undefined reference cross compiling static libraries with LTO under GCC

Question

I am attempting to use GCC 4.9.2 to cross compile an application from Linux (x86_64-pc-linux-gnu) for Windows (x86_64-w64-mingw32).

When building targets that link against static libraries and also using link-time optimisation I get undefined reference errors from the linker for all symbols the target uses from the library.

eg, building bar.a from bar.cpp

int bar (void) {return 42;}

and linking with foo.cpp

extern int bar (void);
int main (int, char**) {bar ();}

using the command line

x86_64-w64-mingw32-g++ -flto -o foo.o -c foo.cpp
x86_64-w64-mingw32-g++ -flto -o bar.o -c bar.cpp
x86_64-w64-mingw32-gcc-ar rc bar.a bar.o
x86_64-w64-mingw32-gcc-ranlib bar.a
x86_64-w64-mingw32-g++ -flto -fuse-linker-plugin foo.o bar.a -o foo

Results in the error

/tmp/ccc3Twsc.lto.o:foo.o:(.text+0x15): undefined reference to `bar()'
collect2: error: ld returned 1 exit status

From above:

• I am using the gcc-wrappers for ar/ranlib
• there are no external dependencies
• all files are compiled with the same options

I have tried using various combinations of -fuse-linker-plugin, gcc-ar vs ar, symbol visibility options, optimsations, etc but I can't get it to link correctly without turning off LTO.

All targets build correctly under the native compiler (x86_64 Linux).

Is there something obvious I'm missing here?

Pane

Answer 1

I'm able to reproduce this linking problem on Mingw32-gcc 4.9.2 under Win7 64-bit. However, I did get it to link successfully by adding -ffat-lto-objects as a workaround:

g++ -flto -o foo.o -c foo.cpp
g++ -flto -ffat-lto-objects -o bar.o -c bar.cpp
ar rc bar.a bar.o
g++ -flto -o foo.exe foo.o bar.a

Dysplasia

Thanks for the confirmation. Unfortunately -ffat-lto-objects allows the linker to fall back to the standard method of linking and more or less negates LTO for my purposes (using lots of convenience libraries). – Pane

Also, note that gcc-ar (as opposed to ar) will likely be a requirement to confirm as ar does not understand the LTO/GIMPLE object sections. gcc-{ar,ranlib,nm} are temporary wrappers that preserve/understand the relevant sections. – Pane