diff mbox

Fix x86_64 / x86 expm1l (-min_subnorm) result sign (bug 18569) [committed]

Message ID alpine.DEB.2.10.1506211843310.9955@digraph.polyomino.org.uk
State New
Headers show

Commit Message

Joseph Myers June 21, 2015, 6:43 p.m. UTC 
In the x86 / x86_64 implementations of expm1l, when expm1l's result
should underflow to 0 (argument minus the least subnormal, in some
rounding modes), it can be a zero of the wrong sign.  This patch fixes
this by returning the argument with underflow forced in that case
(this is a 1ulp error relative to the correctly rounded result of -0,
which is OK in terms of the documented accuracy goals, whereas a
result with the wrong sign never is).

Tested for x86_64 and x86.  Committed.

(auto-libm-test-out changes omitted below.)

2015-06-21  Joseph Myers  <joseph@codesourcery.com>

	[BZ #18569]
	* sysdeps/i386/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]: Force
	underflow and return argument in case of subnormal argument.
	* sysdeps/x86_64/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]:
	Likewise.
	* math/auto-libm-test-in: Add more tests of expm1.
	* math/auto-libm-test-out: Regenerated.
diff mbox

Patch

diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in
index e8e4da6..6b05459 100644
--- a/math/auto-libm-test-in
+++ b/math/auto-libm-test-in
@@ -1364,6 +1364,10 @@  expm1 0x5.fb8dc64e91a74p-4
 expm1 0x3.735f497c4e67535cp-4
 # Bug 16353: underflow exception may be missing
 expm1 0x4.0000000000000028p-16384 missing-underflow
+expm1 min missing-underflow
+expm1 -min missing-underflow
+expm1 min_subnorm missing-underflow
+expm1 -min_subnorm missing-underflow
 
 fma 1.0 2.0 3.0
 fma 1.25 0.75 0.0625
diff --git a/sysdeps/i386/fpu/e_expl.S b/sysdeps/i386/fpu/e_expl.S
index 74968bb..711c86a 100644
--- a/sysdeps/i386/fpu/e_expl.S
+++ b/sysdeps/i386/fpu/e_expl.S
@@ -122,6 +122,12 @@  ENTRY(IEEE754_EXPL)
 	   normal.  */
 	cmpl	$0x0001, %eax
 	jge	2f
+	/* Force underflow and return the argument, to avoid wrong signs
+	   of zero results from the code below in some rounding modes.  */
+	fld	%st
+	fmul	%st
+	fstp	%st
+	jmp	2f
 #else
 	movzwl	4+8(%esp), %eax
 	andl	$0x7fff, %eax
diff --git a/sysdeps/x86_64/fpu/e_expl.S b/sysdeps/x86_64/fpu/e_expl.S
index 866bad2..14dd29d 100644
--- a/sysdeps/x86_64/fpu/e_expl.S
+++ b/sysdeps/x86_64/fpu/e_expl.S
@@ -119,6 +119,12 @@  ENTRY(IEEE754_EXPL)
 	   normal.  */
 	cmpl	$0x0001, %eax
 	jge	2f
+	/* Force underflow and return the argument, to avoid wrong signs
+	   of zero results from the code below in some rounding modes.  */
+	fld	%st
+	fmul	%st
+	fstp	%st
+	jmp	2f
 #else
 	movzwl	8+8(%rsp), %eax
 	andl	$0x7fff, %eax