[llvm-dev] distributed thinlto usage

Teresa Johnson via llvm-dev llvm-dev at lists.llvm.org
Wed Jan 9 07:42:49 PST 2019


Mystery solved - the short answer is to add "-Xclang -fno-lto-unit" to your
compile steps (-c -flto=thin). I had forgotten that we add this for our
internal distributed builds. Another option would be to add
"-Wl,-plugin-opt,obj-path=astar.o" to the thin link step, and add the
resulting (native) astar.o file to the final link.

I am working on getting the default changed (D53890/D53891), since this is
only needed for CFI [1], but currently by default each module is split into
two - a regular LTO module containing vtable defs and a thin LTO module
containing the rest. This is required by CFI so that the vtables can be
globally laid out via regular LTO. If you don't disable this splitting
(via "-Xclang -fno-lto-unit"), then you need to capture the result of this
regular LTO portion via the obj-path plugin option and link it as well.
However, unless you plan to do CFI this is just giving your builds extra
unnecessary overhead (which is why the default will change).

 [1] Also currently for whole program devirtualization which is enabled for
CFI, or separately via options - although I have a patch out for review to
do WPD with ThinLTO alone (D55153).

Teresa

On Tue, Jan 8, 2019 at 5:28 PM Teresa Johnson <tejohnson at google.com> wrote:

