test.c

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#include <stdio.h>
#include <stdint.h>
#include <math.h>
#include <float.h>

#define PG_INT64_MAX INT64_MAX
#define PG_INT64_MIN INT64_MIN

/*
 * Check if a double value can be casted into int64.
 *
 * This macro is assuming that FLT_RADIX == 2 so that the * 2.0 trick works,
 * PG_INT64_MAX is so below DBL_MAX that the doubled value can be represented
 * in double and DBL_MANT_DIG is equal or smaller than DBL_MAX_EXP so that
 * ceil() returns expected result.
 */
#define MIN_DOUBLE_OVER_INT64_MAX (2.0 * (double) (PG_INT64_MAX / 2 + 1))
#define MAX_DOUBLE_UNDER_INT64_MIN (2.0 * (double) (PG_INT64_MIN / 2 - 1))

#if -PG_INT64_MAX != PG_INT64_MIN
#define IS_DOUBLE_SAFE_IN_INT64(x)				\
	((x) < MIN_DOUBLE_OVER_INT64_MAX && ceil(x) >= (double) PG_INT64_MIN)
#else
#define IS_DOUBLE_SAFE_IN_INT64(x)				\
	((x) < MIN_DOUBLE_OVER_INT64_MAX && (x) > MAX_DOUBLE_UNDER_INT64_MIN)
#endif

int main()
{
	double values[] =
	{
		-pow(2, 63) - 1025,
		-pow(2, 63),
		pow(2, 63) - 1024,
		pow(2, 63),
		INFINITY,
		-INFINITY,
		NAN
	};

	for (int i = 0; i < sizeof(values) / sizeof(values[0]); i++)
	{
		double value = values[i];
		printf("value == %lf, Overflow = %s\n", value,
			!IS_DOUBLE_SAFE_IN_INT64(value) ? "true" : "false");
	}
}