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From 4fde88e5dd152fe866a97b12e0f8229970d15cb3 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Fri, 7 Jan 2022 15:21:03 +0000
Subject: [PATCH] libstdc++: Add -nostdinc++ for c++17 sources [PR100017]
When building a build!=host compiler, the just-built gcc can't be used
to build the target libstdc++ (because it is built for the host triplet,
not the build triplet). The top-level configure.ac sets up the build
flags for libstdc++ (and other "raw_cxx" libs) like this:
GCC_TARGET_TOOL(c++ for libstdc++, RAW_CXX_FOR_TARGET, CXX,
[gcc/xgcc -shared-libgcc -B$$r/$(HOST_SUBDIR)/gcc -nostdinc++ -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/src -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/src/.libs -L$$r/$(TARGET_SUBDIR)/libstdc++-v3/libsupc++/.libs],
c++)
The -nostdinc++ flag is only used for the IN-TREE-TOOL, i.e. when using
the just-built gcc/xgcc compiler. This means that the cross-compiler
used to build libstdc++ will add its own libstdc++ headers to the
include path. That results in the #include <cfenv> in
src/c++17/floating_to_chars.cc and src/c++17/floating_from_chars.cc
doing #include_next <fenv.h> and finding the libstdc++ fenv.h wrapper
from the host compiler. Because that has the same include guard as the
<fenv.h> in the libstdc++ we're trying to build, we never reach the
underlying <fenv.h> from libc. That results in several errors of the
form:
error: 'fenv_t' has not been declared in '::'
The most correct fix would be to add -nostdinc++ to the
RAW_CXX_FOR_TARGET variable in configure.ac, or the
RAW_CXX_TARGET_EXPORTS variable in Makefile.tpl.
Another solution would be to make the libstdc++ <fenv.h> wrapper use
_GLIBCXX_INCLUDE_NEXT_C_HEADERS like our <stdlib.h> and other C header
wrappers.
For now though, the simplest and safest solution is to just add
-nostdinc++ to the CXXFLAGS used for src/c++17/*.cc, which is what this
does.
libstdc++-v3/ChangeLog:
PR libstdc++/100017
* src/c++17/Makefile.am (AM_CXXFLAGS): Add -nostdinc++.
* src/c++17/Makefile.in: Regenerate.
---
libstdc++-v3/src/c++17/Makefile.am | 2 +-
libstdc++-v3/src/c++17/Makefile.in | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
--- a/libstdc++-v3/src/c++17/Makefile.am
+++ b/libstdc++-v3/src/c++17/Makefile.am
@@ -79,7 +79,7 @@
# OPTIMIZE_CXXFLAGS on the compile line so that -O2 can be overridden
# as the occasion calls for it.
AM_CXXFLAGS = \
- -std=gnu++17 \
+ -std=gnu++17 -nostdinc++ \
$(glibcxx_lt_pic_flag) $(glibcxx_compiler_shared_flag) \
$(XTEMPLATE_FLAGS) $(VTV_CXXFLAGS) \
$(WARN_CXXFLAGS) $(OPTIMIZE_CXXFLAGS) $(CONFIG_CXXFLAGS) \
--- a/libstdc++-v3/src/c++17/Makefile.in
+++ b/libstdc++-v3/src/c++17/Makefile.in
@@ -455,7 +455,7 @@
# OPTIMIZE_CXXFLAGS on the compile line so that -O2 can be overridden
# as the occasion calls for it.
AM_CXXFLAGS = \
- -std=gnu++17 \
+ -std=gnu++17 -nostdinc++ \
$(glibcxx_lt_pic_flag) $(glibcxx_compiler_shared_flag) \
$(XTEMPLATE_FLAGS) $(VTV_CXXFLAGS) \
$(WARN_CXXFLAGS) $(OPTIMIZE_CXXFLAGS) $(CONFIG_CXXFLAGS) \
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