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performance - What is the fastest way to get the value of π? -

"I'm looking for the fastest way to obtain the value of π, as a personal challenge. More specifically, I'm using ways that don't involve using #define constants like M_PI, or hard-coding the number in.

The program below tests the various ways I know of. The inline assembly version is, in theory, the fastest option, though clearly not portable. I've included it as a baseline to compare against the other versions. In my tests, with built-ins, the 4 * atan(1) version is fastest on GCC 4.2, because it auto-folds the atan(1) into a constant. With -fno-builtin specified, the atan2(0, -1) version is fastest.

Here's the main testing program (pitimes.c):

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

#define ITERS 10000000
#define TESTWITH(x) { \
diff = 0.0; \
time1 = clock(); \
for (i = 0; i < ITERS; ++i) \
diff += (x) - M_PI; \
time2 = clock(); \
printf(""%s\t=> %e, time => %f\n"", #x, diff, diffclock(time2, time1)); \

static inline double
diffclock(clock_t time1, clock_t time0)
return (double) (time1 - time0) / CLOCKS_PER_SEC;

int i;
clock_t time1, time2;
double diff;

/* Warmup. The atan2 case catches GCC's atan folding (which would
* optimise the ``4 * atan(1) - M_PI'' to a no-op), if -fno-builtin
* is not used. */
TESTWITH(4 * atan(1))
TESTWITH(4 * atan2(1, 1))

#if defined(__GNUC__) && (defined(__i386__) || defined(__amd64__))
extern double fldpi();

/* Actual tests start here. */
TESTWITH(atan2(0, -1))
TESTWITH(2 * asin(1))
TESTWITH(4 * atan2(1, 1))
TESTWITH(4 * atan(1))

return 0;

And the inline assembly stuff (fldpi.c) that will only work for x86 and x64 systems:

double pi;
asm(""fldpi"" : ""=t"" (pi));
return pi;

And a build script that builds all the configurations I'm testing (build.sh):

gcc -O3 -Wall -c -m32 -o fldpi-32.o fldpi.c
gcc -O3 -Wall -c -m64 -o fldpi-64.o fldpi.c

gcc -O3 -Wall -ffast-math -m32 -o pitimes1-32 pitimes.c fldpi-32.o
gcc -O3 -Wall -m32 -o pitimes2-32 pitimes.c fldpi-32.o -lm
gcc -O3 -Wall -fno-builtin -m32 -o pitimes3-32 pitimes.c fldpi-32.o -lm
gcc -O3 -Wall -ffast-math -m64 -o pitimes1-64 pitimes.c fldpi-64.o -lm
gcc -O3 -Wall -m64 -o pitimes2-64 pitimes.c fldpi-64.o -lm
gcc -O3 -Wall -fno-builtin -m64 -o pitimes3-64 pitimes.c fldpi-64.o -lm

Apart from testing between various compiler flags (I've compared 32-bit against 64-bit too because the optimizations are different), I've also tried switching the order of the tests around. But still, the atan2(0, -1) version still comes out on top every time.

performance algorithm language-agnostic unix pi

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edited May 15 '19 at 1:33


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asked Aug 1 '08 at 5:21

Chris Jester-YoungChris Jester-Young

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