Reimplement php_round_helper() using modf()

NEWS

@@ -10,5 +10,7 @@ DOM:
 
 Standard:
   . Implement GH-12188 (Indication for the int size in phpinfo()). (timwolla)
+  . Partly fix GH-12143 (Incorrect round() result for 0.49999999999999994).
+    (timwolla)
 
 <<< NOTE: Insert NEWS from last stable release here prior to actual release! >>>

UPGRADING

@@ -50,6 +50,15 @@ PHP 8.4 UPGRADE NOTES
 5. Changed Functions
 ========================================
 
+- Standard:
+  . The internal implementation for rounding to integers has been rewritten
+    to be easier to verify for correctness and to be easier to maintain.
+    Some rounding bugs have been fixed as a result of the rewrite. For
+    example previously rounding 0.49999999999999994 to the nearest integer
+    would have resulted in 1.0 instead of the correct result 0.0. Additional
+    inputs might also be affected and result in different outputs compared to
+    earlier PHP versions.
+
 ========================================
 6. New Functions
 ========================================

ext/standard/math.c

@@ -93,27 +93,73 @@ static inline double php_intpow10(int power) {
 /* {{{ php_round_helper
        Actually performs the rounding of a value to integer in a certain mode */
 static inline double php_round_helper(double value, int mode) {
-	double tmp_value;
+	double integral, fractional;
+
+	/* Split the input value into the integral and fractional part.
+	 *
+	 * Both parts will have the same sign as the input value. We take
+	 * the absolute value of the fractional part (which will not result
+	 * in branches in the assembly) to make the following cases simpler.
+	 */
+	fractional = fabs(modf(value, &integral));
+
+	switch (mode) {
+		case PHP_ROUND_HALF_UP:
+			if (fractional >= 0.5) {
+				/* We must increase the magnitude of the integral part
+				 * (rounding up / towards infinity). copysign(1.0, integral)
+				 * will either result in 1.0 or -1.0 depending on the sign
+				 * of the input, thus increasing the magnitude, but without
+				 * generating branches in the assembly.
+				 *
+				 * This pattern is equally used for all the other modes.
+				 */
+				return integral + copysign(1.0, integral);
+			}
 
-	if (value >= 0.0) {
-		tmp_value = floor(value + 0.5);
-		if ((mode == PHP_ROUND_HALF_DOWN && value == (-0.5 + tmp_value)) ||
-			(mode == PHP_ROUND_HALF_EVEN && value == (0.5 + 2 * floor(tmp_value/2.0))) ||
-			(mode == PHP_ROUND_HALF_ODD  && value == (0.5 + 2 * floor(tmp_value/2.0) - 1.0)))
-		{
-			tmp_value = tmp_value - 1.0;
-		}
-	} else {
-		tmp_value = ceil(value - 0.5);
-		if ((mode == PHP_ROUND_HALF_DOWN && value == (0.5 + tmp_value)) ||
-			(mode == PHP_ROUND_HALF_EVEN && value == (-0.5 + 2 * ceil(tmp_value/2.0))) ||
-			(mode == PHP_ROUND_HALF_ODD  && value == (-0.5 + 2 * ceil(tmp_value/2.0) + 1.0)))
-		{
-			tmp_value = tmp_value + 1.0;
-		}
+			return integral;
+
+		case PHP_ROUND_HALF_DOWN:
+			if (fractional > 0.5) {
+				return integral + copysign(1.0, integral);
+			}
+
+			return integral;
+
+		case PHP_ROUND_HALF_EVEN:
+			if (fractional > 0.5) {
+				return integral + copysign(1.0, integral);
+			}
+
+			if (fractional == 0.5) {
+				bool even = !fmod(integral, 2.0);
+
+				/* If the integral part is not even we can make it even
+				 * by adding one in the direction of the existing sign.
+				 */
+				if (!even) {
+					return integral + copysign(1.0, integral);
+				}
+			}
+
+			return integral;
+		case PHP_ROUND_HALF_ODD:
+			if (fractional > 0.5) {
+				return integral + copysign(1.0, integral);
+			}
+
+			if (fractional == 0.5) {
+				bool even = !fmod(integral, 2.0);
+
+				if (even) {
+					return integral + copysign(1.0, integral);
+				}
+			}
+
+			return integral;
 	}
 
