Dne 2018-02-05 18:44, Richard Biener napsal:
On February 5, 2018 12:26:58 PM GMT+01:00, Allan Sandfeld Jensen
<li...@carewolf.com> wrote:
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?
Hmm. I think you could partially link the static archive contents into
a single relocatable object. Or we could add a mode where you do a
1to1 LTO link of the objects and stop at the ltrans object files. You
could stuff those into an archive again.
I'm not sure how far Honza got partial LTO linking to work?
Parital linking of lto .o files into single non-lto .o file should work
and it will get you cross-module optimization done. The problem is that
without resolution info compiler needs to assume that all symbols
exported by object files are possibly referneced by the later
incremental link and thus the code quality will definitly not be
comparable with what you get for LTO on final binary or DSO. Still
should be better than non-lto build.
I would be curious if it is useful for you in practice.
Honza
Richard.
Best regards
'Allan Jensen
