Best non-trigonometric floating point approximation of tanh(x) in 10 instructions or less

Question

Description

I need a reasonably accurate fast hyperbolic tangent for a machine that has no built-in floating point trigonometry, so e.g. the usual tanh(x) = (exp(2x) - 1) / (exp(2x) + 1) formula is going to need an approximation of exp(2x).
All other instructions like addition, subtraction, multiplication, division, and even FMA (= MUL+ADD in 1 op) are present.

Right now I have several approximations, but none of them are satisfactory in terms of accuracy.

[Update from the comments:]

• The instruction for trunc()/floor() is available
• There is a way to transparently reinterpret floats as integers and do all kinds of bit ops
• There is a family of instructions called SEL.xx (.GT, .LE, etc.) which compare 2 values and choose what to write to the destination
• DIVs are twice as slow, so nothing exceptional, DIVs are okay to use

Approach 1

Accuracy: ±1.2% absolute error, see here.

Pseudocode (A = accumulator register, T = temporary register):

[1] FMA T, 36.f / 73.f, A, A   // T := 36/73 + X^2
[2] MUL A, A, T                // A := X(36/73 + X^2)
[3] ABS T, A                   // T := |X(36/73 + X^2)|
[4] ADD T, T, 32.f / 73.f      // T := |X(36/73 + X^2)| + 32/73
[5] DIV A, A, T                // A := X(36/73 + X^2) / (|X(36/73 + X^2)| + 32/73)

Approach 2

Accuracy: ±0.9% absolute error, see here.

Pseudocode (A = accumulator register, T = temporary register):

[1] FMA T, 3.125f, A, A        // T := 3.125 + X^2
[2] DIV T, 25.125f, T          // T := 25.125/(3.125 + X^2)
[3] MUL A, A, 0.1073f          // A := 0.1073*X
[4] FMA A, A, A, T             // A := 0.1073*X + 0.1073*X*25.125/(3.125 + X^2)
[5] MIN A, A, 1.f              // A := min(0.1073*X + 0.1073*X*25.125/(3.125 + X^2), 1)
[6] MAX A, A, -1.f             // A := max(min(0.1073*X + 0.1073*X*25.125/(3.125 + X^2), 1), -1)

Approach 3

Accuracy: ±0.13% absolute error, see here.

Pseudocode (A = accumulator register, T = temporary register):

[1] FMA T, 14.f, A, A          // T := 14 + X^2
[2] FMA T, -133.f, T, T        // T := (14 + X^2)^2 - 133
[3] DIV T, A, T                // T := X/((14 + X^2)^2 - 133)
[4] FMA A, 52.5f, A, A         // A := 52.5 + X^2
[5] MUL A, A, RSQRT(15.f)      // A := (52.5 + X^2)/sqrt(15)
[6] FMA A, -120.75f, A, A      // A := (52.5 + X^2)^2/15 - 120.75
[7] MUL A, A, T                // A := ((52.5 + X^2)^2/15 - 120.75)*X/((14 + X^2)^2 - 133)
[8] MIN A, A, 1.f              // A := min(((52.5 + X^2)^2/15 - 120.75)*X/((14 + X^2)^2 - 133), 1)
[9] MAX A, A, -1.f             // A := max(min(((52.5 + X^2)^2/15 - 120.75)*X/((14 + X^2)^2 - 133), 1), -1)

The question

Is there anything better that can possibly fit in 10 non-trigonometric float32 instructions?

Answer 1

After doing much exploratory work, I came to the conclusion that approach 2 is the most promising direction. Since division is very fast on the asker's platform, rational approximations are attractive. The platform's support for FMA should be exploited aggressively. Below I am showing C code that implements a fast tanhf() in seven operations and achieves maximum absolute error of less than 2.8e-3.

I used the Remez algorithm to compute the coefficients for the rational approximation and used a heuristic search to reduce these coefficients to as few bits as feasible, which may benefit some processor architectures that are able to incorporate floating-point data into an immediate field of commonly used floating-point instructions.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

/* Fast computation of hyperbolic tangent. Rational approximation with clamping.
   Maximum absolute errror = 2.77074604e-3 @ +/-3.29019976
*/
float fast_tanhf_rat (float x)
{
    const float n0 = -8.73291016e-1f; // -0x1.bf2000p-1
    const float n1 = -2.76107788e-2f; // -0x1.c46000p-6
    const float d0 =  2.79589844e+0f; //  0x1.65e000p+1
    float x2 = x * x;
    float num = fmaf (n0, x2, n1);
    float den = x2 + d0;
    float quot = num / den;
    float res = fmaf (quot, x, x);
    res = fminf (fmaxf (res, -1.0f), 1.0f);
    return res;
}

int main (void)
{
    double ref, err, maxerr = 0;
    float arg, res, maxerrloc = INFINITY;
    maxerr = 0;
    arg = 0.0f;
    while (arg < 0x1.0p64f) {
        res = fast_tanhf_rat (arg);
        ref = tanh ((double)arg);
        err = fabs ((double)res - ref);
        if (err > maxerr) {
            maxerr = err;
            maxerrloc = arg;
        }
        arg = nextafterf (arg, INFINITY);
    }
    arg = -0.0f;
    while (arg > -0x1.0p64f) {
        res = fast_tanhf_rat (arg);
        ref = tanh ((double)arg);
        err = fabs ((double)res - ref);
        if (err > maxerr) {
            maxerr = err;
            maxerrloc = arg;
        }
        arg = nextafterf (arg, -INFINITY);
    }
    printf ("maximum absolute error = %15.8e @ %15.8e\n", maxerr, maxerrloc);
    return EXIT_SUCCESS;
}

