I found sorting large arrays by a comparator that looks at the floating-point difference problematic, especially -ffast-math, (https://stackoverflow.com/questions/24442725/is-floating-point-addition-commutative-in-c.) This is intended to compare 32-bit floats exactly using integer arithmetic. Most references cited concern themselves with radix sort:

Using a comparison sort, like qsort, it only has to decide on a total order of IEEE-754 numbers by comparing two at a time.

#include <stdlib.h> /* EXIT_SUCCESS qsort */
#include <stdio.h>  /* printf */
#include <assert.h> /* assert */
#include <time.h>   /* clock */
#include <limits.h> /* INT_MAX */
#include <math.h>   /* C99 floating point macros */
#include <stdint.h> /* C99 uint32_t */

struct Foo { float x; };

/** Compares float {x} values of {Foo} exactly. Assumes IEEE-754-ish 32-bit
 float has the same endianness as {uint32_t}. References:
 Nicholas Chapman \url{ https://www.forwardscattering.org/post/34 },
 Michael Herf \url{ http://stereopsis.com/radix.html },
 Pierre Terdiman \url{ http://codercorner.com/RadixSortRevisited.htm },
 \url{ https://hbfs.wordpress.com/2010/03/09/radix-sort-on-floating-point-numbers/ },
 \url{ https://randomascii.wordpress.com/category/floating-point/ },
 \url{ https://stackoverflow.com/questions/10632237/any-c-compiler-where-evaluates-to-larger-than-one }.
 @implements <Foo>Comparator
 @return Greater than, equal to, or less than 0, if the {Foo.x} pointed to by
 {av} is greater than, equal to, or less than the {Foo.x} pointed to by {bv}. */
static int x_cmp(const void *av, const void *bv) {
    const struct Foo *const a = av, *const b = bv;
    union { float f; uint32_t u; } ax, bx;
    ax.f = a->x, bx.f = b->x;
        /* @chux unsigned -> uint32_t critical fix */
        const uint32_t ax_abs = ax.u & 0x7fffffff, ax_sign = ax.u & 0x80000000;
        const uint32_t bx_abs = bx.u & 0x7fffffff, bx_sign = bx.u & 0x80000000;
        const uint32_t same = !(ax_sign ^ bx_sign);
        /* if(!same) return ax_sign^1; else return ax_sign^(ax_abs - bx_abs); */
        return ax_sign ^ (!same + same * (ax_abs - bx_abs));

/** Tests {x_cmp}. */
int main(void) {
    struct Foo foos[32]; /* could be bigger */
    const size_t foos_size = sizeof foos / sizeof *foos;
    unsigned i;
    union { float f; uint32_t u; } x;

    assert(sizeof(float) == 4);
    srand((unsigned)clock()), rand();
    foos[0].x = 0.0f;
    foos[1].x = 0.0f;
    foos[2].x = -0.0f;
    foos[3].x = INFINITY; /* C99 */
    foos[4].x = -INFINITY; /* C99 */
    foos[5].x = NAN; /* C99 */
    foos[6].x = (x.u = 0x807fffff, x.f); /* subnormal */
    foos[7].x = (x.u = 1, x.f); /* subnormal */
    for(i = 8; i < 2 * foos_size / 3; i++) {
        foos[i].x = rand() / (0.5f * RAND_MAX / INT_MAX) - INT_MAX;
    } for( ; i < foos_size; i++) {
        foos[i].x = rand() / (0.5f * RAND_MAX / 100.0f) - 100.0f;
    qsort(foos, foos_size, sizeof foos[0], &x_cmp);
    for(i = 1; i < foos_size; i++) {
        printf("%g <= %g\n", foos[i - 1].x, foos[i].x);
        /* isnan C99 */
        assert(isnan(foos[i - 1].x) || isnan(foos[i].x)
            || foos[i - 1].x <= foos[i].x);
    return EXIT_SUCCESS;

I compiled it on GCC 4.2.1 MacOS, GCC 5.4.0 Linux, and MSVC15, (the order of the NaN macro switched, thus isnan,) I think it's valid? Could the comparator be done in less steps?


int32_t: increasing injection mod the branch cut. int32_t

IEEE 754-ish float: (one way) to get rid of the singularity and make it monotonic is to invert the negative values and flip the sign bit on positive values, Radix Sort, Sorting a float data.

IEEE 754 floating point.

