The present disclosure relates generally to techniques for enhancing recurrent neural networks (RNNs) implemented on an integrated circuit. In particular, approximations of activation functions used in an RNN, such as sigmoid and hyperbolic tangent, may be implemented in an integrated circuit, which may result in increased efficiencies, reduced latency, increased accuracy, and reduced resource consumption involved with implementing machine learning.

In described examples, an apparatus is arranged to generate a linear term, a quadratic term, and a constant term of a transcendental function with, respectively, a first circuit, a second circuit, and a third circuit in response to least significant bits of an input operand and in response to, respectively, a first, a second, and a third table value that is retrieved in response to, respectively, a first, a second, and a third index generated in response to most significant bits of the input operand. The third circuit is further arranged to generate a mantissa of an output operand in response to a sum of the linear term, the quadratic term, and the constant term.

In described examples, an apparatus is arranged to generate a linear term, a quadratic term, and a constant term of a transcendental function with, respectively, a first circuit, a second circuit, and a third circuit in response to least significant bits of an input operand and in response to, respectively, a first, a second, and a third table value that is retrieved in response to, respectively, a first, a second, and a third index generated in response to most significant bits of the input operand. The third circuit is further arranged to generate a mantissa of an output operand in response to a sum of the linear term, the quadratic term, and the constant term.

A functional unit for a data processor comprises an input register to store a variable X; a first circuit, having an input connected to the input register and an output, to generate a value e.sup.X on its output; a second circuit, having an input connected to the input register and an output, to generate an output which is a value (tan h(X/2)+1)/2 on its output; a comparator, having an input connected to the input register and an output, to generate a line on its output based on a comparison between X and a constant; and a selector to select between inputs connected to the outputs of the first circuit and the second circuit, in response to the output of the comparator, and provide an output representing a value sigmoid(X).

A first multiplier multiplies a first input with a first coefficient and a first adder sums an output of the first multiplier and a second input to generate a first output. A second multiplier multiplies a third input with a second coefficient, a third multiplier multiplies a fourth input with a third coefficient, and a second adder sums outputs of the second and third multipliers to generate a second output. The second and third inputs are derived from the first output and the first and fourth inputs are derived from the second output. The first and second outputs generate digital values for first and second digital sinusoids, respectively.

A first multiplier multiplies a first input with a first coefficient and a first adder sums an output of the first multiplier and a second input to generate a first output. A second multiplier multiplies a third input with a second coefficient, a third multiplier multiplies a fourth input with a third coefficient, and a second adder sums outputs of the second and third multipliers to generate a second output. The second and third inputs are derived from the first output and the first and fourth inputs are derived from the second output. The first and second outputs generate digital values for first and second digital sinusoids, respectively.

A system and an accelerator circuit including a register file comprising instruction registers to store an instruction for evaluating an elementary function, and data registers comprising a first data register to store an input value. The accelerator circuit further includes a successive cumulative rotation circuit comprising a reconfigurable inner stage to perform a successive cumulative rotation recurrence, and a determination circuit to determine a type of the elementary function based on the instruction, and responsive to determining that the input value is a fixed-point number, configure the reconfigurable inner stage to a configuration for evaluating the type of the elementary function, wherein the successive cumulative rotation circuit is to calculate an evaluation of the elementary function using the reconfigurable inner stage performing the successive cumulative rotation recurrence.

A trigonometric function calculating device includes: an address generator that generates an address signal that is formed from plural bit strings and corresponds to a phase; a trigonometric function table that stores first sines and first cosines that respectively correspond to phases expressed by upper bits of the address signals, and second sines and a second cosines that respectively correspond to phases expressed by lower bits of the address signals; a calculation circuit that outputs, as a calculated value, a sine that corresponds to the address signal by calculating processing using the first sine, the first cosine, the second sine and the second cosine that correspond to the address signal and have been extracted by referring to the trigonometric function table; and a correcting section that corrects the calculated value on the basis of a correction value corresponding to the address signal.

A trigonometric function calculating device includes: an address generator that generates an address signal that is formed from plural bit strings and corresponds to a phase; a trigonometric function table that stores first sines and first cosines that respectively correspond to phases expressed by upper bits of the address signals, and second sines and a second cosines that respectively correspond to phases expressed by lower bits of the address signals; a calculation circuit that outputs, as a calculated value, a sine that corresponds to the address signal by calculating processing using the first sine, the first cosine, the second sine and the second cosine that correspond to the address signal and have been extracted by referring to the trigonometric function table; and a correcting section that corrects the calculated value on the basis of a correction value corresponding to the address signal.

Apparatus for evaluating a mathematical function for a received input value includes a polynomial block configured to identify a domain interval containing the received input value over which the mathematical function can be evaluated, the mathematical function over the identified interval being approximated by a polynomial function; and evaluate the polynomial function for the received input value using a set of one or more stored values representing the polynomial function over the identified interval to calculate a first evaluation of the mathematical function for the received input value; and a CORDIC block for performing a CORDIC algorithm, configured to initialise the CORDIC algorithm using the first evaluation of the mathematical function for the received input value calculated by the polynomial block; and implement the CORDIC algorithm to calculate a refined evaluation of the mathematical function for the received input value.