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On Fri, 2020-07-24 at 14:27 -0600, Jeff Law wrote:
On Fri, 2020-07-24 at 22:24 +0200, Igor Raits wrote:
> On Fri, 2020-07-24 at 13:29 -0600, Jeff Law wrote:
> > On Fri, 2020-07-24 at 19:12 +0000, Artem Tim wrote:
> > > Hi. In rare cases building packages with LTO producing binaries
> > > or
> > > libraries which have bigger size then if they have built
> > > without
> > > LTO. For example 'kitty' package:
> > >
> > > * with LTO:
> > > - koji task
> > >
https://koji.fedoraproject.org/koji/taskinfo?taskID=47762998
> > > 1.79 MB glfw-wayland.so
> > > 1.99 MB glfw-x11.so
> > > 4.78 MB fast_data_types.so
> > > 8.56 MB total
> > >
> > > * no LTO
> > > - koji
> > >
https://koji.fedoraproject.org/koji/taskinfo?taskID=47769854
> > > 1.65 MB glfw-wayland.so
> > > 1.84 MB glfw-x11.so
> > > 4.51 MB fast_data_types.so
> > > 8.00 MB total
> > >
> > > Difference is 7%. What we should do in such case? Should we
> > > disable
> > > LTO for such packages? Or there is still could be gains from
> > > faster
> > > code execution speed?
> > I'd tend to leave LTO on, but it's totally your call. Without
> > looking at the
> > binaries, sources and compiler dumps I'd hazard a guess you're
> > getting a lot of
> > cross module inlining, but very little identical code folding.
> > THe
> > former tends
> > to make things bigger, but faster. The latter tends to shrink
> > code
> > with little
> > impact on runtime performance.
>
> From what I see in this case, -ffat-lto-objects generates code that
> is
> bigger than without -flto. -flto alone generates smaller code than
> without -flto.
The fat-lto-objects bits are not used during an LTO link. They exist
solely to
cover the case where there's a .o/.a that ends up installed.
Well, I tell what I see :)
Compiling kitty with settings below produces this big
/usr/lib64/kitty/kitty/fast_data_types.so:
* Without any LTO-related flags: 4.52 MB
* With -flto: 4.30 MB
* With -flto -ffat-lto-objects: 4.79 MB
Well, I did not run compilation multiple times but don't think it will
change much.
jeff
>
- --
Igor Raits <ignatenkobrain(a)fedoraproject.org>
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