diff mbox

Fix nextafter overflow in non-default rounding modes (bug 16677)

Message ID Pine.LNX.4.64.1403112058540.23789@digraph.polyomino.org.uk
State Committed
Headers show

Commit Message

Joseph Myers March 11, 2014, 8:59 p.m. UTC
ISO C requires the result of nextafter to be independent of the
rounding mode, even when underflow or overflow occurs.  This patch
fixes the bug in various nextafter implementations that, having done
an overflowing computation to force an overflow exception (correct),
they then return the result of that computation rather than an
infinity computed some other way (incorrect, when the overflowing
result of arithmetic with that sign and rounding mode is finite but
the correct result is infinite) - generally by falling through to
existing code to return a value that in fact is correct for this case
(but was computed by an integer increment and so without generating
the exceptions required).  Having fixed the bug, the previously
deferred conversion of nextafter testing in libm-test.inc to
ALL_RM_TEST is also included.

Tested x86_64 and x86; also spot-checked results of nextafter tests
for powerpc32 and mips64 to test the ldbl-128ibm and ldbl-128
changes.  (The m68k change is untested.)

2014-03-11  Joseph Myers  <joseph@codesourcery.com>

	[BZ #16677]
	* math/s_nextafter.c (__nextafter): Do not return value from
	overflowing computation.
	* sysdeps/i386/fpu/s_nextafterl.c (__nextafterl): Likewise.
	* sysdeps/ieee754/flt-32/s_nextafterf.c (__nextafterf): Likewise.
	* sysdeps/ieee754/ldbl-128/s_nextafterl.c (__nextafterl):
	Likewise.
	* sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c (__nextafterl):
	Likewise.
	* sysdeps/m68k/m680x0/fpu/s_nextafterl.c (__nextafterl): Likewise.
	* math/libm-test.inc (nextafter_test): Use ALL_RM_TEST.

Comments

Andreas Jaeger March 11, 2014, 9:21 p.m. UTC | #1
On 03/11/2014 09:59 PM, Joseph S. Myers wrote:
> ISO C requires the result of nextafter to be independent of the
> rounding mode, even when underflow or overflow occurs.  This patch
> fixes the bug in various nextafter implementations that, having done
> an overflowing computation to force an overflow exception (correct),
> they then return the result of that computation rather than an
> infinity computed some other way (incorrect, when the overflowing
> result of arithmetic with that sign and rounding mode is finite but
> the correct result is infinite) - generally by falling through to
> existing code to return a value that in fact is correct for this case
> (but was computed by an integer increment and so without generating
> the exceptions required).  Having fixed the bug, the previously
> deferred conversion of nextafter testing in libm-test.inc to
> ALL_RM_TEST is also included.

Thanks,
Andreas
diff mbox

Patch

diff --git a/math/libm-test.inc b/math/libm-test.inc
index 0fe0f69..574654e 100644
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -9553,10 +9553,7 @@  static const struct test_ff_f_data nextafter_test_data[] =
 static void
 nextafter_test (void)
 {
-
-  START (nextafter, 1);
-  RUN_TEST_LOOP_ff_f (nextafter, nextafter_test_data, );
-  END;
+  ALL_RM_TEST (nextafter, 1, nextafter_test_data, RUN_TEST_LOOP_ff_f, END);
 }
 