> Ok thanks for checking. I'll try to repro tomorrow morning probably and
> get back. Teresa
>
> On Tue, Jan 8, 2019, 5:03 PM David Callahan <dcallahan at fb.com> wrote:
>
>> Fails with gold too:
>>
>> Library-native.o:Library.cpp:regway: error: undefined reference to
>> 'vtable for regwayobj'
>>
>> /home/dcallahan/fbsource/fbcode/third-party-buck/platform007/tools/binutils/bin/gold/ld:
>> the vtable symbol may be undefined because the class is missing its key
>> function
>>
>> clang-8: error: linker command failed with exit code 1 (use -v to see
>> invocation)
>>
>>
>>
>> *From: *Teresa Johnson <tejohnson at google.com>
>> *Date: *Tuesday, January 8, 2019 at 4:57 PM
>>
>>
>> *To: *David Callahan <dcallahan at fb.com>
>> *Cc: *LLVM Dev Mailing list <llvm-dev at lists.llvm.org>
>> *Subject: *Re: [llvm-dev] distributed thinlto usage
>>
>>
>>
>>
>>
>>
>>
>> On Tue, Jan 8, 2019 at 4:36 PM David Callahan <dcallahan at fb.com> wrote:
>>
>> Thanks Teresa
>>
>> Yes it is astar, happen to send a tar of the sources but they are just
>> copies from the spec distribution
>>
>> The ld command is:
>>
>> GNU ld (GNU Binutils) 2.29.1
>>
>>
>>
>> Could you try with either a recent gold or lld to see if it still
>> reproduces? We don't test llvm LTO with GNU ld, so I can't be sure there
>> are no issues there.
>>
>>
>>
>>
>>
>> Thanks for the guidance on path names. The prefix-replace just effects
>> the string written to the object files right? So we could post-process that
>> file with other tools as well, correct?
>>
>>
>>
>> Correct.
>>
>> Teresa
>>
>>
>>
>>
>>
>> Thanks again
>>
>> --david
>>
>>
>>
>> *From: *Teresa Johnson <tejohnson at google.com>
>> *Date: *Tuesday, January 8, 2019 at 4:11 PM
>> *To: *David Callahan <dcallahan at fb.com>
>> *Cc: *LLVM Dev Mailing list <llvm-dev at lists.llvm.org>
>> *Subject: *Re: [llvm-dev] distributed thinlto usage
>>
>>
>>
>> Hi David,
>>
>>
>>
>> That looks basically right, so I am not sure offhand what is wrong. Looks
>> like this is the astar spec binary, so I will try to repro it tomorrow
>> morning. What linker are you using?
>>
>>
>>
>> Note that if your link involves archives of .a files, you will want to
>> ensure that the final native link includes only those files selected by the
>> linker. That is output in the file given to the
>> "-Wl,-plugin-opt,thinlto-index-only=files.out" option. What we do is use
>> that in combination with the '-Wl,-plugin-opt,thinlto-prefix-replace=path1;path2'
>> option to simplify the final link invocation. Specifically, if you do
>> something like:
>>
>>  "-Wl,-plugin-opt,thinlto-index-only=files.out -Wl,-plugin-opt,thinlto
>> -prefix-replace=path1;path2"
>>
>> for the indexing link step, the output files from the thin link will be
>> written to a directory tree with the original path substring "path1"
>> replaced with "path2", and the object names in the file emitted by
>> thinlto-index-only= ("files.out" in the above example), will have their
>> path names adjusted to the new path as well - if your native .o files use
>> the new path then this is a simple way of getting a list of input files for
>> the final link, which can be passed via @files.out to gold (note the gold
>> manpage does not document "@" but the gnu ld manpage does).
>>
>>
>>
>> I.e. something like:
>>
>> clang++ -flto=thin -O3 -c -o path1/foo.o foo.cpp
>>
>> ...
>> clang++ -flto=thin -O3 -Wl,-plugin-opt,thinlto-index-only=files.out
>> -Wl,-plugin-opt,thinlto-prefix-replace=path1;path2" foo.o ...
>>
>> clang++ -c -x ir path1/foo.o -O3 -o path2/foo.o
>> -fthinlto-index=path2/foo.o.thinlto.bc
>>
>> ...
>>
>> clang++ -flto=thin -O3 -o a.out @files.out
>>
>>
>>
>>
>>
>> But again, that should only come into play if you are linking archives of
>> bitcode files...
>>
>>
>>
>> Teresa
>>
>>
>>
>>
>>
>> On Tue, Jan 8, 2019 at 2:38 PM David Callahan via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>         I am trying to work through the usage of thinlto for distributed
>> builds.
>>
>>
>>
>>         Here is the simple thinlto usage, just add -flto=thin everywhere,
>> easy:
>>
>>
>>
>>         clang++  -flto=thin -O3 -c -o CreateWay_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else CreateWay_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Places_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Places_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o RegBounds_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else RegBounds_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o RegMng_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else RegMng_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Way2_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Way2_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o WayInit_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else WayInit_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Library.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Library.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Random.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Random.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Region_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Region_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o RegWay_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else RegWay_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Way_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Way_.cpp
>>
>>         clang++ -flto=thin -O3 -o astar.thin  CreateWay_.o Places_.o
>> RegBounds_.o RegMng_.o Way2_.o WayInit_.o Library.o Random.o Region_.o
>> RegWay_.o Way_.o
>>
>>
>>
>>
>>
>>         My understanding is for the distributed, you need to do the
>> inital compiles as above,
>>
>>         then a special link to get the thinlto index files, then compile
>> again to get
>>
>>         native files, and then a final link. This looks like the
>> following but
>>
>>         it does not work:
>>
>>
>>
>>         clang++  -flto=thin -O3 -c -o CreateWay_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else CreateWay_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Places_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Places_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o RegBounds_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else RegBounds_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o RegMng_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else RegMng_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Way2_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Way2_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o WayInit_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else WayInit_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Library.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Library.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Random.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Random.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Region_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Region_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o RegWay_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else RegWay_.cpp
>>
>>         clang++  -flto=thin -O3 -c -o Way_.o -DSPEC_CPU -DNDEBUG
>> -DSPEC_CPU_LITTLE_ENDIAN -Wno-dangling-else Way_.cpp
>>
>>         clang++ -flto=thin -O3 -o thinlto.objects
>> -Wl,-plugin-opt,thinlto-index-only=thinlto.objects CreateWay_.o
>> Places_.o RegBounds_.o RegMng_.o Way2_.o WayInit_.o Library.o Random.o
>> Region_.o RegWay_.o Way_.o
>>
>>         clang++ -c -x ir CreateWay_.o -O3 -o CreateWay_-native.o
>> -fthinlto-index=CreateWay_.o.thinlto.bc
>>
>>         clang++ -c -x ir Places_.o -O3 -o Places_-native.o
>> -fthinlto-index=Places_.o.thinlto.bc
>>
>>         clang++ -c -x ir RegBounds_.o -O3 -o RegBounds_-native.o
>> -fthinlto-index=RegBounds_.o.thinlto.bc
>>
>>         clang++ -c -x ir RegMng_.o -O3 -o RegMng_-native.o
>> -fthinlto-index=RegMng_.o.thinlto.bc
>>
>>         clang++ -c -x ir Way2_.o -O3 -o Way2_-native.o
>> -fthinlto-index=Way2_.o.thinlto.bc
>>
>>         clang++ -c -x ir WayInit_.o -O3 -o WayInit_-native.o
>> -fthinlto-index=WayInit_.o.thinlto.bc
>>
>>         clang++ -c -x ir Library.o -O3 -o Library-native.o
>> -fthinlto-index=Library.o.thinlto.bc
>>
>>         clang++ -c -x ir Random.o -O3 -o Random-native.o
>> -fthinlto-index=Random.o.thinlto.bc
>>
>>         clang++ -c -x ir Region_.o -O3 -o Region_-native.o
>> -fthinlto-index=Region_.o.thinlto.bc
>>
>>         clang++ -c -x ir RegWay_.o -O3 -o RegWay_-native.o
>> -fthinlto-index=RegWay_.o.thinlto.bc
>>
>>         clang++ -c -x ir Way_.o -O3 -o Way_-native.o
>> -fthinlto-index=Way_.o.thinlto.bc
>>
>>         clang++ -o astar CreateWay_-native.o Places_-native.o
>> RegBounds_-native.o RegMng_-native.o Way2_-native.o WayInit_-native.o
>> Library-native.o Random-native.o Region_-native.o RegWay_-native.o
>> Way_-native.o
>>
>>         Library-native.o:(.data+0x0): undefined reference to `vtable for
>> regwayobj'
>>
>>         clang-8: error: linker command failed with exit code 1 (use -v to
>> see invocation)
>>
>>         make: *** [astar] Error 1
>>
>>
>>
>>         Did I miss a step?
>>
>>
>>
>>
>>
>> _______________________________________________
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>>
>>
>>
>>
>> --
>>
>> Teresa Johnson |
>>
>>  Software Engineer |
>>
>>  tejohnson at google.com |
>>
>>
>>
>>
>>
>>
>> --
>>
>> Teresa Johnson |
>>
>>  Software Engineer |
>>
>>  tejohnson at google.com |
>>
>>
>>
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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