, make_op("mul"), a, x);
        auto c = m.insert_instruction(ins, make_op("mul"), b, x);
        auto u = m.insert_instruction(ins, make_op("add"), c, xb);
        auto neg_u = m.insert_instruction(ins, make_op("neg"), u);
        auto d = m.insert_instruction(ins, make_op("sub"), neg_u, u);
        auto emu = m.insert_instruction(ins, make_op("exp"), d);
        auto e = insert_common_op(m, ins, make_op("add"), {one, emu});
        auto cdf = insert_common_op(m, ins, make_op("div"), {one, e});
        auto y = m.insert_instruction(ins, make_op("mul"), x, cdf);
        m.replace_instruction(ins, y);
    }
};

void rewrite_gelu::apply(module& m) const {
    match::find_matches(m,
            find_gelu_erf{},
            find_tanh_fast_gelu{}  
        );
}

} // namespace MIGRAPHX_INLINE_NS
} // namespace migraphx