Re: [PATCH] Try LTO partial linking. (Was: Speed of compiling gimple-match.c)

[David Edelsohn <dje.gcc@gmail.com>]

On Fri, May 21, 2021 at 5:25 AM Martin Liška <mliska@suse.cz> wrote:
>
> On 5/20/21 2:54 PM, Richard Biener wrote:
> > On Thu, May 20, 2021 at 2:34 PM Martin Liška <mliska@suse.cz> wrote:
> >>
> >> Hello.
> >>
> >> I've got a patch candidate that leverages partial linking for a couple of selected object files.
> >>
> >> I'm sending make all-host- jX results for my machine:
> >>
> >> before: 3m18s (user 32m52s)
> >> https://gist.githubusercontent.com/marxin/223890df4d8d8e490b6b2918b77dacad/raw/1dd5eae5001295ba0230a689f7edc67284c9b742/gcc-all-host.svg
> >>
> >> after: 2m57m (user 35m)
> >> https://gist.githubusercontent.com/marxin/223890df4d8d8e490b6b2918b77dacad/raw/d659b2187cf622167841efbbe6bc93cb33855fa9/gcc-all-host-partial-lto.svg
> >>
> >> One can utilize it with:
> >> make -j16 all-host PARTIAL_LTO=1
> >>
> >> @Segher, Andrew: Can you please measure time improvement for your slow bootstrap?
> >> One can also tweak --param=lto-partitions=16 param value.
> >>
> >> Thoughts?
> >
> > You're LTO linking multiple objects here - that's almost as if you
> > were doing this
> > for the whole of libbackend.a ... so $(OBJS)_CLFAGS += -flto and in the
> > libbackend.a rule do a similar partial link trick.
>
> Yeah, apart from that one can't likely do partial linking for an archive:
>
> $ g++ -no-pie -flto=auto --param=lto-partitions=16 -flinker-output=nolto-rel -r libbackend.a
> collect2: fatal error: ld terminated with signal 11 [Segmentation fault], core dumped
> compilation terminated.
>
> while ld.bfd immediately finishes.
>
> >
> > That gets you half of a LTO bootstrap then.
> >
> > So why did you go from applying this per-file to multiple files?  Does $(LINKER)
> > have a proper rule to pick up a jobserver?
> >
> > When upstreaming in any form you probably have to gate it on bootstrap-lto
> > being not active.
>
> Sure, that's reasonable, we can likely detect a -flto option in $(COMPILE), right?
>
> One more thing I face is broken dependency:
> $ make clean && make -j32 PARTIAL_LTO=1
>
> g++ -fcf-protection -fno-PIE -c   -g   -DIN_GCC -fPIC    -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wno-error=format-diag -Wmissing-format-attribute -Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -fno-common -Wno-unused -DHAVE_CONFIG_H -I. -I. -I/home/marxin/Programming/gcc/gcc -I/home/marxin/Programming/gcc/gcc/. -I/home/marxin/Programming/gcc/gcc/../include -I/home/marxin/Programming/gcc/gcc/../libcpp/include -I/home/marxin/Programming/gcc/gcc/../libcody  -I/home/marxin/Programming/gcc/gcc/../libdecnumber -I/home/marxin/Programming/gcc/gcc/../libdecnumber/bid -I../libdecnumber -I/home/marxin/Programming/gcc/gcc/../libbacktrace   -o gimple-match-lto.o -MT gimple-match-lto.o -MMD -MP -MF ./.deps/gimple-match-lto.TPo gimple-match.c -flto
> g++ -fcf-protection -fno-PIE -c   -g   -DIN_GCC -fPIC    -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wno-error=format-diag -Wmissing-format-attribute -Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -fno-common -Wno-unused -DHAVE_CONFIG_H -I. -I. -I/home/marxin/Programming/gcc/gcc -I/home/marxin/Programming/gcc/gcc/. -I/home/marxin/Programming/gcc/gcc/../include -I/home/marxin/Programming/gcc/gcc/../libcpp/include -I/home/marxin/Programming/gcc/gcc/../libcody  -I/home/marxin/Programming/gcc/gcc/../libdecnumber -I/home/marxin/Programming/gcc/gcc/../libdecnumber/bid -I../libdecnumber -I/home/marxin/Programming/gcc/gcc/../libbacktrace   -o generic-match-lto.o -MT generic-match-lto.o -MMD -MP -MF ./.deps/generic-match-lto.TPo generic-match.c -flto
>
> In file included from ./tm.h:26,
>                   from /home/marxin/Programming/gcc/gcc/backend.h:28,
>                   from /home/marxin/Programming/gcc/gcc/generic-match-head.c:23,
>                   from generic-match.c:4:
> /home/marxin/Programming/gcc/gcc/config/i386/i386.h:2286:10: fatal error: insn-attr-common.h: No such file or directory
>   2286 | #include "insn-attr-common.h"
>        |          ^~~~~~~~~~~~~~~~~~~~
> compilation terminated.
> make: *** [Makefile:2678: generic-match-lto.o] Error 1
> make: *** Waiting for unfinished jobs....
>
> In file included from ./tm.h:26,
>                   from /home/marxin/Programming/gcc/gcc/backend.h:28,
>                   from /home/marxin/Programming/gcc/gcc/gimple-match-head.c:23,
>                   from gimple-match.c:4:
> /home/marxin/Programming/gcc/gcc/config/i386/i386.h:2286:10: fatal error: insn-attr-common.h: No such file or directory
>   2286 | #include "insn-attr-common.h"
>        |          ^~~~~~~~~~~~~~~~~~~~
>
> I explicitly added:
> gimple-match.o: gimple-match.c $(generated_files)
> generic-match.o: generic-match.c $(generated_files)
>
> But it's not obeyed.

Please remember that not all targets support LTO so a fallback to a
non-partial-LTO build needs to be provided and automatically invoked
for those targets.

Thanks, David