Hi,
Currently, when the option --enable-s390-excess-float-precision is
omitted, configure checks the libc's behavior only for native builds yet
defaults to the new model for cross compiles. This patch extends the
configure-time check to cross-compiles that have target headers
available (e.g., in the second stage when manually bootstrapping a cross
toolchain or in the build process for cross compilers in some
distributions).
Built cross-compilers for target s390x on x86_64 and checked configure
results and behavior wrt excess float precision (1) with target headers
with old float_t behavior, (2) with new float_t behavior, and (3)
without target headers.
Bootstrapped and checked configure results on s390x.
Tested configure and build on x86_64.
Marius
----------->8------------>8------------>8------------>8------------
When cross-compiling GCC with target libc headers available and
configure option --enable-s390-excess-float-precision has been omitted,
identify whether they clamp float_t to double or respect
__FLT_EVAL_METHOD__ via a compile test that coerces the build-system
compiler to use the target headers. Then derive the setting from that.
gcc/ChangeLog:
2020-12-14 Marius Hillenbrand <mhil...@linux.ibm.com>
* configure.ac: Change --enable-s390-excess-float-precision
default behavior for cross compiles with target headers.
* configure: Regenerate.
* doc/install.texi: Adjust documentation.
---
gcc/configure.ac | 52 +++++++++++++++++++++++++++++++-------------
gcc/doc/install.texi | 7 +++---
2 files changed, 41 insertions(+), 18 deletions(-)
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 062f57fa12b..0964bab6fec 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -7372,9 +7372,9 @@ AC_MSG_RESULT($ld_pushpopstate_support)
# gcc describes its behavior via the macro __FLT_EVAL_METHOD__ and glibc
derives
# float_t from that, this behavior can be configured with
# --enable-s390-excess-float-precision. When given as enabled, that flag
selects
-# the old model. When omitted, native builds will derive the flag from the
-# behavior of glibc. When glibc clamps float_t to double, gcc follows the old
-# model. In any other case, it defaults to the new model.
+# the old model. When omitted, native builds and cross compiles that have
target
+# libc headers will detect whether libc clamps float_t to double and in that
+# case maintain the old model. Otherwise, they will default to the new model.
AC_ARG_ENABLE(s390-excess-float-precision,
[AS_HELP_STRING([--enable-s390-excess-float-precision],
[on s390 targets, evaluate float with double precision
@@ -7383,20 +7383,42 @@ AC_ARG_ENABLE(s390-excess-float-precision,
case $target in
s390*-linux*)
- if test "$target" = "$host" -a "$host" = "$build" -a \
- x"$enable_s390_excess_float_precision" = xauto; then
- AC_CACHE_CHECK([for glibc clamping float_t to double],
- gcc_cv_float_t_clamped_to_double,
- [AC_RUN_IFELSE([AC_LANG_SOURCE([
+ if test x"$enable_s390_excess_float_precision" = xauto; then
+ # Can we autodetect the behavior of the target libc?
+ if test "$target" = "$host" -a "$host" = "$build"; then
+ enable_s390_excess_float_precision=autodetect
+ elif test "x$with_headers" != xno; then
+ # cross build. are target headers available?
+ # carefully coerce the build-system compiler to use target headers
+ saved_CXXFLAGS="$CXXFLAGS"
+ CROSS_TEST_CXXFLAGS="-nostdinc ${XGCC_FLAGS_FOR_TARGET//-B/-idirafter/}"
+ CXXFLAGS="$CROSS_TEST_CXXFLAGS"
+ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+#include <math.h>
+]])], [enable_s390_excess_float_precision=autodetect], [])
+ CXXFLAGS="$saved_CXXFLAGS"
+ fi
+
+ if test x"$enable_s390_excess_float_precision" = xautodetect; then
+ saved_CXXFLAGS="$CXXFLAGS"
+ if ! test "$target" = "$host" -a "$host" = "$build"; then
+ CXXFLAGS="$CROSS_TEST_CXXFLAGS"
+ unset CROSS_TEST_CXXFLAGS
+ fi
+ AC_CACHE_CHECK([for glibc clamping float_t to double],
+ gcc_cv_float_t_clamped_to_double, [
+ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
#define __FLT_EVAL_METHOD__ 0
#include <math.h>
-int main() {
- return !(sizeof(float_t) == sizeof(double));
-}])],
- [gcc_cv_float_t_clamped_to_double=yes],
- [gcc_cv_float_t_clamped_to_double=no])])
- if test x"$gcc_cv_float_t_clamped_to_double" = xyes; then
- enable_s390_excess_float_precision=yes
+int dummy[sizeof(float_t) == sizeof(double) ? 1 : -1];
+]])],
+ [gcc_cv_float_t_clamped_to_double=yes],
+ [gcc_cv_float_t_clamped_to_double=no])])
+ CXXFLAGS="$saved_CXXFLAGS"
+ enable_s390_excess_float_precision="$gcc_cv_float_t_clamped_to_double"
+ else
+ # no way to detect behavior of target libc, default to new model
+ enable_s390_excess_float_precision=no
fi
fi
diff --git a/gcc/doc/install.texi b/gcc/doc/install.texi
index a38ca3e3ce8..e30d2fc95bc 100644
--- a/gcc/doc/install.texi
+++ b/gcc/doc/install.texi
@@ -2277,9 +2277,10 @@ On s390(x) targets, enable treatment of float
expressions with double precision
when in standards-compliant mode (e.g., when @code{--std=c99} or
@code{-fexcess-precision=standard} are given).
-For a native build, the option's default is derived from glibc's behavior. When
-glibc clamps float_t to double, gcc follows and enables the option. In all
other
-cases, it defaults to off.
+For a native build and cross compiles that have target headers, the option's
+default is derived from glibc's behavior. When glibc clamps float_t to double,
+GCC follows and enables the option. For other cross compiles, the default is
+disabled.
@end table
@subheading Cross-Compiler-Specific Options
--
2.26.2
