[PATCH] D80391: [Driver] Don't make -gsplit-dwarf imply -g2

[David Blaikie via cfe-commits]

On Wed, Jan 13, 2021 at 6:26 PM Fangrui Song via Phabricator <
reviews at reviews.llvm.org> wrote:

> MaskRay added a comment.
>
> In D80391#2497018 <https://reviews.llvm.org/D80391#2497018>, @inglorion
> wrote:
>
> > For Chrome on Chrome OS, this is https://crbug.com/1158215
> >
> > Here, we saw our links fail with "output file too large". Investigation
> revealed that debug info was being included in the binary, instead of in
> .dwo files as expected. That turned out to be caused by this change.
> >
> > It seems that there are cases where -gsplit-dwarf is passed, but it does
> not take effect because there isn't also a -g (or -g<level>) option:
> >
> >   clang -c -fthinlto-index=foo.o.thinlto.bc -gsplit-dwarf foo.o -o
> foo.bin.o
> >
> > Shows no .dwo file and debug info in foo.bin.o, whereas
> >
> >   clang -c -fthinlto-index=foo.o.thinlto.bc -gsplit-dwarf -g foo.o -o
> foo.bin.o
> >
> > shows a .dwo file which is referenced from foo.bin.o
> >
> > The only thing that is different here is that the latter command has
> "-g" whereas the former doesn't.
> >
> > In other words, the new implementation still doesn't make -g and
> -gsplit-dwarf fully orthogonal; instead of -gsplit-dwarf always turning on
> debug fission, it *only* turns it on when there is also a -g or -g<level>
> option - at least in distributed ThinLTO code generation.
> >
> > It seems that orthogonality was the motivation for this change, and that
> -gsplit-dwarf without -g is supposed to work in the linker (per MaskRay's
> comment about -g being ignored by the linker).
> >
> > Can we make it so that -gsplit-dwarf turns on debug fission and we then
> use that even if no -g options were passed?
>
> I am still confused what the problem is. I added some comments to the
> following distributed ThinLTO example.
> Both -gsplit-dwarf and -g are needed for the ThinLTO backend compile step
> (`-fthinlto-index=`)


^ this I would consider to be a bug. -g should be needed to generate debug
info into the IR, and -gsplit-dwarf should be needed to choose how debug
info already in the IR should be placed into object files or dwo files.

Usually (non-LTO) these two steps are in the same compile, so -g and
-gsplit-dwarf go together. But for LTO cases, I think it's suitable for -g
to be needed for IR generation, and -gsplit-dwarf to be needed for object
code generation, without needing -g as well in that latter step (the -g was
already specified at IR generation time, and that shoul be enough).

We can see that behavior I'm describing as desirable in implicit ThinLTO I
showed earlier in the thread.

I suggest this should be the desired behavior, because -g generally has no
effect on IR->object code generation, so it seems strange/unnecessary to me
to gate -gsplit-dwarf behind -g in that case. The -g was already specified
and used during IR generation (& the -gN level, -gmlt behavior-etc has all
been handled in IR generation (or embedded in the IR for use later) - so to
me, specifying -g again during object code generation would be/is strange,
because any special values would be ignored (-g1/g2/g3/etc would not be
respected - the N value would only be respected when passed to the IR
generation step))


> :
>
>   cat > ./a.c <<eof
>   int foo();
>   int main() { return foo();}
>   eof
>   echo 'int foo() {return 42; }' > ./b.c
>   cat > ./Makefile <<eof
>   clang := /tmp/RelA/bin/clang
>   all: final_link
>
>   compile a.o b.o a.indexing.o b.indexing.o: a.c b.c
>   # -gsplit-dwarf in the initial compile is a no-op.
>           $(clang) -gsplit-dwarf -g -O2 -c -flto=thin
> -fthin-link-bitcode=a.indexing.o a.c
>           $(clang) -gsplit-dwarf -g -O2 -c -flto=thin
> -fthin-link-bitcode=b.indexing.o b.c
>
>   thin_link lto/a.o.thinlto.bc lto/b.o.thinlto.bc a.rsp: a.indexing.o
> b.indexing.o
>           $(clang) -fuse-ld=lld -Wl,--thinlto-index-only=a.rsp
> -Wl,--thinlto-prefix-replace=';lto/'
> -Wl,--thinlto-object-suffix-replace='.indexing.o;.o' a.indexing.o
> b.indexing.o
>
>   thinlto_backend lto/a.o lto/b.o: a.o b.o lto/a.o.thinlto.bc
> lto/b.o.thinlto.bc
>   # This does not produce lto/a.dwo
>   # Both -gsplit-dwarf and -g are needed to get lto/a.dwo
>           $(clang) -g -O2 -c -fthinlto-index=lto/a.o.thinlto.bc a.o -o
> lto/a.o
>           $(clang) -g -O2 -c -fthinlto-index=lto/b.o.thinlto.bc b.o -o
> lto/b.o
>
>   final_link exe: lto/a.o lto/b.o a.rsp
>           $(clang) -fuse-ld=lld @a.rsp -o exe
>
>   clean:
>           $(RM) -f *.o lto/*.o lto/*.bc lto/*.dwo
>   eof
>   mkdir -p ./lto
>
>
> Repository:
>   rG LLVM Github Monorepo
>
> CHANGES SINCE LAST ACTION
>   https://reviews.llvm.org/D80391/new/
>
> https://reviews.llvm.org/D80391
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20210113/467213cb/attachment.html>