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Re: [PATCH v3 1/9] Add configure check to test if gcc supports attribute ifunc.
On 08/29/2016 11:34 AM, Florian Weimer wrote:
On 08/24/2016 04:04 PM, Stefan Liebler wrote:
+For multi-arch support it is recommended to use a GCC with
gnu-indirect-function
+support as it is used to generate ifunc'ed symbols with correct
+debug-information. This support can either be enabled by configuring
GCC with
+@samp{--enable-gnu-indirect-function} or by enabling it by default by
setting
+@samp{default_gnu_indirect_function} variable for a particular
architecture in
+gcc source file @file{gcc/config.gcc}.
Maybe use this text instead?
6808c16e2d067c8137fa9eae6078dc55c5d355c6
For multi-arch support it is recommended to use a GCC which has been
built with support for GNU indirect functions. This ensures that
correct debugging information is generated for functions selected by
IFUNC resolvers. This support can either be enabled by configuring GCC
with @samp{--enable-gnu-indirect-function}, or by enabling it by default
by setting @samp{default_gnu_indirect_function} variable for a
particular architecture in the GCC source file @file{gcc/config.gcc}.
Okay. I've updated install-files and I'll use this updated text also for
the NEWS entry.
The configure check looks okay to me. Some existing checks use
AC_TRY_COMPILE, others use the manual expansion you provided.
Thanks,
Florian
commit 3cfc378ca6f95feaa35b99b4b5897fa35dbff68f
Author: Stefan Liebler <stli@linux.vnet.ibm.com>
Date: Wed Aug 24 10:18:14 2016 +0200
Add configure check to test if gcc supports attribute ifunc.
This patch adds a configure check to test if gcc supports attribute ifunc.
The support can either be enabled in <gcc-src>/gcc/config.gcc for one
architecture in general by setting default_gnu_indirect_function variable to yes
or by configuring gcc with --enable-gnu-indirect-function.
The next patch rewrites libc_ifunc macro to use gcc attribute ifunc instead
of inline assembly to generate the IFUNC symbols due to false debuginfo.
If gcc does not support attribute ifunc, the old approach for generating
ifunc'ed symbols is used. Then the debug-information is false. Thus it is
recommended to use a gcc with indirect function support (See notes in INSTALL).
After this patch-series these inline assemblies for ifunc-handling are not
scattered in multiple files but are used only indirect via ifunc-macros
and can simply removed in libc-symbols.h in future.
If glibc is configured with --enable-multi-arch and gcc does not support
attribute ifunc, a configure warning is dumped!
This NEWS entry will be added:
* For multi-arch support it is recommended to use a GCC which has
been built with support for GNU indirect functions. This ensures
that correct debugging information is generated for functions
selected by IFUNC resolvers. This support can either be enabled by
configuring GCC with '--enable-gnu-indirect-function', or by
enabling it by default by setting 'default_gnu_indirect_function'
variable for a particular architecture in the GCC source file
'gcc/config.gcc'.
ChangeLog:
* config.h.in (HAVE_GCC_IFUNC): New undef.
* configure.ac: Add check if gcc supports attribute ifunc feature.
* configure: Regenerated.
* manual/install.texi: Add recommendation for gcc with
indirect-function support.
* INSTALL: Regenerated.
diff --git a/INSTALL b/INSTALL
index ec3445f..66be341 100644
--- a/INSTALL
+++ b/INSTALL
@@ -359,6 +359,15 @@ build the GNU C Library:
better code. As of release time, GCC 5.3 is the newest compiler
verified to work to build the GNU C Library.
+ For multi-arch support it is recommended to use a GCC which has
+ been built with support for GNU indirect functions. This ensures
+ that correct debugging information is generated for functions
+ selected by IFUNC resolvers. This support can either be enabled by
+ configuring GCC with '--enable-gnu-indirect-function', or by
+ enabling it by default by setting 'default_gnu_indirect_function'
+ variable for a particular architecture in the GCC source file
+ 'gcc/config.gcc'.
+
You can use whatever compiler you like to compile programs that use
the GNU C Library.
