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authorJoseph Myers <joseph@codesourcery.com>2018-02-06 21:55:08 +0000
committerJoseph Myers <joseph@codesourcery.com>2018-02-06 21:55:08 +0000
commit0d40d0ecba3b1e5b8c3b8da01c708fea3948e193 (patch)
treeb50f3b8f023316f12f2352ff93053dd3b1a6e8d2 /string
parent383e87c96b9eee3b4b2d78dbeeaa8e2c0db35feb (diff)
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Unify and simplify bits/byteswap.h, bits/byteswap-16.h headers (bug 14508, bug 15512, bug 17082, bug 20530).
We have a general principle of preferring optimizations for library facilities to use compiler built-in functions rather than being located in library headers, where the compiler can reasonably optimize code without needing to know glibc implementation details. This patch applies this principle to bits/byteswap.h, eliminating all the architecture-specific variants and bits/byteswap-16.h. The __bswap_16, __bswap_32 and __bswap_64 interfaces all become inline functions, never macros, using the GCC built-in functions where available and otherwise a single architecture-independent definition using shifts and masking (which compilers may well be able to detect and optimize; GCC has detection of various byte-swapping idioms). The __bswap_constant_32 macro needs to stay around because of uses in static initializers within glibc and its tests, and so for consistency all __bswap_constant_* are kept rather than just being inlined into the old-GCC-or-non-GCC parts of the __bswap_* inline function definitions. Various open bugs are addressed by this cleanup, with caveats about exactly what is covered by those bugs and when the bugs applied at all. Bug 14508 reports -Wformat warnings building glibc because __bswap_* sometimes returned the wrong types. Obviously we already don't have such warnings any more or the build would be failing, given -Werror, and I suspect that bug was originally for wrong types for x86_64, as fixed by commit d394eb742a3565d7fe7a4b02710a60b5f219ee64 (glibc 2.17). The only case I saw removed by this patch where the types would still have been wrong was the non-__GNUC__ case of __bswap_64 in the s390 header (using unsigned long long int, but uint64_t would be unsigned long int for 64-bit). In any case, the single header consistently uses __uintN_t types after this patch, thereby eliminating all such bugs. The existing string/test-endian-types.c test already suffices to verify that the types are correct with the compiler used to build glibc and its tests. Bug 15512 reports an error from __bswap_constant_16 with -Werror -Wsign-conversion. I am unable to reproduce this with any GCC version supporting -Wsign-conversion - all seem to be able to avoid warning for ((x) >> 8) & 0xffu, where x is uint16_t, which while it formally does involve an implicit conversion from int to unsigned int, is also a case where it should be easy for the compiler to see that the value converted is never negative. But in this patch __bswap_constant_16 is changed to use signed 0xff so that no such implicit conversion occurs at all, and a test with -Werror -Wsign-conversion is added. Bug 17082 objects to the use of ({}) statement expressions in these macros preventing use at file scope (in C, that's in sizeof etc.; in C++, more generally in static initializers). The particular case of these interfaces is fixed by this patch as it changes them to inline functions, eliminating all uses of ({}) in bits/byteswap.h, and a corresponding testcase is added. The bug tries to raise a more general policy question about use of ({}) in macros in installed headers, referring to "many other libc functions" (unspecified which functions are being considered). Since such policy questions belong on libc-alpha, and since there *are* macros in installed headers which can't really avoid using ({}) (where they are type-generic, so can't use an inline function, but need a temporary variable, and a few where the interface involves returning memory from alloca so can't use an inline function either), I propose to consider that bug fixed with this change. That is without prejudice to any other new bugs anyone wishes to file *for precisely defined sets of macros* requesting moving away from ({}) *where it is clearly possible for those interfaces*. Where ({}) can be avoided, typically by use of an inline function, I think that's a good idea - that inline functions are typically to be preferred to ({}) for header interfaces where such optimizations are useful but the interface is suited to being defined using an inline function. Bug 20530 requests use of __builtin_bswap16 when available (GCC 4.8 and later), which this patch implements. Tested for x86_64, and with build-many-glibcs.py. Also did an x86_64 test with the __GNUC_PREREQ conditionals changed to "#if 0" to verify the old-GCC/non-GCC case in the headers. (There are already existing tests for correctness of results of these interfaces.) [BZ #14508] [BZ #15512] [BZ #17082] [BZ #20530] * bits/byteswap.h: Update file comment. Do not include <bits/byteswap-16.h>. (__bswap_constant_16): Cast result to __uint16_t. Use signed 0xff constant. (__bswap_16): Define as inline function. (__bswap_constant_32): Reformat definition. (__bswap_32): Always define as inline function, not macro, using __uint32_t. Use __builtin_bswap32 if [__GNUC_PREREQ (4, 3)], otherwise __bswap_constant_32. (__bswap_constant_64): Reformat definition. Do not use __extension__ here. (__bswap_64): Always define as inline function, not macro. Use __extension__ on function definition. Use __builtin_bswap64 if [__GNUC_PREREQ (4, 3)], otherwise __bswap_constant_64. * string/test-endian-file-scope.c: New file. * string/test-endian-sign-conversion.c: Likewise. * string/Makefile (headers): Remove bits/byteswap-16.h. (tests): Add test-endian-file-scope and test-endian-sign-conversion. (CFLAGS-test-endian-sign-conversion.c): New variable. * bits/byteswap-16.h: Remove file. * sysdeps/ia64/bits/byteswap-16.h: Likewise. * sysdeps/ia64/bits/byteswap.h: Likewise. * sysdeps/m68k/bits/byteswap.h: Likewise. * sysdeps/s390/bits/byteswap-16.h: Likewise. * sysdeps/s390/bits/byteswap.h: Likewise. * sysdeps/tile/bits/byteswap.h: Likewise. * sysdeps/x86/bits/byteswap-16.h: Likewise. * sysdeps/x86/bits/byteswap.h: Likewise.
