[Bug target/56533] Linker problem on avr with lto and main function inside archive

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56533



--- Comment #1 from Richard Biener rguenth at gcc dot gnu.org 2013-03-05 
12:38:51 UTC ---

+ /usr/bin/avr-gcc -mmcu=atmega328p -Os -flto -fwhole-program realmain.o

main.a



please provide the console output produced when adding -v to the above

commandline.  I suspect that you fall foul of using -fwhole-program without

making the whole program visible to gcc via -flto by means of not using

a compiler that can handle LTO of static archives.



In general, don't use -fwhole-program with -flto, -flto is smart enough

to discover things more intelligently.

[Bug target/56533] Linker problem on avr with lto and main function inside archive

[matthijs at stdin dot nl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56533



--- Comment #2 from Matthijs Kooijman matthijs at stdin dot nl 2013-03-05 
12:55:47 UTC ---

+ /usr/bin/avr-gcc -v -mmcu=atmega328p -Os -flto -fwhole-program realmain.o

main.a

Using built-in specs.

COLLECT_GCC=/usr/bin/avr-gcc

COLLECT_LTO_WRAPPER=/usr/lib/gcc/avr/4.7.2/lto-wrapper

Target: avr

Configured with: ../src/configure -v --enable-languages=c,c++ --prefix=/usr/lib

--infodir=/usr/share/info --mandir=/usr/share/man --bindir=/usr/bin

--libexecdir=/usr/lib --libdir=/usr/lib --enable-shared --with-system-zlib

--enable-long-long --enable-nls --without-included-gettext --disable-libssp

--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=avr

Thread model: single

gcc version 4.7.2 (GCC) 

COMPILER_PATH=/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/:/usr/lib/gcc/avr/4.7.2/../../../avr/bin/

LIBRARY_PATH=/usr/lib/gcc/avr/4.7.2/avr5/:/usr/lib/gcc/avr/4.7.2/../../../avr/lib/avr5/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/4.7.2/../../../avr/lib/

COLLECT_GCC_OPTIONS='-v' '-mmcu=atmega328p' '-Os' '-flto' '-fwhole-program'

 /usr/lib/gcc/avr/4.7.2/collect2 -flto -m avr5 -Tdata 0x800100

/usr/lib/gcc/avr/4.7.2/../../../avr/lib/avr5/crtm328p.o

-L/usr/lib/gcc/avr/4.7.2/avr5 -L/usr/lib/gcc/avr/4.7.2/../../../avr/lib/avr5

-L/usr/lib/gcc/avr/4.7.2 -L/usr/lib/gcc/avr/4.7.2/../../../avr/lib realmain.o

main.a -lgcc -lc -lgcc

 /usr/bin/avr-gcc @/tmp/ccYrSTvi.args

Using built-in specs.

COLLECT_GCC=/usr/bin/avr-gcc

Target: avr

Configured with: ../src/configure -v --enable-languages=c,c++ --prefix=/usr/lib

--infodir=/usr/share/info --mandir=/usr/share/man --bindir=/usr/bin

--libexecdir=/usr/lib --libdir=/usr/lib --enable-shared --with-system-zlib

--enable-long-long --enable-nls --without-included-gettext --disable-libssp

--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=avr

Thread model: single

gcc version 4.7.2 (GCC) 

COLLECT_GCC_OPTIONS='-c' '-v' '-mmcu=atmega328p' '-Os' '-fwhole-program'

'-fltrans-output-list=/tmp/ccZEu3t3.ltrans.out' '-fwpa'

 /usr/lib/gcc/avr/4.7.2/lto1 -quiet -dumpbase realmain.o -mmcu=atmega328p

-auxbase realmain -Os -version -fwhole-program

-fltrans-output-list=/tmp/ccZEu3t3.ltrans.out -fwpa @/tmp/ccOsOe32

GNU GIMPLE (GCC) version 4.7.2 (avr)

compiled by GNU C version 4.7.2, GMP version 5.0.5, MPFR version

3.1.0-p10, MPC version 0.9

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

GNU GIMPLE (GCC) version 4.7.2 (avr)

compiled by GNU C version 4.7.2, GMP version 5.0.5, MPFR version

3.1.0-p10, MPC version 0.9

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

COMPILER_PATH=/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/:/usr/lib/gcc/avr/4.7.2/../../../avr/bin/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/:/usr/lib/gcc/avr/4.7.2/../../../avr/bin/

LIBRARY_PATH=/usr/lib/gcc/avr/4.7.2/avr5/:/usr/lib/gcc/avr/4.7.2/../../../avr/lib/avr5/:/usr/lib/gcc/avr/4.7.2/:/usr/lib/gcc/avr/4.7.2/../../../avr/lib/

COLLECT_GCC_OPTIONS='-c' '-v' '-mmcu=atmega328p' '-Os' '-fwhole-program'

'-fltrans-output-list=/tmp/ccZEu3t3.ltrans.out' '-fwpa'

 /usr/bin/avr-gcc @/tmp/ccoysJBM.args

Using built-in specs.

