On Tue, 2021-06-22 at 16:52 +0100, Jonathan Wakely wrote: > On Tue, 22 Jun 2021 at 16:14, Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> > wrote: > > > > On Tue, 2021-06-22 at 14:02 +0100, Jonathan Wakely via Gcc-help > > wrote: > > > On Mon, 21 Jun 2021 at 14:42, Chris S wrote: > > > > Is it possible to build static libraries that have LTO > > > > optimizations > > > > applied to the object code they contain (that is, all the code > > > > in > > > > that > > > > library is optimized together with LTO), but when built together > > > > into a > > > > final binary, no additional LTO is performed? We have several > > > > large, > > > > static libraries that are mostly unrelated, and are looking for > > > > ways > > > > to > > > > reduce a massive increase in build times after moving to g++10, > > > > where > > > > almost 80% of the time is spent in LTO. Having optimized > > > > individual > > > > libraries but not a global optimized binary might be a > > > > reasonable > > > > tradeoff. Is this possible? > > > > I tried "gcc -Wl,--whole-archive lib.a -Wl,-r -nostdlib -flinker- > > output=nolto-rel -o lib.o", which seems working for a very simple > > testcase. But I'm not sure if it's really correct. > > > > > A static library is just an archive file containing object files. > > > No > > > LTO is done "between" those files, they're just added to the > > > archive. > > > That's because creating a static library is not "linking". > > > > > > If you do not use -flto when doing the final link, you might as > > > well > > > not use -flto when compiling the objects that go into your static > > > library, because otherwise you're adding all the extra LTO > > > information > > > to the objects and then ignoring it when linking. > > > > It's not "extra" information. Object files created with -flto only > > contains GIMPLE which can only be linked with LTO enabled, the > > "normal" > > object code is not there. For example, if lib.a contains several > > object > > files compiled with -flto: > > > > cc main.c lib.a -flto # main.c is compiled with LTO, and LTO > > will > > # run for main.o and object files in > > lib.a > > > > cc main.c lib.a # main.c is not compiled with LTO, but > > LTO > > # will still run for object files in > > lib.a > > > > cc main.c lib.a -fno-lto # error, linker will say > > # "plugin needed to handle lto object" > > > > (Unless -ffat-lto-objects is used.) > > Thanks for the correction. So then maybe that's what Chris wants: the > objects in the static libs can be LTO'd but the objects in the main > executable won't be. And if it's still too slow, only enable LTO for > some static libs, where it gives significant performance gain. I think he means: cc main.c lib1.a lib2.a lib3.a lib4.a lib[1-4].a are all built with -flto. In this case, unfortunately gcc will still run a whole LTO pass for all object files in lib[1-4].a. But Chris wants 4 LTO passes, each for one static library and abandon the optimization oppertunity crossing library files. -- Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
