Fix exp() losing precision on 32-bit x86

Zend/zend_float.h

@@ -20,6 +20,8 @@
 
 #include "zend_portability.h"
 
+#include <math.h>
+
 BEGIN_EXTERN_C()
 
 /*
@@ -413,4 +415,22 @@ END_EXTERN_C()
 
 #endif /* FPU CONTROL */
 
+/* zend_init_fpu() clamps the x87 FPU to double precision (53-bit mantissa) for
+ * the whole request. libm's exp() relies on the FPU running at extended
+ * precision internally, so while that clamp is in place its result is off by
+ * one or more ULP compared to platforms whose FPU has no precision control.
+ * Restore extended precision for the call and truncate the result back to
+ * double. Compiles down to a plain exp() everywhere XPFPA_HAVE_CW is 0, which
+ * includes x86-64 and arm64. */
+static zend_always_inline double zend_exp(double num)
+{
+#if XPFPA_HAVE_CW
+	XPFPA_DECLARE
+	XPFPA_SWITCH_DOUBLE_EXTENDED();
+	XPFPA_RETURN_DOUBLE(exp(num));
+#else
+	return exp(num);
+#endif
+}
+
 #endif

ext/standard/math.c

@@ -667,7 +667,7 @@ PHP_FUNCTION(exp)
 		Z_PARAM_DOUBLE(num)
 	ZEND_PARSE_PARAMETERS_END();
 
-	RETURN_DOUBLE(exp(num));
+	RETURN_DOUBLE(zend_exp(num));
 }
 /* }}} */
 

ext/standard/tests/math/exp_fpu_precision.phpt

@@ -0,0 +1,48 @@
+--TEST--
+exp(): results must not lose precision when the FPU is clamped to double precision
+--INI--
+serialize_precision=-1
+--FILE--
+<?php
+/* On i386 the x87 FPU is the platform default and zend_init_fpu() clamps it to
+ * double precision (53-bit mantissa) for the whole request. libm's exp()
+ * relies on the FPU running at extended precision internally, so while that
+ * clamp is in place its result is off by one or more ULP compared to platforms
+ * whose FPU has no precision control.
+ *
+ * Each expected value below is the correctly rounded double, printed as the
+ * shortest decimal that round-trips back to it. */
+
+$x = 3;
+
+echo "-- runtime calls --\n";
+var_dump(exp($x));
+var_dump(exp($x + 2));
+var_dump(exp($x + 3));
+var_dump(exp($x + 4));
+var_dump(exp($x + 5));
+var_dump(exp($x + 6));
+var_dump(exp($x + 7));
+var_dump(exp($x + 9));
+var_dump(exp($x + 12));
+var_dump(exp($x + 17));
+
+echo "-- constant argument gives the same result --\n";
+var_dump(exp(10) === exp($x + 7));
+var_dump(exp(20) === exp($x + 17));
+?>
+--EXPECT--
+-- runtime calls --
+float(20.085536923187668)
+float(148.4131591025766)
+float(403.4287934927351)
+float(1096.6331584284585)
+float(2980.9579870417283)
+float(8103.083927575384)
+float(22026.465794806718)
+float(162754.79141900392)
+float(3269017.3724721107)
+float(485165195.4097903)
+-- constant argument gives the same result --
+bool(true)
+bool(true)