Diffstat (limited to 'string')
-rw-r--r--string/Makefile6
-rw-r--r--string/test-endian-file-scope.c44
-rw-r--r--string/test-endian-sign-conversion.c48
3 files changed, 96 insertions, 2 deletions
diff --git a/string/Makefile b/string/Makefile
index 1bcc9bb5d8c..680431f9218 100644
--- a/string/Makefile
+++ b/string/Makefile
@@ -24,7 +24,7 @@ include ../Makeconfig
headers := string.h bits/string_fortified.h \
strings.h bits/strings_fortified.h \
- byteswap.h bits/byteswap.h bits/byteswap-16.h \
+ byteswap.h bits/byteswap.h \
endian.h bits/endian.h bits/uintn-identity.h \
memory.h argz.h envz.h
@@ -58,7 +58,8 @@ tests := tester inl-tester noinl-tester testcopy test-ffs \
bug-strtok1 $(addprefix test-,$(strop-tests)) \
bug-envz1 tst-strxfrm2 tst-endian tst-svc2 \
tst-strtok_r bug-strcoll2 tst-cmp tst-xbzero-opt \
- test-endian-types
+ test-endian-types test-endian-file-scope \
+ test-endian-sign-conversion
# This test allocates a lot of memory and can run for a long time.
xtests = tst-strcoll-overflow
@@ -76,6 +77,7 @@ CFLAGS-stratcliff.c += -fno-builtin
CFLAGS-test-ffs.c += -fno-builtin
CFLAGS-tst-inlcall.c += -fno-builtin
CFLAGS-tst-xbzero-opt.c += -O3
+CFLAGS-test-endian-sign-conversion.c += -Werror -Wsign-conversion
# BZ 21006: Resolve all functions but at least explicit_bzero at startup.
# Otherwise the test fails on s390x as the memcpy in prepare_test_buffer is
# done by loading r4 / r5 with the test_pattern and using store multiple
diff --git a/string/test-endian-file-scope.c b/string/test-endian-file-scope.c
new file mode 100644
index 00000000000..e03063cfbb5
--- /dev/null
+++ b/string/test-endian-file-scope.c
@@ -0,0 +1,44 @@
+/* Test endian.h endian-conversion macros accepted at file scope.
+ Copyright (C) 2018 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+#include <endian.h>
+#include <stddef.h>
+
+int i;
+
+size_t s0 = sizeof (htobe16 (i));
+size_t s1 = sizeof (htole16 (i));
+size_t s2 = sizeof (be16toh (i));
+size_t s3 = sizeof (le16toh (i));
+size_t s4 = sizeof (htobe32 (i));
+size_t s5 = sizeof (htole32 (i));
+size_t s6 = sizeof (be32toh (i));
+size_t s7 = sizeof (le32toh (i));
+size_t s8 = sizeof (htobe64 (i));
+size_t s9 = sizeof (htole64 (i));
+size_t s10 = sizeof (be64toh (i));
+size_t s11 = sizeof (le64toh (i));
+
+static int
+do_test (void)
+{
+ /* This is a compilation test. */
+ return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/string/test-endian-sign-conversion.c b/string/test-endian-sign-conversion.c
new file mode 100644
index 00000000000..cc6b5168b63
--- /dev/null
+++ b/string/test-endian-sign-conversion.c
@@ -0,0 +1,48 @@
+/* Test endian.h endian-conversion macros work with -Wsign-conversion.
+ Copyright (C) 2018 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+#include <endian.h>
+#include <stdint.h>
+
+uint16_t u16;
+uint32_t u32;
+uint64_t u64;
+
+static int
+do_test (void)
+{
+ /* This is a compilation test. */
+ u16 = (htobe16 (u16));
+ u16 = (htole16 (u16));
+ u16 = (be16toh (u16));
+ u16 = (le16toh (u16));
+ u32 = (htobe32 (u32));
+ u32 = (htole32 (u32));
+ u32 = (be32toh (u32));
+ u32 = (le32toh (u32));
+ u64 = (htobe64 (u64));
+ u64 = (htole64 (u64));
+ u64 = (be64toh (u64));
+ u64 = (le64toh (u64));
+ (void) u16;
+ (void) u32;
+ (void) u64;
+ return 0;
+}
+
+#include <support/test-driver.c>