Re: GCC 10 LTO documentation

[Chris S <cs2991@gmail.com>]

Many thanks! This was extremely helpful, and reduced link time for that
binary by an hour. :)

Chris

On Wed, Jun 23, 2021 at 5:05 AM Kewen.Lin <linkw@linux.ibm.com> wrote:

> Hi,
>
> on 2021/6/21 下午9:41, Chris S via Gcc-help wrote:
> > Are the capabilities and/or limitations of GCC10 LTO documented anywhere?
> > I understand it only at a high level but not with details, and am having
> > trouble finding any current information that describes it very clearly.
> > Some questions came up recently that I'd like to be able to answer, but
> it
> > boils down to this:
> >
> > 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.
>
> If the compiling time is the concern, maybe it's worth to trying with
> LTO parallel build, such as -flto=auto or -flto=n.
>
>
> https://gcc.gnu.org/onlinedocs/gcc-10.3.0/gcc/Optimize-Options.html#Optimize-Options
>
> BR,
> Kewen
>
> > Having optimized individual
> > libraries but not a global optimized binary might be a reasonable
> > tradeoff.  Is this possible?
> >
> > I don't have a good mental model of when "extra information" (GIMPLE) is
> > merely included in the code for later use, and when that GIMPLE
> information
> > is actually used to perform LTO optimizations.  (My suspicion is that
> it's
> > only when building the final binary.)  However, if we can build static
> > libraries that are already optimized within themselves, a  hint of what
> > command line options to use would also be very appreciated.
> >
> > Thanks.
> > Chris
> >
>
>