-	return tmp_value;
+	ZEND_UNREACHABLE();
 }
 /* }}} */
 

ext/standard/tests/math/round_gh12143_1.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-12143: Test rounding of 0.49999999999999994.
+--FILE--
+<?php
+foreach ([
+    0.49999999999999994,
+    -0.49999999999999994,
+] as $number) {
+    foreach ([
+        'PHP_ROUND_HALF_UP',
+        'PHP_ROUND_HALF_DOWN',
+        'PHP_ROUND_HALF_EVEN',
+        'PHP_ROUND_HALF_ODD',
+    ] as $mode) {
+        printf("%-20s: %+.17g -> %+.17g\n", $mode, $number, round($number, 0, constant($mode)));
+    }
+}
+?>
+--EXPECT--
+PHP_ROUND_HALF_UP   : +0.49999999999999994 -> +0
+PHP_ROUND_HALF_DOWN : +0.49999999999999994 -> +0
+PHP_ROUND_HALF_EVEN : +0.49999999999999994 -> +0
+PHP_ROUND_HALF_ODD  : +0.49999999999999994 -> +0
+PHP_ROUND_HALF_UP   : -0.49999999999999994 -> -0
+PHP_ROUND_HALF_DOWN : -0.49999999999999994 -> -0
+PHP_ROUND_HALF_EVEN : -0.49999999999999994 -> -0
+PHP_ROUND_HALF_ODD  : -0.49999999999999994 -> -0

ext/standard/tests/math/round_gh12143_2.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-12143: Test rounding of 0.50000000000000011.
+--FILE--
+<?php
+foreach ([
+    0.50000000000000011,
+    -0.50000000000000011,
+] as $number) {
+    foreach ([
+        'PHP_ROUND_HALF_UP',
+        'PHP_ROUND_HALF_DOWN',
+        'PHP_ROUND_HALF_EVEN',
+        'PHP_ROUND_HALF_ODD',
+    ] as $mode) {
+        printf("%-20s: %+.17g -> %+.17g\n", $mode, $number, round($number, 0, constant($mode)));
+    }
+}
+?>
+--EXPECT--
+PHP_ROUND_HALF_UP   : +0.50000000000000011 -> +1
+PHP_ROUND_HALF_DOWN : +0.50000000000000011 -> +1
+PHP_ROUND_HALF_EVEN : +0.50000000000000011 -> +1
+PHP_ROUND_HALF_ODD  : +0.50000000000000011 -> +1
+PHP_ROUND_HALF_UP   : -0.50000000000000011 -> -1
+PHP_ROUND_HALF_DOWN : -0.50000000000000011 -> -1
+PHP_ROUND_HALF_EVEN : -0.50000000000000011 -> -1
+PHP_ROUND_HALF_ODD  : -0.50000000000000011 -> -1

ext/standard/tests/math/round_gh12143_3.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-12143: Test rounding of 0.5.
+--FILE--
+<?php
+foreach ([
+    0.5,
+    -0.5,
+] as $number) {
+    foreach ([
+        'PHP_ROUND_HALF_UP',
+        'PHP_ROUND_HALF_DOWN',
+        'PHP_ROUND_HALF_EVEN',
+        'PHP_ROUND_HALF_ODD',
+    ] as $mode) {
+        printf("%-20s: %+.17g -> %+.17g\n", $mode, $number, round($number, 0, constant($mode)));
+    }
+}
+?>
+--EXPECT--
+PHP_ROUND_HALF_UP   : +0.5 -> +1
+PHP_ROUND_HALF_DOWN : +0.5 -> +0
+PHP_ROUND_HALF_EVEN : +0.5 -> +0
+PHP_ROUND_HALF_ODD  : +0.5 -> +1
+PHP_ROUND_HALF_UP   : -0.5 -> -1
+PHP_ROUND_HALF_DOWN : -0.5 -> -0
+PHP_ROUND_HALF_EVEN : -0.5 -> -0
+PHP_ROUND_HALF_ODD  : -0.5 -> -1

ext/standard/tests/math/round_gh12143_4.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-12143: Test rounding of 1.5.
+--FILE--
+<?php
+foreach ([
+    1.5,
+    -1.5,
+] as $number) {
+    foreach ([
+        'PHP_ROUND_HALF_UP',
+        'PHP_ROUND_HALF_DOWN',
+        'PHP_ROUND_HALF_EVEN',
+        'PHP_ROUND_HALF_ODD',
+    ] as $mode) {
+        printf("%-20s: %+.17g -> %+.17g\n", $mode, $number, round($number, 0, constant($mode)));
+    }
+}
+?>
+--EXPECT--
+PHP_ROUND_HALF_UP   : +1.5 -> +2
+PHP_ROUND_HALF_DOWN : +1.5 -> +1
+PHP_ROUND_HALF_EVEN : +1.5 -> +2
+PHP_ROUND_HALF_ODD  : +1.5 -> +1
+PHP_ROUND_HALF_UP   : -1.5 -> -2
+PHP_ROUND_HALF_DOWN : -1.5 -> -1
+PHP_ROUND_HALF_EVEN : -1.5 -> -2
+PHP_ROUND_HALF_ODD  : -1.5 -> -1