GCC 10 LTO documentation

[Kewen.Lin]

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
>