Given that asker budgeted for up to ten operations, we can increase the degree of both numerator and denominator polynomials by one to achieve a fast tanhf() implementation comprising nine operations that has significantly lower maximum absolute error, less than 5.8e-5:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

/* Fast computation of hyperbolic tangent. Rational approximation with clamping.
   Maximum absolute error = 5.77514052e-5 @ +/-=2.22759748
 */
float fast_tanhf_rat2 (float x)
{
    const float n0 = -9.49066162e-1f; // -0x1.e5ec00p-1
    const float n1 = -2.68447266e+1f; // -0x1.ad8400p+4
    const float n2 = -2.01115608e-2f; // -0x1.498200p-6
    const float d0 =  3.49853516e+1f; //  0x1.17e200p+5
    const float d1 =  8.07031250e+1f; //  0x1.42d000p+6
    float x2 = x * x;
    float num = fmaf (fmaf (n0, x2, n1), x2, n2);
    float den = fmaf (x2 + d0, x2, d1);
    float quot = num / den;
    float res = fmaf (quot, x, x);
    res = fminf (fmaxf (res, -1.0f), 1.0f);
    return res;
}

int main (void)
{
    double ref, err, maxerr = 0;
    float arg, res, maxerrloc = INFINITY;
    maxerr = 0;
    arg = 0.0f;
    while (arg < 0x1.0p32f) {
        res = fast_tanhf_rat2 (arg);
        ref = tanh ((double)arg);
        err = fabs ((double)res - ref);
        if (err > maxerr) {
            maxerr = err;
            maxerrloc = arg;
        }
        arg = nextafterf (arg, INFINITY);
    }
    arg = -0.0f;
    while (arg > -0x1.0p32f) {
        res = fast_tanhf_rat2 (arg);
        ref = tanh ((double)arg);
        err = fabs ((double)res - ref);
        if (err > maxerr) {
            maxerr = err;
            maxerrloc = arg;
        }
        arg = nextafterf (arg, -INFINITY);
    }
    printf ("maximum absolute error = %15.8e @ %15.8e\n", maxerr, maxerrloc);
    return EXIT_SUCCESS;
}

Clamping the output of the approximation to the interval [-1, 1] is unnecessary if we can guarantee that the approximation can produces values outside this range. Single-precision implementations can be tested exhaustively, so one can show that by adjusting the coefficients of the approximation slightly this can be successfully enforces. By clamping the argument to a specific single-precision number for which the approximation returns the value ±1, the correct asymptotic behavior is achieved. This requires that all basic arithmetic operations and in particular the division are compliant with IEEE-754 and thus correctly rounded, all operands are IEEE-754 binary32 operands, and that rounding to nearest-or-even is in effect. Using the maximum of 10 operations allowed by the asker, maximum absolute and relative error of less than 2.0e-5 can be achieved:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

/* Fast computation of hyperbolic tangent. Rational approximation with clamping
   of the argument. Maximum absolute error = 1.98537030e-5, maximum relative
   error = 1.98540995e-5, maximum ulp error = 333.089863.
*/
float fast_tanhf_rat3 (float x) // 10 operations
{
    const float cutoff = 5.76110792f; //  0x1.70b5fep+2
    const float n0 = -1.60153955e-4f; // -0x1.4fde00p-13
    const float n1 = -9.34448242e-1f; // -0x1.de7000p-1
    const float n2 = -2.19176636e+1f; // -0x1.5eaec0p+4
    const float d0 =  2.90915985e+1f; //  0x1.d17730p+4
    const float d1 =  6.57667847e+1f; //  0x1.071130p+6
    float y = fminf (fmaxf (x, -cutoff), cutoff);
    float y2 = y * y;
    float num = fmaf (fmaf (n0, y2, n1), y2, n2) * y2;
    float den = fmaf (y2 + d0, y2, d1);
    float quot = num / den;
    float res = fmaf (quot, y, y);
    return res;
}

int main (void)
{
    double ref, abserr, relerr, maxabserr = 0, maxrelerr = 0;
    float arg, res, maxabserrloc = INFINITY, maxrelerrloc = INFINITY;

    maxabserr = 0;
    maxrelerr = 0;
    arg = 0.0f;
    while (arg < INFINITY) {
        res = fast_tanhf_rat3 (arg);
        if (res > 1) { 
            printf ("error at %15.8e: result out of bounds\n", arg);
            return EXIT_FAILURE;
        }
        ref = tanh ((double)arg);
        abserr = fabs ((double)res - ref);
        if (abserr > maxabserr) {
            maxabserr = abserr;
            maxabserrloc = arg;
        }
        relerr = fabs (((double)res - ref) / ref);
        if (relerr > maxrelerr) {
            maxrelerr = relerr;
            maxrelerrloc = arg;
        }
        arg = nextafterf (arg, INFINITY);
    }
    arg = -0.0f;
    while (arg > -INFINITY) {
        res = fast_tanhf_rat3 (arg);
        if (res < -1) { 
            printf ("error at %15.8e: result out of bounds\n", arg);
            return EXIT_FAILURE;
        }
        ref = tanh ((double)arg);
        abserr = fabs ((double)res - ref);
        if (abserr > maxabserr) {
            maxabserr = abserr;
            maxabserrloc = arg;
        }
        relerr = fabs (((double)res - ref) / ref);
        if (relerr > maxrelerr) {
            maxrelerr = relerr;
            maxrelerrloc = arg;
        }
        arg = nextafterf (arg, -INFINITY);
    }
    printf ("maximum absolute error = %15.8e @ %15.8e\n", maxabserr, maxabserrloc);
    printf ("maximum relative error = %15.8e @ %15.8e\n", maxrelerr, maxrelerrloc);
    return EXIT_SUCCESS;
}