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optimize math (#5287)

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* optimize math

Results in considerable flash saving

* log_approx, exp_approx, pow_approx

Taken from https://github.com/jhjourdan/SIMD-math-prims/blob/master/simd_math_prims.h

* Fix pow in rangefinder

* Use approximate function in baro calculation

Maximum error is < 20cm

* fixup! Fix pow in rangefinder
This commit is contained in:
Petr Ledvina 2018-03-03 13:26:33 +01:00 • committed by Michael Keller
parent b64802d931
commit c11d016bc7
7 changed files with 117 additions and 7 deletions
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src/main/common/explog_approx.c Normal file
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/*
The MIT License (MIT)
Copyright (c) 2015 Jacques-Henri Jourdan <jourgun@gmail.com>
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
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
Taken from https://github.com/jhjourdan/SIMD-math-prims/blob/master/simd_math_prims.h
Stripped down for BF use
*/
#include <math.h>
#include <stdint.h>
/* Workaround a lack of optimization in gcc */
float exp_cst1 = 2139095040.f;
float exp_cst2 = 0.f;
/* Relative error bounded by 1e-5 for normalized outputs
Returns invalid outputs for nan inputs
Continuous error */
float exp_approx(float val) {
union { int32_t i; float f; } xu, xu2;
float val2, val3, val4, b;
int32_t val4i;
val2 = 12102203.1615614f*val+1065353216.f;
val3 = val2 < exp_cst1 ? val2 : exp_cst1;
val4 = val3 > exp_cst2 ? val3 : exp_cst2;
val4i = (int32_t) val4;
xu.i = val4i & 0x7F800000; // mask exponent / round down to neareset 2^n (implicit mantisa bit)
xu2.i = (val4i & 0x7FFFFF) | 0x3F800000; // force exponent to 0
b = xu2.f;
/* Generated in Sollya with:
> f=remez(1-x*exp(-(x-1)*log(2)),
[|(x-1)*(x-2), (x-1)*(x-2)*x, (x-1)*(x-2)*x*x|],
[1.000001,1.999999], exp(-(x-1)*log(2)));
> plot(exp((x-1)*log(2))/(f+x)-1, [1,2]);
> f+x;
*/
return
xu.f * (0.509871020343597804469416f + b *
(0.312146713032169896138863f + b *
(0.166617139319965966118107f + b *
(-2.19061993049215080032874e-3f + b *
1.3555747234758484073940937e-2f))));
}
/* Absolute error bounded by 1e-6 for normalized inputs
Returns a finite number for +inf input
Returns -inf for nan and <= 0 inputs.
Continuous error. */
float log_approx(float val) {
union { float f; int32_t i; } valu;
float exp, addcst, x;
valu.f = val;
exp = valu.i >> 23;
/* 89.970756366f = 127 * log(2) - constant term of polynomial */
addcst = val > 0 ? -89.970756366f : -(float)INFINITY;
valu.i = (valu.i & 0x7FFFFF) | 0x3F800000;
x = valu.f;
/* Generated in Sollya using:
> f = remez(log(x)-(x-1)*log(2),
[|1,(x-1)*(x-2), (x-1)*(x-2)*x, (x-1)*(x-2)*x*x,
(x-1)*(x-2)*x*x*x|], [1,2], 1, 1e-8);
> plot(f+(x-1)*log(2)-log(x), [1,2]);
> f+(x-1)*log(2)
*/
return
x * (3.529304993f + x * (-2.461222105f +
x * (1.130626167f + x * (-0.288739945f +
x * 3.110401639e-2f))))
+ (addcst + 0.69314718055995f*exp);
}
float pow_approx(float a, float b)
{
return exp_approx(b * log_approx(a));
}