PR/56937 CVS commit: [netbsd-10] src

[gnats-admin%netbsd.org@localhost]

The following reply was made to PR lib/56937; it has been noted by GNATS.

From: "Martin Husemann" <martin%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/56937 CVS commit: [netbsd-10] src
Date: Thu, 22 Aug 2024 19:59:47 +0000

 Module Name:	src
 Committed By:	martin
 Date:		Thu Aug 22 19:59:46 UTC 2024
 
 Modified Files:
 	src/lib/libc/gdtoa [netbsd-10]: hdtoa.c
 	src/tests/lib/libc/stdio [netbsd-10]: t_printf.c
 
 Log Message:
 Pull up following revision(s) (requested by riastradh in ticket #787):
 
 	tests/lib/libc/stdio/t_printf.c: revision 1.17
 	tests/lib/libc/stdio/t_printf.c: revision 1.18
 	lib/libc/gdtoa/hdtoa.c: revision 1.13
 	tests/lib/libc/stdio/t_printf.c: revision 1.11
 	tests/lib/libc/stdio/t_printf.c: revision 1.12
 	tests/lib/libc/stdio/t_printf.c: revision 1.13
 	tests/lib/libc/stdio/t_printf.c: revision 1.14
 	tests/lib/libc/stdio/t_printf.c: revision 1.15
 	tests/lib/libc/stdio/t_printf.c: revision 1.16
 
 tests/lib/libc/stdio/t_printf: Add a couple simple %La tests.
 
 PR lib/56937: printf(3) long double %a formatting is broken
 tests/lib/libc/stdio/t_printf: Fix %La test.
 0xa.99ap+0 is closer to (long double)10.6 in x86 ld80 and in
 binary128 (and possibly more formats, haven't verified).
 tests/lib/libc/stdio/t_printf: Fix %a test the same way.
 tests/lib/libc/stdio/t_printf: Add another %La test.
 
 This one was adapted from the screw case shown in
 https://mail-index.netbsd.org/tech-userlevel/2020/04/11/msg012329.html
 which wasn't broken in our libc, but which nevertheless prompted us
 to commit a wrong and apparently untested patch that has rendered
 printf %La broken for the last four years, which is a little
 embarrassing.  (The part of that patch that led to a buffer overrun
 has been worked around, so now the output is just incorrect.)
 
 PR lib/56937: printf(3) long double %a formatting is broken
 
 Revert various broken changes to printf %La (hldtoa).
 This reverts:
 hdtoa.c 1.12 (PR/56247: Greg A. Woods: printf("%La", LDBL_MIN) dumps core)
 hdtoa.c 1.11 (fix tyop)
 hdtoa.c 1.10 (Via enh at google dot com in tech-userlevel. Fix handling of
     EXT_FRAC{H,L}BITS (although we don't need to since we don't have them).)
 
 The underlying motivation for this change was that when ld128 is
 decomposed into 4x32 words, this hldtoa logic is broken.
 
 But we don't decompose ld128 into 4x32 words; we decompose it into
 6x64 words.
 
 And the change, which was supposed to be a noop in our case of 2x64
 words (or similar for x87 80-bit floating-point), broke it to the
 point of causing buffer overruns (PR 56247) which when worked around
 led to just incorrect output output (PR 56937).
 If we want to make the #ifdefs for 4x32 words work, that's fine, but
 we absolutely must have automatic test cases to detect this kind of
 regression because %La formatting is extremely important for
 diagnosing details of floating-point data since it doesn't involve
 rounding in binary formats.  For now I've added some trivial tests;
 there is a more extensive test suite inside gdtoa that we need to
 wire up before anyone tries any other shenanigans in this code.
 
 PR lib/56937: printf(3) long double %a formatting is broken
 tests/lib/libc/stdio/t_printf: Fix another rounding error.
 Noted by kre.
 
 This doesn't break a passing test or fix a failed test, at least on
 x86 -- our printf produces `0x1.533p+3' for the double case and
 `0xa.99ap+0' for the long double case.  But of the hexadecimal number
 literals that that start with 0x5 having three hexadigits to the
 right of the fractional point, 0x5.4cdp+1 closest to the IEEE 754
 binary64, VAX D, x86 extended precision, and IEEE 754 binary128
 floating-point numbers closest to 10.6.
 
 The reason is that the number 10.6 (or the nearest floating-point
 number in any format with enough precision) is:
 101.0100 1100 1100|1100... * 2^1 = 0x5.4cc|c...p+1
 If we round at the vertical bar to the _nearest_ output with three
 hexadigits of precision, the result is:
 101.0100 1100 1101 * 2^1 = 0x5.4cdp+1
 tests/lib/libc/stdio/t_printf: Fix typo in ld128 case.
 printf %La does not write the `L' suffix.
 tests/lib/libc/stdio/t_printf: Fix sign error in ld128 case.
 
 Also link back to where the test case came from.
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.12 -r1.12.2.1 src/lib/libc/gdtoa/hdtoa.c
 cvs rdiff -u -r1.8.42.1 -r1.8.42.2 src/tests/lib/libc/stdio/t_printf.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.