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xz-archive/m4/tuklib_integer.m4
Lasse Collin 850620468b Revise tuklib_integer.h and .m4.
Add a configure option --enable-unsafe-type-punning to get the
old non-conforming memory access methods. It can be useful with
old compilers or in some other less typical situations but
shouldn't normally be used.

Omit the packed struct trick for unaligned access. While it's
best in some cases, this is simpler. If the memcpy trick doesn't
work, one can request unsafe type punning from configure.

Because CRC32/CRC64 code needs fast aligned reads, if no very
safe way to do it is found, type punning is used as a fallback.
This sucks but since it currently works in practice, it seems to
be the least bad option. It's never needed with GCC >= 4.7 or
Clang >= 3.6 since these support __builtin_assume_aligned and
thus fast aligned access can be done with the memcpy trick.

Other things:
  - Support GCC/Clang __builtin_bswapXX
  - Cleaner bswap fallback macros
  - Minor cleanups
2019-12-31 22:34:10 +02:00

117 lines
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Text

#
# SYNOPSIS
#
# TUKLIB_INTEGER
#
# DESCRIPTION
#
# Checks for tuklib_integer.h:
# - Endianness
# - Does operating system provide byte swapping macros
# - Does the hardware support fast unaligned access to 16-bit
# and 32-bit integers
#
# COPYING
#
# Author: Lasse Collin
#
# This file has been put into the public domain.
# You can do whatever you want with this file.
#
AC_DEFUN_ONCE([TUKLIB_INTEGER], [
AC_REQUIRE([TUKLIB_COMMON])
AC_REQUIRE([AC_C_BIGENDIAN])
AC_CHECK_HEADERS([byteswap.h sys/endian.h sys/byteorder.h], [break])
# Even if we have byteswap.h, we may lack the specific macros/functions.
if test x$ac_cv_header_byteswap_h = xyes ; then
m4_foreach([FUNC], [bswap_16,bswap_32,bswap_64], [
AC_MSG_CHECKING([if FUNC is available])
AC_LINK_IFELSE([AC_LANG_SOURCE([
#include <byteswap.h>
int
main(void)
{
FUNC[](42);
return 0;
}
])], [
AC_DEFINE(HAVE_[]m4_toupper(FUNC), [1],
[Define to 1 if] FUNC [is available.])
AC_MSG_RESULT([yes])
], [AC_MSG_RESULT([no])])
])dnl
fi
AC_MSG_CHECKING([if __builtin_bswap16/32/64 are supported])
AC_LINK_IFELSE([AC_LANG_PROGRAM([[]],
[[__builtin_bswap16(1);
__builtin_bswap32(1);
__builtin_bswap64(1);]])],
[
AC_DEFINE([HAVE___BUILTIN_BSWAPXX], [1],
[Define to 1 if the GNU C extensions
__builtin_bswap16/32/64 are supported.])
AC_MSG_RESULT([yes])
], [
AC_MSG_RESULT([no])
])
AC_MSG_CHECKING([if unaligned memory access should be used])
AC_ARG_ENABLE([unaligned-access], AS_HELP_STRING([--enable-unaligned-access],
[Enable if the system supports *fast* unaligned memory access
with 16-bit and 32-bit integers. By default, this is enabled
only on x86, x86_64, and big endian PowerPC.]),
[], [enable_unaligned_access=auto])
if test "x$enable_unaligned_access" = xauto ; then
# TODO: There may be other architectures, on which unaligned access
# is OK.
case $host_cpu in
i?86|x86_64|powerpc|powerpc64)
enable_unaligned_access=yes
;;
*)
enable_unaligned_access=no
;;
esac
fi
if test "x$enable_unaligned_access" = xyes ; then
AC_DEFINE([TUKLIB_FAST_UNALIGNED_ACCESS], [1], [Define to 1 if
the system supports fast unaligned access to 16-bit and
32-bit integers.])
AC_MSG_RESULT([yes])
else
AC_MSG_RESULT([no])
fi
AC_MSG_CHECKING([if unsafe type punning should be used])
AC_ARG_ENABLE([unsafe-type-punning],
AS_HELP_STRING([--enable-unsafe-type-punning],
[This introduces strict aliasing violations and may result
in broken code. However, this might improve performance in
some cases, especially with old compilers (e.g.
GCC 3 and early 4.x on x86, GCC < 6 on ARMv6 and ARMv7).]),
[], [enable_unsafe_type_punning=no])
if test "x$enable_unsafe_type_punning" = xyes ; then
AC_DEFINE([TUKLIB_USE_UNSAFE_TYPE_PUNNING], [1], [Define to 1 to use
unsafe type punning, e.g. char *x = ...; *(int *)x = 123;
which violates strict aliasing rules and thus is
undefined behavior and might result in broken code.])
AC_MSG_RESULT([yes])
else
AC_MSG_RESULT([no])
fi
AC_MSG_CHECKING([if __builtin_assume_aligned is supported])
AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[__builtin_assume_aligned("", 1);]])],
[
AC_DEFINE([HAVE___BUILTIN_ASSUME_ALIGNED], [1],
[Define to 1 if the GNU C extension
__builtin_assume_aligned is supported.])
AC_MSG_RESULT([yes])
], [
AC_MSG_RESULT([no])
])
])dnl