Added edge case generation logic for cases where the precision is so fine that adding 0.5 does not change the value.

Author: SakiTakamachi

ext/standard/math.c

@@ -51,9 +51,21 @@ static inline double php_intpow10(int power) {
 
 static zend_always_inline double php_round_get_basic_edge_case(double integral, double exponent, int places)
 {
-	return (places > 0)
-		? fabs((integral + copysign(0.5, integral)) / exponent)
-		: fabs((integral + copysign(0.5, integral)) * exponent);
+	/* Integer + `0.5` can be represented exactly in a double, so in most cases, there is no rounding error at this point. */
+	double edge_case = integral + copysign(0.5, integral);
+
+	/* When the calculation precision is as high as `1e16`, adding `0.5` may be lost due to rounding.
+	 * In such cases, the exponent is applied individually before performing the addition.
+	 * This approach is not used all the time in order to avoid introducing rounding errors when adding `0.5`.
+	 * It is only used when the precision is so fine that adding `0.5` would have no meaningful effect. */
+	if (UNEXPECTED(integral == edge_case)) {
+		return (places > 0)
+			? fabs((integral / exponent) + (copysign(0.5, integral) / exponent))
+			: fabs((integral * exponent) + (copysign(0.5, integral) * exponent));
+	}
+
+	/* Returns a value adjusted with the exponent so that it can be compared with the input. */
+	return (places > 0) ? fabs(edge_case / exponent) : fabs(edge_case * exponent);
 }
 
 static zend_always_inline double php_round_get_zero_edge_case(double integral, double exponent, int places)

ext/standard/tests/math/round_gh12143_expand_rounding_target.phpt

@@ -13,6 +13,9 @@ $testCases = [
         [-12345678901234565, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
     'PHP_ROUND_HALF_DOWN' => [
         [0.12345678901234565, 16],
@@ -21,6 +24,9 @@ $testCases = [
         [-12345678901234565, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
     'PHP_ROUND_HALF_EVEN' => [
         [0.12345678901234565, 16],
@@ -29,6 +35,9 @@ $testCases = [
         [-12345678901234565, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
     'PHP_ROUND_HALF_ODD' => [
         [0.12345678901234565, 16],
@@ -37,6 +46,9 @@ $testCases = [
         [-12345678901234565, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
     'RoundingMode::AwayFromZero' => [
         [0.12345678901234560, 16],
@@ -45,6 +57,9 @@ $testCases = [
         [-12345678901234567, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
     'RoundingMode::TowardsZero' => [
         [0.12345678901234566, 16],
@@ -53,6 +68,9 @@ $testCases = [
         [-12345678901234565, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
     'RoundingMode::PositiveInfinity' => [
         [0.12345678901234560, 16],
@@ -61,6 +79,9 @@ $testCases = [
         [-12345678901234564, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
     'RoundingMode::NegativeInfinity' => [
         [0.12345678901234560, 16],
@@ -69,6 +90,9 @@ $testCases = [
         [-12345678901234564, -1],
         [4503599627370495.5, 0],
         [-4503599627370495.5, 0],
+        [5.0, 15],
+        [5000000000000000.0, 0],
+        [5000000000000000.5, 0],
     ],
 ];
 
@@ -88,6 +112,9 @@ float(12345678901234570)
 float(-12345678901234570)
 float(4503599627370496)
 float(-4503599627370496)
+float(5)
+float(5000000000000001)
+float(5000000000000001)
 
 ========== PHP_ROUND_HALF_DOWN ==========
 float(0.1234567890123456)
@@ -96,6 +123,9 @@ float(12345678901234560)
 float(-12345678901234560)
 float(4503599627370495)
 float(-4503599627370495)
+float(5)
+float(5000000000000000)
+float(5000000000000000)
 
 ========== PHP_ROUND_HALF_EVEN ==========
 float(0.1234567890123456)
@@ -104,6 +134,9 @@ float(12345678901234560)
 float(-12345678901234560)
 float(4503599627370496)
 float(-4503599627370496)
+float(5)
+float(5000000000000000)
+float(5000000000000000)
 
 ========== PHP_ROUND_HALF_ODD ==========
 float(0.1234567890123457)
@@ -112,6 +145,9 @@ float(12345678901234570)
 float(-12345678901234570)
 float(4503599627370495)
 float(-4503599627370495)
+float(5)
+float(5000000000000001)
+float(5000000000000001)
 
 ========== RoundingMode::AwayFromZero ==========
 float(0.1234567890123456)
@@ -120,6 +156,9 @@ float(12345678901234570)
 float(-12345678901234570)
 float(4503599627370496)
 float(-4503599627370496)
+float(5)
+float(5000000000000000)
+float(5000000000000000)
 
 ========== RoundingMode::TowardsZero ==========
 float(0.1234567890123456)
@@ -128,6 +167,9 @@ float(12345678901234560)
 float(-12345678901234560)
 float(4503599627370495)
 float(-4503599627370495)
+float(5)
+float(5000000000000000)
+float(5000000000000000)
 
 ========== RoundingMode::PositiveInfinity ==========
 float(0.1234567890123456)
@@ -136,6 +178,9 @@ float(12345678901234570)
 float(-12345678901234560)
 float(4503599627370496)
 float(-4503599627370495)
+float(5)
+float(5000000000000000)
+float(5000000000000000)
 
 ========== RoundingMode::NegativeInfinity ==========
 float(0.1234567890123456)
@@ -144,3 +189,6 @@ float(12345678901234560)
 float(-12345678901234570)
 float(4503599627370495)
 float(-4503599627370496)
+float(5)
+float(5000000000000000)
+float(5000000000000000)