Re: [PATCH] libstdc++: Do not overwrite module definition files during build [PR126786]

On Tue, Aug 18, 2026 at 10:46 AM Jonathan Wakely <[email protected]> wrote:

> On Tue, 18 Aug 2026 at 09:20, Tomasz Kaminski <[email protected]> wrote:
> >
> >
> >
> > On Mon, Aug 17, 2026 at 10:27 PM Jonathan Wakely <[email protected]>
> wrote:
> >>
> >> As described in the bug report, when compilation of std.cc or
> >> std.compat.cc fails we overwrite the source file with an empty file,
> and
> >> then compile that (so that bootstrap doesn't fail). Then when the module
> >> definition files are installed, we install the empty file.
> >>
> >> We should install the original source, not an empty file. We can create
> >> an empty file and compile it, but leave the original source untouched so
> >> that it can still be installed.
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >>         PR libstdc++/126786
> >>         * src/c++23/Makefile.am (std.lo, std.compat.lo): Do not
> >>         overwrite original sources in error-recovery steps.
> >>         * src/c++23/Makefile.in: Regenerate.
> >> ---
> >>
> >> Tested x86_64-linux.
> >>
> >> Also checked by injecting errors into std.cc.in and std.compat.cc.in to
> >> ensure that the build completes as expected.
> >>
> >>  libstdc++-v3/src/c++23/Makefile.am | 12 ++++--------
> >>  libstdc++-v3/src/c++23/Makefile.in | 12 ++++--------
> >>  2 files changed, 8 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/libstdc++-v3/src/c++23/Makefile.am
> b/libstdc++-v3/src/c++23/Makefile.am
> >> index 92691c502a94..87c0108b52d0 100644
> >> --- a/libstdc++-v3/src/c++23/Makefile.am
> >> +++ b/libstdc++-v3/src/c++23/Makefile.am
> >> @@ -104,29 +104,25 @@ std.lo: std.cc
> >>         if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(LTCXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >
> > Any reason for switching the name of the helper file from std.cc.tmp to
> xstd.cc?
>
> That's not what the patch does.
>
> The code that's on trunk now writes to a new file called std.cc.tmp
> but then moves that to std.cc
> That means the file has the correct .cc extension, so will be treated
> as C++ source by g++. If it was called std.cc.tmp it would assume it's
> a library or object file to be passed to the linker:
>
> $ g++ std.cc.tmp -c
> g++: warning: std.cc.tmp: linker input file unused because linking not done
>
We could pass -x c++ to override the extension-based selector, but I think
it is
better to use the same compilation flags.

>
> So I'm switching the name of the helper file from std.cc to xstd.cc
> and std.compat.cc to xstd.compat.cc, which avoids overwriting the
> original std.cc and std.compat.cc files, which is the point of the
> patch.
>
> > I think the later is more likely to conflict with something, so would
> preffer to use $<.tmp
> > here. Or is this common practice for these makefiles?
>
> Using a .tmp extension won't work.
>
> It could be tmp-std.cc or empty-std.cc or something like that, but
> whatever change we make needs to be a prefix not a suffix.
>
"empty-" prefix sounds better to me.

>
> >>
> >>         fi
> >>  std.o: std.cc
> >>         if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(CXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >>         fi
> >>  std.compat.lo: std.compat.cc std.lo
> >>         if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std.compat module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(LTCXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >>         fi
> >>  std.compat.o: std.compat.cc std.o
> >>         if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std.compat module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(CXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >>         fi
> >>
> >>  # AM_CXXFLAGS needs to be in each subdirectory so that it can be
> >> diff --git a/libstdc++-v3/src/c++23/Makefile.in
> b/libstdc++-v3/src/c++23/Makefile.in
> >> index 0ce93f812d25..63e5cb4e2a1c 100644
> >> --- a/libstdc++-v3/src/c++23/Makefile.in
> >> +++ b/libstdc++-v3/src/c++23/Makefile.in
> >> @@ -888,29 +888,25 @@ std.lo: std.cc
> >>         if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(LTCXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >>         fi
> >>  std.o: std.cc
> >>         if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(CXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >>         fi
> >>  std.compat.lo: std.compat.cc std.lo
> >>         if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std.compat module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(LTCXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >>         fi
> >>  std.compat.o: std.compat.cc std.o
> >>         if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
> >>           echo "Cannot compile std.compat module" >&2; \
> >>           echo "Module initialization function will be missing" >&2; \
> >> -         echo > $<.tmp && mv $<.tmp $< && \
> >> -         $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
> >> +         echo > x$< && $(CXXCOMPILE) $(MODULES_FLAGS) -c x$< -o $@; \
> >>         fi
> >>
> >>  # Tell versions [3.59,3.63) of GNU make to not export all variables.
> >> --
> >> 2.55.0
> >>
>
>