 
diff --git a/math/s_nextafter.c b/math/s_nextafter.c
index 7b026f0..28962e5 100644
--- a/math/s_nextafter.c
+++ b/math/s_nextafter.c
@@ -70,10 +70,8 @@  double __nextafter(double x, double y)
 	}
 	hy = hx&0x7ff00000;
 	if(hy>=0x7ff00000) {
-	  x = x+x;	/* overflow  */
-	  if (FLT_EVAL_METHOD != 0 && FLT_EVAL_METHOD != 1)
-	    asm ("" : "+m"(x));
-	  return x;	/* overflow  */
+	  double u = x+x;	/* overflow  */
+	  math_force_eval (u);
 	}
 	if(hy<0x00100000) {
 	    double u = x*x;			/* underflow */
diff --git a/sysdeps/i386/fpu/s_nextafterl.c b/sysdeps/i386/fpu/s_nextafterl.c
index bafe743..66d903f 100644
--- a/sysdeps/i386/fpu/s_nextafterl.c
+++ b/sysdeps/i386/fpu/s_nextafterl.c
@@ -106,7 +106,10 @@  long double __nextafterl(long double x, long double y)
 	    }
 	}
 	esy = esx&0x7fff;
-	if(esy==0x7fff) return x+x;	/* overflow  */
+	if(esy==0x7fff) {
+	    long double u = x + x;	/* overflow  */
+	    math_force_eval (u);
+	}
 	if(esy==0) {
 	    long double u = x*x;		/* underflow */
 	    math_force_eval (u);		/* raise underflow flag */
diff --git a/sysdeps/ieee754/flt-32/s_nextafterf.c b/sysdeps/ieee754/flt-32/s_nextafterf.c
index b0de3d9..22e0b3d 100644
--- a/sysdeps/ieee754/flt-32/s_nextafterf.c
+++ b/sysdeps/ieee754/flt-32/s_nextafterf.c
@@ -57,10 +57,8 @@  float __nextafterf(float x, float y)
 	}
 	hy = hx&0x7f800000;
 	if(hy>=0x7f800000) {
-	  x = x+x;	/* overflow  */
-	  if (FLT_EVAL_METHOD != 0)
-	    asm ("" : "+m"(x));
-	  return x;	/* overflow  */
+	  float u = x+x;	/* overflow  */
+	  math_force_eval (u);
 	}
 	if(hy<0x00800000) {
 	    float u = x*x;			/* underflow */
diff --git a/sysdeps/ieee754/ldbl-128/s_nextafterl.c b/sysdeps/ieee754/ldbl-128/s_nextafterl.c
index e345bc8..d5eaa1c 100644
--- a/sysdeps/ieee754/ldbl-128/s_nextafterl.c
+++ b/sysdeps/ieee754/ldbl-128/s_nextafterl.c
@@ -67,7 +67,10 @@  long double __nextafterl(long double x, long double y)
 	    }
 	}
 	hy = hx&0x7fff000000000000LL;
-	if(hy==0x7fff000000000000LL) return x+x;/* overflow  */
+	if(hy==0x7fff000000000000LL) {
+	    long double u = x + x;		/* overflow  */
+	    math_force_eval (u);
+	}
 	if(hy==0) {
 	    long double u = x*x;		/* underflow */
 	    math_force_eval (u);		/* raise underflow flag */
diff --git a/sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c b/sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c
index c050944..30b1540 100644
--- a/sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c
@@ -66,8 +66,11 @@  long double __nextafterl(long double x, long double y)
 	       long double with a 106 bit mantissa, and nextafterl
 	       is insane with variable precision.  So to make
 	       nextafterl sane we assume 106 bit precision.  */
-	    if((hx==0xffefffffffffffffLL)&&(lx==0xfc8ffffffffffffeLL))
-	      return x+x;	/* overflow, return -inf */
+	    if((hx==0xffefffffffffffffLL)&&(lx==0xfc8ffffffffffffeLL)) {
+	      u = x+x;	/* overflow, return -inf */
+	      math_force_eval (u);
+	      return y;
+	    }
 	    if (hx >= 0x7ff0000000000000LL) {
 	      u = 0x1.fffffffffffff7ffffffffffff8p+1023L;
 	      return u;
@@ -93,8 +96,11 @@  long double __nextafterl(long double x, long double y)
 	    }
 	    return x - u;
 	} else {				/* x < y, x += ulp */
-	    if((hx==0x7fefffffffffffffLL)&&(lx==0x7c8ffffffffffffeLL))
-	      return x+x;	/* overflow, return +inf */
+	    if((hx==0x7fefffffffffffffLL)&&(lx==0x7c8ffffffffffffeLL)) {
+	      u = x+x;	/* overflow, return +inf */
+	      math_force_eval (u);
+	      return y;
+	    }
 	    if ((uint64_t) hx >= 0xfff0000000000000ULL) {
 	      u = -0x1.fffffffffffff7ffffffffffff8p+1023L;
 	      return u;
diff --git a/sysdeps/m68k/m680x0/fpu/s_nextafterl.c b/sysdeps/m68k/m680x0/fpu/s_nextafterl.c
index 03c136b..ad77ca4 100644
--- a/sysdeps/m68k/m680x0/fpu/s_nextafterl.c
+++ b/sysdeps/m68k/m680x0/fpu/s_nextafterl.c
@@ -89,7 +89,10 @@  long double __nextafterl(long double x, long double y)
 	    }
 	}
 	esy = esx&0x7fff;
-	if(esy==0x7fff) return x+x;	/* overflow  */
+	if(esy==0x7fff) {
+	    long double u = x + x;	/* overflow  */
+	    math_force_eval (u);
+	}
 	if(esy==0 && (hx & 0x80000000) == 0) { /* underflow */
 	    y = x*x;
 	    math_force_eval (y);		/* raise underflow flag */