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Resolving LTO symbols for static library boundary


Hello GCC

In trying to make it possible to use LTO for distro-builds of Qt, I have again 
hit the problem of static libraries. In Qt in general we for LTO rely on a 
library boundary, where LTO gets resolved when generating the library but no 
LTO-symbols are exported in the shared library. This ensure the library has a 
well defined binary compatible interface and gets LTO optimizations within 
each library. For some private libraries we use static libraries however, 
because we don't need binary compatibility, but though we don't need BC 
between Qt versions, the libraries should still be linkable with different gcc 
versions (and with different compilers). However when LTO is enabled the 
static libraries will contain definitions that depend on a single gcc version 
making it unsuitable for distribution.

One solution is to enable fat-lto object files for static libraries but that 
is both a waste of space and compile time, and disables any LTO optimization 
within the library. Ideally I would like to have the static library do LTO 
optimizations internally just like a shared library, but then exported as 
static library.

I suspect this is more of gcc task than ar/ld task, as it basically boils down 
to gcc doing for a static library what it does for shared library, but maybe 
export the result as a single combined .o file, that can then be ar'ed into a 
compatible static library.

Is this possible?

Best regards
'Allan Jensen


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