  • 4
    \$\begingroup\$ You shouldn't rely on internal FP representation like that, it won't work properly with every target CPU/FPU. \$\endgroup\$ Apr 19, 2017 at 0:27
  • \$\begingroup\$ Hmmmm: Idea: If x_cmp() only did uint32_t *a = av; uint32_t *b = bv; return (*a > *b ) - (*a < *b);, then the array would be quickly "sorted" per its binary image. Assuming a matching float/int32_t.endian and binary32 FP, a post analysis could fold, in O(n) time, the resultant array as needed. I suspect it would be faster. Perhaps I'll try it later. \$\endgroup\$ Apr 19, 2017 at 2:18
  • 1
    \$\begingroup\$ If you care about performance, don't use qsort. Here is an old post on the performance of sorting. qsort was very slow. I don't know how much it stands nowadays, but calling function pointers that frequently hurts anyway. In addition, qsort is badly designed. Generic sorting algorithms can implemented with a single "less than" operator (like in C++ STL). You don't need any arithmetic operations. \$\endgroup\$
    – user172818
    Apr 21, 2017 at 18:02
  • \$\begingroup\$ @πάνταῥεῖ I think I'm relying on: (1) float is 32 bits (I check this with an assert,) (2) negative sign bit is 31, (3) ~int32_t(float)++ is 1 ULP bigger. Specifically, this applies to IBM floats as well. The answer by @chux made no such assumption. @user172818 true; I assume this applies to all comparison sorts, on average. \$\endgroup\$
    – Neil
    Apr 21, 2017 at 20:52

1 Answer 1

  1. ax_sign ^ (!same + same * (ax_abs - bx_abs)); returns the incorrect signed result if int is not 32-bit. Certainly a problem if int is 16-bit and likely if int is 64-bit. If unsigned/int needs to be 32-bit, use (u)int32_t

  2. , operator reduces clarity here. Suggest 2 lines of code

    // ax.f = a->x, bx.f = b->x;
    ax.f = a->x;
    bx.f = b->x;
  3. I ran test cases and found OP's x_cmp() functionally correct for finite float over a 2,000,000,000 test cases.

  4. As a test case, I tried OP's original x_cmp() versus the below and was at least 10% faster with the new code. Of course, that is just one platform comparison, yet aside from NaN issues, the below code is functionally similar to OP's and as a plus, is highly portable - unlike OP's. The point being that OP's compare method needs some reference point to justify the bit magic.

    static int x_cmp_ref(const void *av, const void *bv) {
      return (*(float*)av > *(float*)bv) - (*(float*)av < *(float*)bv);
  5. OP's has not stated the compare functionality of Not-a-number floats. A desirable aspect is that all NaN sort to one side, either all greater or all less than any other number, regardless of the NaN's "sign". x_cmp() considers sign first without regard to NaN-ness.

As a reference, I used the following to generate random float

float randf() {
  union {
      float f;
      unsigned char uc[sizeof (float)];
  } u;
  do {
    for (unsigned i=0; i<sizeof u.uc; i++) {
      u.uc[i] = (unsigned char) rand();
  } while (!isfinite(u.f));
  return u.f;

At a later time, I may try to implement the idea in this comment

  • \$\begingroup\$ 1. Good catch, it should be all uint32_t to match float; 2. thanks; 4. that would be great! but it didn't work for negative values; stackoverflow.com/questions/4640906/…; 5. I agree that it's arguably best to keep them all NaNs one place; otoh, why change arbitrarily no-work placement without good reason? \$\endgroup\$
    – Neil
    Apr 19, 2017 at 10:18
  • 1
    \$\begingroup\$ @NeilEdelman Thanks for your thoughts. #4 returns the correct int when comparing float of which 1 or both are negative. Unclear on "it didn't work for negative values" as it applies to qsort() used here. #5 There is a good reason: A typical application for a list with NaNs is to sort them out to the end, that way a simple reduction in N can lop them off the array. As I see it, a robust compare concenring NaN can be done with low cost. Yet as you imply, NaN handling is not universal. \$\endgroup\$ Apr 19, 2017 at 13:35
  • \$\begingroup\$ I was reinterpreting them as uint32_t, which did not sort the negative values correctly; leaving them as floats produced the right output. I measure it to be 25% faster on my machine. I did a sequence of values that differed by 1 ulp, and it appears to be stable, and looks cool. \$\endgroup\$
    – Neil
    Apr 20, 2017 at 3:43
  • \$\begingroup\$ *As you said, the placement of NaNs in a comparison sort with float is different than int32_t; comparisons with float NaNs are not treated as equivalence relations. \$\endgroup\$
    – Neil
    Apr 20, 2017 at 5:12

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