diff --git a/config.h.in b/config.h.in
index 856ef6a..9f5c5ff 100644
--- a/config.h.in
+++ b/config.h.in
@@ -171,6 +171,9 @@
/* Define to 1 if STT_GNU_IFUNC support actually works. */
#define HAVE_IFUNC 0
+/* Define if gcc supports attribute ifunc. */
+#undef HAVE_GCC_IFUNC
+
/* Define if the linker defines __ehdr_start. */
#undef HAVE_EHDR_START
diff --git a/configure b/configure
index 17625e1..8ffc2b5 100755
--- a/configure
+++ b/configure
@@ -3914,6 +3914,36 @@ fi
{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_ld_gnu_indirect_function" >&5
$as_echo "$libc_cv_ld_gnu_indirect_function" >&6; }
+# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute ifunc support" >&5
+$as_echo_n "checking for gcc attribute ifunc support... " >&6; }
+if ${libc_cv_gcc_indirect_function+:} false; then :
+ $as_echo_n "(cached) " >&6
+else
+ cat > conftest.c <<EOF
+extern int func (int);
+int used_func (int a)
+{
+ return a;
+}
+static void *resolver ()
+{
+ return &used_func;
+}
+extern __typeof (func) func __attribute__ ((ifunc ("resolver")));
+EOF
+libc_cv_gcc_indirect_function=no
+if ${CC-cc} -c conftest.c -o conftest.o 1>&5 \
+ 2>&5 ; then
+ if $READELF -s conftest.o | grep IFUNC >/dev/null 2>&5; then
+ libc_cv_gcc_indirect_function=yes
+ fi
+fi
+rm -f conftest*
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_indirect_function" >&5
+$as_echo "$libc_cv_gcc_indirect_function" >&6; }
+
if test x"$libc_cv_ld_gnu_indirect_function" != xyes; then
if test x"$multi_arch" = xyes; then
as_fn_error $? "--enable-multi-arch support requires assembler and linker support" "$LINENO" 5
@@ -3921,6 +3951,13 @@ if test x"$libc_cv_ld_gnu_indirect_function" != xyes; then
multi_arch=no
fi
fi
+if test x"$libc_cv_gcc_indirect_function" != xyes &&
+ test x"$multi_arch" = xyes; then
+ { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: --enable-multi-arch support recommends a gcc with gnu-indirect-function support.
+Please use a gcc which supports it by default or configure gcc with --enable-gnu-indirect-function" >&5
+$as_echo "$as_me: WARNING: --enable-multi-arch support recommends a gcc with gnu-indirect-function support.
+Please use a gcc which supports it by default or configure gcc with --enable-gnu-indirect-function" >&2;}
+fi
multi_arch_d=
if test x"$multi_arch" != xno; then
multi_arch_d=/multiarch
@@ -6504,6 +6541,11 @@ if test x"$libc_cv_ld_gnu_indirect_function" = xyes; then
fi
+if test x"$libc_cv_gcc_indirect_function" = xyes; then
+ $as_echo "#define HAVE_GCC_IFUNC 1" >>confdefs.h
+
+fi
+
# This is far from the AC_ARG_ENABLE that sets it so that a sysdeps
# configure fragment can override the value to prevent this AC_DEFINE.
diff --git a/configure.ac b/configure.ac
index 33bcd62..643002f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -634,6 +634,30 @@ if ${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS \
fi
rm -f conftest*])
+# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
+AC_CACHE_CHECK([for gcc attribute ifunc support],
+ libc_cv_gcc_indirect_function, [dnl
+cat > conftest.c <<EOF
+extern int func (int);
+int used_func (int a)
+{
+ return a;
+}
+static void *resolver ()
+{
+ return &used_func;
+}
+extern __typeof (func) func __attribute__ ((ifunc ("resolver")));
+EOF
+libc_cv_gcc_indirect_function=no
+if ${CC-cc} -c conftest.c -o conftest.o 1>&AS_MESSAGE_LOG_FD \
+ 2>&AS_MESSAGE_LOG_FD ; then
+ if $READELF -s conftest.o | grep IFUNC >/dev/null 2>&AS_MESSAGE_LOG_FD; then
+ libc_cv_gcc_indirect_function=yes
+ fi
+fi
+rm -f conftest*])
+
if test x"$libc_cv_ld_gnu_indirect_function" != xyes; then
if test x"$multi_arch" = xyes; then
AC_MSG_ERROR([--enable-multi-arch support requires assembler and linker support])
@@ -641,6 +665,11 @@ if test x"$libc_cv_ld_gnu_indirect_function" != xyes; then
multi_arch=no
fi
fi
+if test x"$libc_cv_gcc_indirect_function" != xyes &&
+ test x"$multi_arch" = xyes; then
+ AC_MSG_WARN([--enable-multi-arch support recommends a gcc with gnu-indirect-function support.
+Please use a gcc which supports it by default or configure gcc with --enable-gnu-indirect-function])
+fi
multi_arch_d=
if test x"$multi_arch" != xno; then
multi_arch_d=/multiarch
@@ -1770,6 +1799,10 @@ if test x"$libc_cv_ld_gnu_indirect_function" = xyes; then
AC_DEFINE(HAVE_IFUNC)
fi
+if test x"$libc_cv_gcc_indirect_function" = xyes; then
+ AC_DEFINE(HAVE_GCC_IFUNC)
+fi
+
# This is far from the AC_ARG_ENABLE that sets it so that a sysdeps
# configure fragment can override the value to prevent this AC_DEFINE.
AC_SUBST(use_nscd)
diff --git a/manual/install.texi b/manual/install.texi
index 79ee45f..5f5705e 100644
--- a/manual/install.texi
+++ b/manual/install.texi
@@ -402,6 +402,14 @@ the newest version of the compiler that is known to work for building
release time, GCC 5.3 is the newest compiler verified to work to build
@theglibc{}.
+For multi-arch support it is recommended to use a GCC which has been built with
+support for GNU indirect functions. This ensures that correct debugging
+information is generated for functions selected by IFUNC resolvers. This
+support can either be enabled by configuring GCC with
+@samp{--enable-gnu-indirect-function}, or by enabling it by default by setting
+@samp{default_gnu_indirect_function} variable for a particular architecture in
+the GCC source file @file{gcc/config.gcc}.
+
You can use whatever compiler you like to compile programs that use
@theglibc{}.