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Merge bitcoin#689: Remove "except in benchmarks" exception for fp math
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bde2a32 Convert bench.h to fixed-point math (Wladimir J. van der Laan)

Pull request description:

  Convert `bench.h` to fixed-point math, removing all use of float math from the repository:

  - Use 64-bit integer microsecond timestamps
  - Use decimal fixed-point math for formatting numbers

  It turned out to be a little trickier than I expected because of formatting and rounding. But, output should be the same before and after.

  I used the following to test the number formatting: https://gist.github.com/laanwj/f971bfbe018e39c19677a21ff954d0c7

ACKs for top commit:
  real-or-random:
    ACK bde2a32 I've read the code in detail and I've tested it. I haven't explicitly tested the formatting function with known/hardcoded inputs.

Tree-SHA512: 41ab6024b88c65a4b194272097c70d527bedb396dc7ab9d3d93165f1a19d31092798370f66399443a8d5393d0a6dcf5825679de5a325550865cfdef3586bf64c
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real-or-random committed Dec 13, 2019
2 parents 387d723 + bde2a32 commit d644dda
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2 changes: 1 addition & 1 deletion README.md
Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ Implementation details
* Extensive testing infrastructure.
* Structured to facilitate review and analysis.
* Intended to be portable to any system with a C89 compiler and uint64_t support.
* No use of floating types, except in benchmarks.
* No use of floating types.
* Expose only higher level interfaces to minimize the API surface and improve application security. ("Be difficult to use insecurely.")
* Field operations
* Optimized implementation of arithmetic modulo the curve's field size (2^256 - 0x1000003D1).
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82 changes: 62 additions & 20 deletions src/bench.h
Original file line number Diff line number Diff line change
Expand Up @@ -7,43 +7,85 @@
#ifndef SECP256K1_BENCH_H
#define SECP256K1_BENCH_H

#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include "sys/time.h"

static double gettimedouble(void) {
static int64_t gettime_i64(void) {
struct timeval tv;
gettimeofday(&tv, NULL);
return tv.tv_usec * 0.000001 + tv.tv_sec;
return (int64_t)tv.tv_usec + (int64_t)tv.tv_sec * 1000000LL;
}

void print_number(double x) {
double y = x;
int c = 0;
if (y < 0.0) {
y = -y;
#define FP_EXP (6)
#define FP_MULT (1000000LL)

/* Format fixed point number. */
void print_number(const int64_t x) {
int64_t x_abs, y;
int c, i, rounding;
size_t ptr;
char buffer[30];

if (x == INT64_MIN) {
/* Prevent UB. */
printf("ERR");
return;
}
while (y > 0 && y < 100.0) {
y *= 10.0;
x_abs = x < 0 ? -x : x;

/* Determine how many decimals we want to show (more than FP_EXP makes no
* sense). */
y = x_abs;
c = 0;
while (y > 0LL && y < 100LL * FP_MULT && c < FP_EXP) {
y *= 10LL;
c++;
}
printf("%.*f", c, x);

/* Round to 'c' decimals. */
y = x_abs;
rounding = 0;
for (i = c; i < FP_EXP; ++i) {
rounding = (y % 10) >= 5;
y /= 10;
}
y += rounding;

/* Format and print the number. */
ptr = sizeof(buffer) - 1;
buffer[ptr] = 0;
if (c != 0) {
for (i = 0; i < c; ++i) {
buffer[--ptr] = '0' + (y % 10);
y /= 10;
}
buffer[--ptr] = '.';
}
do {
buffer[--ptr] = '0' + (y % 10);
y /= 10;
} while (y != 0);
if (x < 0) {
buffer[--ptr] = '-';
}
printf("%s", &buffer[ptr]);
}

void run_benchmark(char *name, void (*benchmark)(void*), void (*setup)(void*), void (*teardown)(void*), void* data, int count, int iter) {
int i;
double min = HUGE_VAL;
double sum = 0.0;
double max = 0.0;
int64_t min = INT64_MAX;
int64_t sum = 0;
int64_t max = 0;
for (i = 0; i < count; i++) {
double begin, total;
int64_t begin, total;
if (setup != NULL) {
setup(data);
}
begin = gettimedouble();
begin = gettime_i64();
benchmark(data);
total = gettimedouble() - begin;
total = gettime_i64() - begin;
if (teardown != NULL) {
teardown(data);
}
Expand All @@ -56,11 +98,11 @@ void run_benchmark(char *name, void (*benchmark)(void*), void (*setup)(void*), v
sum += total;
}
printf("%s: min ", name);
print_number(min * 1000000.0 / iter);
print_number(min * FP_MULT / iter);
printf("us / avg ");
print_number((sum / count) * 1000000.0 / iter);
print_number(((sum * FP_MULT) / count) / iter);
printf("us / max ");
print_number(max * 1000000.0 / iter);
print_number(max * FP_MULT / iter);
printf("us\n");
}

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