Use LibFuzzer to look for violations of the result of floating point addition being subnormal implying

fuzz_subnormal_result_implies_exact.c

#include <assert.h>
#include <fenv.h>
#include <inttypes.h>
#include <math.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>

int is_subnormal(float f) {
  return fpclassify(f) == FP_SUBNORMAL;
}

void dump_float(float f, const char* name) {
  printf("Dumping float \"%s\"\n", name);
  printf("Bad value: %f\n", f);
  printf("as hex float: %a\n", f);
  // Show as bits
  uint32_t as_bits = 0;
  memcpy(&as_bits, &f, sizeof(float));
  printf("as bits: 0x%.8" PRIx32 "\n", as_bits);
}

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
  float a = 0.0;
  float b = 0.0;

  if (Size < (sizeof(a) + sizeof(b)))
    return 0;

  // Big enough
  memcpy(&a, Data, sizeof(a));
  memcpy(&b, Data + sizeof(a), sizeof(b));

  int failure = feclearexcept(FE_ALL_EXCEPT);
  assert(!failure);

  __asm__ __volatile__ (""); // HACK: Don't move my code plz.

  float result = a + b;

  __asm__ __volatile__ (""); // HACK: Don't move my code plz.

  bool inexact = fetestexcept(FE_INEXACT);

  if (!is_subnormal(result))
    return 0;

  if (inexact) {
    dump_float(a, "a");
    dump_float(b, "b");
    dump_float(result, "result");
    abort();
  }
  return 0;
}