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Implement fast_erf() and fast_gelu() #45

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Implement fast_erf() and fast_gelu() #45

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15 changes: 15 additions & 0 deletions gpt2.f90
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Expand Up @@ -43,6 +43,21 @@ elemental real(sp) function fast_tanh(x) result(y)
end if
end function

elemental real function fast_erf(x) result(y)
real(sp), intent(in) :: x
real(sp) :: abs_x
abs_x = abs(x)
y = 1 - 1 / (1+ abs_x * (0.278393 + abs_x * (0.230389 + abs_x * (0.000972 + 0.078108*abs_x))))**4
y = merge(y, -y, x >= 0)
end function

elemental real function fast_gelu(x) result(y)
real(sp), intent(in) :: x
real(sp), parameter :: inverse_root_2 = 1 / sqrt(2._sp)
y = 0.5_sp * x * (1 + fast_erf(x * inverse_root_2))
end function


elemental real(sp) function gelu(x) result(y)
real(sp), intent(in) :: x
y = 0.5_sp * x * (1 + tanh(sqrt(2 / pi) * (x + 0.044715_sp * x**3)))
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