COLLECT_GCC=/usr/bin/avr-gcc

Target: avr

Configured with: ../src/configure -v --enable-languages=c,c++ --prefix=/usr/lib

--infodir=/usr/share/info --mandir=/usr/share/man --bindir=/usr/bin

--libexecdir=/usr/lib --libdir=/usr/lib --enable-shared --with-system-zlib

--enable-long-long --enable-nls --without-included-gettext --disable-libssp

--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=avr

Thread model: single

gcc version 4.7.2 (GCC) 

COLLECT_GCC_OPTIONS='-c' '-v' '-mmcu=atmega328p' '-Os' '-fwhole-program'

'-fltrans-output-list=/tmp/ccZEu3t3.ltrans.out' '-fltrans' '-o'

'/tmp/ccZEu3t3.ltrans0.ltrans.o'

 /usr/lib/gcc/avr/4.7.2/lto1 -quiet -dumpbase ccZEu3t3.ltrans0.o

-mmcu=atmega328p -auxbase-strip /tmp/ccZEu3t3.ltrans0.ltrans.o -Os -version

-fwhole-program -fltrans-output-list=/tmp/ccZEu3t3.ltrans.out -fltrans

@/tmp/ccAudUT3 -o /tmp/ccyDScYi.s

GNU GIMPLE (GCC) version 4.7.2 (avr)

compiled by GNU C version 4.7.2, GMP version 5.0.5, MPFR version

3.1.0-p10, MPC version 0.9

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

GNU GIMPLE (GCC) version 4.7.2 (avr)

compiled by GNU C version 4.7.2, GMP version 5.0.5, MPFR version

3.1.0-p10, MPC version 0.9

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

COLLECT_GCC_OPTIONS='-c' '-v' '-mmcu=atmega328p' '-Os' '-fwhole-program'

'-fltrans-output-list=/tmp/ccZEu3t3.ltrans.out' '-fltrans' '-o'

'/tmp/ccZEu3t3.ltrans0.ltrans.o'

 /usr/lib/gcc/avr/4.7.2/../../../avr/bin/as -mmcu=atmega328p -mno-skip-bug -o

/tmp/ccZEu3t3.ltrans0.ltrans.o 

[Bug target/56533] Linker problem on avr with lto and main function inside archive

[matthijs at stdin dot nl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56533



--- Comment #3 from Matthijs Kooijman matthijs at stdin dot nl 2013-03-05 
13:06:18 UTC ---

Seems I made a wrong observation in my original report: When I link main.o

instead of main.a, the problem does _not_ go away. In fact, I can remove a few

more flags then, while still keeping the problem around:



$ ./do

+ rm -f main.a main.o realmain.o

+ /usr/bin/avr-gcc -c main.c -o main.o

+ /usr/bin/avr-gcc -c -flto realmain.c -o realmain.o

+ /usr/bin/avr-gcc -flto -fwhole-program realmain.o main.o

main.o: In function `main':

main.c:(.text+0x8): undefined reference to `realmain'

collect2: error: ld returned 1 exit status



main.c and realmain.c are the same as before.



However, adding -flto to the main.c compilation makes the problem disappear

again. I suspect that this means that without -flto, main.o is passed straight

to the linker and with -flto it is included in link-time optimization, which

would mean your previous analysis still holds.



$ ./do

+ rm -f *.a main.o realmain.o

+ /usr/bin/avr-gcc -c -flto main.c -o main.o

+ /usr/bin/avr-gcc -c -flto realmain.c -o realmain.o

+ /usr/bin/avr-gcc -flto -fwhole-program realmain.o main.o

[Bug target/56533] Linker problem on avr with lto and main function inside archive

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56533



Richard Biener rguenth at gcc dot gnu.org changed:



   What|Removed |Added



 Status|UNCONFIRMED |RESOLVED

 Resolution||INVALID



--- Comment #4 from Richard Biener rguenth at gcc dot gnu.org 2013-03-05 
13:41:01 UTC ---

Yes.  You are not using a linker-plugin which would end up basically

ignoring -fwhole-program.  With -fwhole-program you are basically lieing to

GCC.

[Bug target/56533] Linker problem on avr with lto and main function inside archive

[matthijs at stdin dot nl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56533



Matthijs Kooijman matthijs at stdin dot nl changed:



   What|Removed |Added



 Status|RESOLVED|UNCONFIRMED

 Resolution|INVALID |



--- Comment #5 from Matthijs Kooijman matthijs at stdin dot nl 2013-03-05 
14:38:36 UTC ---

Just for future reference, the problem here seems to be that I'm using

-fwhole-program, but the GCC LTO cannot actually look at the whole program. In

particular, .a archives and .o object files that were compiled without -flto,

are passed directly to the linker and not included in LTO. Since

-fwhole-program makes the compiler assume that all files that are included in

LTO compose the whole program, the compiler removes symbols that look unused,

but then turn up missing in the final link.



So, I shouldn't have been using -fwhole-program, or I should be aware of the

above and set externally_visible attributes as needed if I insist on using

-fwhole-program.



Ideally, the compiler would ask the linker about which symbols are used in

these non-LTO objects, which is done by -fuse-linker-plugin (which is implied

by -flto). However, on the AVR target, it seems there is no linker plugin (at

least not in this particular case), which means that without -fwhole-program,

the compiler cannot optimize as much (since it has to assume that all symbols

are externally visible).

[Bug target/56533] Linker problem on avr with lto and main function inside archive

[matthijs at stdin dot nl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56533



Matthijs Kooijman matthijs at stdin dot nl changed:



   What|Removed |Added



 Status|UNCONFIRMED |RESOLVED

 Resolution||INVALID



--- Comment #6 from Matthijs Kooijman matthijs at stdin dot nl 2013-03-05 
14:40:15 UTC ---

w00ps, didn't mean to change the resolution.