NetBSD Problem Report #58236

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Date: Wed,  8 May 2024 16:14:22 +0000 (UTC)
From: campbell+netbsd@mumble.net
Reply-To: campbell+netbsd@mumble.net
To: gnats-bugs@NetBSD.org
Subject: nexttoward(3) is broken on subnormals
X-Send-Pr-Version: www-1.0

>Number:         58236
>Category:       lib
>Synopsis:       nexttoward(3) is broken on subnormals
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    lib-bug-people
>State:          closed
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed May 08 16:15:01 +0000 2024
>Closed-Date:    Sun Oct 13 20:26:42 +0000 2024
>Last-Modified:  Sun Oct 13 20:26:42 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, ...
>Organization:
The nexttowarbsd(3) Foundation
>Environment:
>Description:
nexttoward(k*n, INFINITY) should return (k + 1)*n, where n is the smallest positive subnormal and k is any integer below 2^p.  But instead it returns (k - 1)*n.

Similarly, nexttoward(k*n, -INFINITY) should return (k - 1)*n, but it returns (k + 1)*n.
>How-To-Repeat:
$ cat test.c
#include <math.h>
#include <stdio.h>

int
main(void)
{
	volatile double n = __DBL_DENORM_MIN__;
	volatile double d = INFINITY;
	printf("123*n goes up to %.0f*n\n", nextafter(123*n, d)/n);
	printf("123*n goes down to %.0f*n\n", nextafter(123*n, -d)/n);
	fflush(stdout);
	return ferror(stdout);
}
$ make test LDLIBS=-lm
cc -O2   -o test test.c -lm
$ ./test
123*n goes up to 124*n
123*n goes down to 122*n

>Fix:
Yes, please!

>Release-Note:

>Audit-Trail:
From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/58236 CVS commit: src/tests/lib/libm
Date: Wed, 8 May 2024 17:27:04 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Wed May  8 17:27:04 UTC 2024

 Modified Files:
 	src/tests/lib/libm: t_next.c

 Log Message:
 tests/lib/libm/t_next: Expand substantially.

 This covers many more potential problem areas -- and includes a new
 xfail test for PR lib/58236: nexttoward(3) is broken on subnormals.


 To generate a diff of this commit:
 cvs rdiff -u -r1.3 -r1.4 src/tests/lib/libm/t_next.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/58236 CVS commit: src
Date: Sat, 11 May 2024 02:07:54 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Sat May 11 02:07:54 UTC 2024

 Modified Files:
 	src/lib/libm/src: s_nexttoward.c
 	src/tests/lib/libm: t_next.c

 Log Message:
 nexttoward(3): Fix high-word test on small positive subnormals.

 By this point in the logic, x can't be zero, so it's either positive
 or negative.

 The high word hx, however, can be zero, when x is a small positive
 subnormal.  This means x is a small positive subnormal, so if x > y
 we are computing nextDown, and if x < y we are computing nextUp.

 hx is a (signed 32-bit) integer, not a double floating-point number,
 so it's a little silly to compare hx > 0.0.  But that on its own
 isn't enough to trigger the bug because all signed 32-bit integers
 can be represented by double on all NetBSD architectures.

 PR lib/58236


 To generate a diff of this commit:
 cvs rdiff -u -r1.2 -r1.3 src/lib/libm/src/s_nexttoward.c
 cvs rdiff -u -r1.5 -r1.6 src/tests/lib/libm/t_next.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

State-Changed-From-To: open->needs-pullups
State-Changed-By: riastradh@NetBSD.org
State-Changed-When: Tue, 23 Jul 2024 21:17:45 +0000
State-Changed-Why:
fixed in HEAD, needs pullup-10 and pullup-9


State-Changed-From-To: needs-pullups->pending-pullups
State-Changed-By: riastradh@NetBSD.org
State-Changed-When: Sun, 13 Oct 2024 00:57:20 +0000
State-Changed-Why:
pullup-10 #963 https://releng.netbsd.org/cgi-bin/req-10.cgi?show=963
pullup-9 #1906 https://releng.netbsd.org/cgi-bin/req-9.cgi?show=1906


From: "Martin Husemann" <martin@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/58236 CVS commit: [netbsd-10] src
Date: Sun, 13 Oct 2024 15:05:18 +0000

 Module Name:	src
 Committed By:	martin
 Date:		Sun Oct 13 15:05:18 UTC 2024

 Modified Files:
 	src/distrib/sets/lists/debug [netbsd-10]: mi
 	src/distrib/sets/lists/tests [netbsd-10]: mi
 	src/lib/libm/src [netbsd-10]: s_nexttoward.c
 	src/tests/lib/libm [netbsd-10]: Makefile
 Added Files:
 	src/tests/lib/libm [netbsd-10]: t_next.c

 Log Message:
 Pull up following revision(s) (requested by riastradh in ticket #963):

 	tests/lib/libm/Makefile: revision 1.49
 	distrib/sets/lists/tests/mi: revision 1.1315
 	tests/lib/libm/t_next.c: revision 1.1
 	tests/lib/libm/t_next.c: revision 1.2
 	distrib/sets/lists/debug/mi: revision 1.435
 	tests/lib/libm/t_next.c: revision 1.3
 	tests/lib/libm/t_next.c: revision 1.4
 	tests/lib/libm/t_next.c: revision 1.5
 	tests/lib/libm/t_next.c: revision 1.6
 	lib/libm/src/s_nexttoward.c: revision 1.3

 tests/lib/libm: Test nextafter/nexttoward and variants.

