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Re: Another GLIBC build error with GCC6
- From: Paul Eggert <eggert at cs dot ucla dot edu>
- To: Roland McGrath <roland at hack dot frob dot com>, sellcey at imgtec dot com
- Cc: Jeff Law <law at redhat dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Tue, 21 Jul 2015 22:56:49 -0700
- Subject: Re: Another GLIBC build error with GCC6
- Authentication-results: sourceware.org; auth=none
- References: <1437496279 dot 19674 dot 126 dot camel at ubuntu-sellcey> <1437507995 dot 19674 dot 136 dot camel at ubuntu-sellcey> <55AEA71B dot 2020405 at redhat dot com> <1437509796 dot 19674 dot 138 dot camel at ubuntu-sellcey> <55AEB7FF dot 1000405 at redhat dot com> <1437518197 dot 19674 dot 152 dot camel at ubuntu-sellcey> <20150721224637 dot 1E0102C3B32 at topped-with-meat dot com>
Roland McGrath wrote:
the left shift doesn't even lose any bits, because the high
bit is already zero (and it's a constant, so the compiler knows that).
But it does lose a bit. It loses a zero bit, which can't be recovered the way
that a non-overflowing signed left shift is recovered by shifting right the same
amount.
that's the whole point of the thing: to sign-extend the 31-bit value
to 32 bits. I don't think it should complain about 0x7fffffff << 1.
Such complaints are useful for compilers that take advantage of C's overflow
rules to generate efficient-but-weird code when you violate the rules (something
that describes GCC more and more nowadays...).
Anyway, if I understand this one correctly it should be easy enough to fix
portably. Something like the attached patch, say. (I haven't tested it.)
>From 64586c5448f6f5537bd36bb5f271324790055c52 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Tue, 21 Jul 2015 22:50:29 -0700
Subject: [PATCH] Port the 0x7efe...feff pattern to GCC 6.
See Steve Ellcey's bug report in:
https://sourceware.org/ml/libc-alpha/2015-07/msg00673.html
* string/memrchr.c (MEMRCHR):
* string/rawmemchr.c (RAWMEMCHR):
* string/strchr.c (strchr):
* string/strchrnul.c (STRCHRNUL):
Rewrite code to avoid issues with signed shift overflow.
---
ChangeLog | 11 +++++++++++
string/memrchr.c | 11 ++---------
string/rawmemchr.c | 11 ++---------
string/strchr.c | 9 ++-------
string/strchrnul.c | 9 ++-------
5 files changed, 19 insertions(+), 32 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index 652d968..9f87cd9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+2015-07-21 Paul Eggert <eggert@cs.ucla.edu>
+
+ Port the 0x7efe...feff pattern to GCC 6.
+ See Steve Ellcey's bug report in:
+ https://sourceware.org/ml/libc-alpha/2015-07/msg00673.html
+ * string/memrchr.c (MEMRCHR):
+ * string/rawmemchr.c (RAWMEMCHR):
+ * string/strchr.c (strchr):
+ * string/strchrnul.c (STRCHRNUL):
+ Rewrite code to avoid issues with signed shift overflow.
+
2015-07-22 Mike Frysinger <vapier@gentoo.org>
* sysdeps/unix/sysv/linux/ia64/bits/msq.h: Change sys/types.h include
diff --git a/string/memrchr.c b/string/memrchr.c
index 0c8fd84..86cd5b9 100644
--- a/string/memrchr.c
+++ b/string/memrchr.c
@@ -96,15 +96,8 @@ MEMRCHR
The 1-bits make sure that carries propagate to the next 0-bit.
The 0-bits provide holes for carries to fall into. */
-
- if (sizeof (longword) != 4 && sizeof (longword) != 8)
- abort ();
-
-#if LONG_MAX <= LONG_MAX_32_BITS
- magic_bits = 0x7efefeff;
-#else
- magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
-#endif
+ magic_bits = -1;
+ magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
/* Set up a longword, each of whose bytes is C. */
charmask = c | (c << 8);
diff --git a/string/rawmemchr.c b/string/rawmemchr.c
index 05b22be..228ca9d 100644
--- a/string/rawmemchr.c
+++ b/string/rawmemchr.c
@@ -86,15 +86,8 @@ RAWMEMCHR (s, c_in)
The 1-bits make sure that carries propagate to the next 0-bit.
The 0-bits provide holes for carries to fall into. */
-
- if (sizeof (longword) != 4 && sizeof (longword) != 8)
- abort ();
-
-#if LONG_MAX <= LONG_MAX_32_BITS
- magic_bits = 0x7efefeff;
-#else
- magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
-#endif
+ magic_bits = -1;
+ magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
/* Set up a longword, each of whose bytes is C. */
charmask = c | (c << 8);
diff --git a/string/strchr.c b/string/strchr.c
index 5f90075..f13b2b3 100644
--- a/string/strchr.c
+++ b/string/strchr.c
@@ -60,13 +60,8 @@ strchr (const char *s, int c_in)
The 1-bits make sure that carries propagate to the next 0-bit.
The 0-bits provide holes for carries to fall into. */
- switch (sizeof (longword))
- {
- case 4: magic_bits = 0x7efefeffL; break;
- case 8: magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL; break;
- default:
- abort ();
- }
+ magic_bits = -1;
+ magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
/* Set up a longword, each of whose bytes is C. */
charmask = c | (c << 8);
diff --git a/string/strchrnul.c b/string/strchrnul.c
index 2678f1d..daf0b3f 100644
--- a/string/strchrnul.c
+++ b/string/strchrnul.c
@@ -66,13 +66,8 @@ STRCHRNUL (s, c_in)
The 1-bits make sure that carries propagate to the next 0-bit.
The 0-bits provide holes for carries to fall into. */
- switch (sizeof (longword))
- {
- case 4: magic_bits = 0x7efefeffL; break;
- case 8: magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL; break;
- default:
- abort ();
- }
+ magic_bits = -1;
+ magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
/* Set up a longword, each of whose bytes is C. */
charmask = c | (c << 8);
--
2.1.0