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

Richard Biener richard.guenther@gmail.com
Tue Jun 1 07:42:51 GMT 2021


On Tue, Jun 1, 2021 at 9:33 AM Martin Liška <mliska@suse.cz> wrote:
>
> @Richi: Can you please reply to this email?

Not sure what I should add here?  Honza suggested to mangle the
promoted symbol names.  I don't
really like the idea to compile multiple TUs into one object.  Also

+LTO_LINKER_FLAGS = -flto=auto --param=lto-partitions=16
-flinker-output=nolto-rel -r

why hard-code to 16 partitions?  You're side-stepping the driver
diagnostic by doing
compile & link separately, but in the end we're going to want sth like Giulianos
-fparallel-compile that works transparently from within the driver, so
the "manual"
operation should try to follow that or alternatively a driver-only
wrapper around the
"manual" processing could be added whose implementation can be optimized later.

Why do you use -flto=auto?  There should be a jobserver active.

> On 5/21/21 10:43 AM, Martin Liška 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.
> >
> > Martin
> >
> >>
> >> Richard.
> >>
> >>> Thanks,
> >>> Martin
> >
>


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