 The tests are fairly trivial but should work without any conditionals
 about floating-point formats.
 tests/lib/libm/t_next: Disable this test on VAX.

 But leave a replacement xfail test that fails unconditionally, to
 leave a reminder in the tests of PR 57881: vax libm is missing
 various symbols.

 tests/lib/libm/t_next: Fix stub on VAX.
 Tested building the wrong tree, oops.

 tests/lib/libm/t_next: Expand substantially.

 This covers many more potential problem areas -- and includes a new
 xfail test for PR lib/58236: nexttoward(3) is broken on subnormals.
 tests/lib/libm/t_next: nexttoward works if it's just nextafter.

 It's broken on platforms where long double and double aren't the same
 and nexttoward isn't an alias for nextafter.
 nexttoward(3): Fix high-word test on small positive subnormals.

 By this point in the logic, x can't be zero, so it's either positive
 or negative.

 The high word hx, however, can be zero, when x is a small positive
 subnormal.  This means x is a small positive subnormal, so if x > y
 we are computing nextDown, and if x < y we are computing nextUp.
 hx is a (signed 32-bit) integer, not a double floating-point number,
 so it's a little silly to compare hx > 0.0.  But that on its own
 isn't enough to trigger the bug because all signed 32-bit integers
 can be represented by double on all NetBSD architectures.
 PR lib/58236


 To generate a diff of this commit:
 cvs rdiff -u -r1.394.2.12 -r1.394.2.13 src/distrib/sets/lists/debug/mi
 cvs rdiff -u -r1.1238.2.14 -r1.1238.2.15 src/distrib/sets/lists/tests/mi
 cvs rdiff -u -r1.2 -r1.2.36.1 src/lib/libm/src/s_nexttoward.c
 cvs rdiff -u -r1.48.2.2 -r1.48.2.3 src/tests/lib/libm/Makefile
 cvs rdiff -u -r0 -r1.7.4.2 src/tests/lib/libm/t_next.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

From: "Martin Husemann" <martin@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/58236 CVS commit: [netbsd-9] src
Date: Sun, 13 Oct 2024 15:09:58 +0000

 Module Name:	src
 Committed By:	martin
 Date:		Sun Oct 13 15:09:58 UTC 2024

 Modified Files:
 	src/distrib/sets/lists/debug [netbsd-9]: mi
 	src/distrib/sets/lists/tests [netbsd-9]: mi
 	src/lib/libm/src [netbsd-9]: s_nexttoward.c
 	src/tests/lib/libm [netbsd-9]: Makefile
 Added Files:
 	src/tests/lib/libm [netbsd-9]: t_next.c

 Log Message:
 Pull up following revision(s) (requested by riastradh in ticket #963):

 	tests/lib/libm/Makefile: revision 1.49
 	distrib/sets/lists/tests/mi: revision 1.1315
 	tests/lib/libm/t_next.c: revision 1.1
 	tests/lib/libm/t_next.c: revision 1.2
 	distrib/sets/lists/debug/mi: revision 1.435
 	tests/lib/libm/t_next.c: revision 1.3
 	tests/lib/libm/t_next.c: revision 1.4
 	tests/lib/libm/t_next.c: revision 1.5
 	tests/lib/libm/t_next.c: revision 1.6
 	lib/libm/src/s_nexttoward.c: revision 1.3
 	(all via patch)

 tests/lib/libm: Test nextafter/nexttoward and variants.

 The tests are fairly trivial but should work without any conditionals
 about floating-point formats.
 tests/lib/libm/t_next: Disable this test on VAX.

 But leave a replacement xfail test that fails unconditionally, to
 leave a reminder in the tests of PR 57881: vax libm is missing
 various symbols.

 tests/lib/libm/t_next: Fix stub on VAX.
 Tested building the wrong tree, oops.

 tests/lib/libm/t_next: Expand substantially.

 This covers many more potential problem areas -- and includes a new
 xfail test for PR lib/58236: nexttoward(3) is broken on subnormals.
 tests/lib/libm/t_next: nexttoward works if it's just nextafter.

 It's broken on platforms where long double and double aren't the same
 and nexttoward isn't an alias for nextafter.
 nexttoward(3): Fix high-word test on small positive subnormals.

 By this point in the logic, x can't be zero, so it's either positive
 or negative.

 The high word hx, however, can be zero, when x is a small positive
 subnormal.  This means x is a small positive subnormal, so if x > y
 we are computing nextDown, and if x < y we are computing nextUp.
 hx is a (signed 32-bit) integer, not a double floating-point number,
 so it's a little silly to compare hx > 0.0.  But that on its own
 isn't enough to trigger the bug because all signed 32-bit integers
 can be represented by double on all NetBSD architectures.
 PR lib/58236


 To generate a diff of this commit:
 cvs rdiff -u -r1.285.2.9 -r1.285.2.10 src/distrib/sets/lists/debug/mi
 cvs rdiff -u -r1.818.2.7 -r1.818.2.8 src/distrib/sets/lists/tests/mi
 cvs rdiff -u -r1.2 -r1.2.28.1 src/lib/libm/src/s_nexttoward.c
 cvs rdiff -u -r1.46 -r1.46.2.1 src/tests/lib/libm/Makefile
 cvs rdiff -u -r0 -r1.7.6.2 src/tests/lib/libm/t_next.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

State-Changed-From-To: pending-pullups->closed
State-Changed-By: riastradh@NetBSD.org
State-Changed-When: Sun, 13 Oct 2024 20:26:42 +0000
State-Changed-Why:
fixed in HEAD, pulled up to 9 and 10


>Unformatted:

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