Fuzzing "subnormal result implies exact result with LibFuzzer"

fuzz_subnormal_result_implies_exact.c

#include <assert.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;
}

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));


  float result = a + b;
  if (!is_subnormal(result))
    return 0;


  // Approximately check the addition
  // was exact. Not sure how good this
  // approximation is.
  int condition = (result - b) == a;
  if (!condition) {
    printf("Bad value: %f\n", result);
    printf("as hex float: %a\n", result);
    // Show as bits
    uint32_t result_bits = 0;
    memcpy(&result_bits, &result, sizeof(float));
    printf("as bits: 0x%.8" PRIx32 "\n", result_bits);
    abort();
  }
  return 0;
}