Re: [Minios-devel] [UNIKRAFT PATCH v2 2/2] build: Provide LTO as build option

[Sharan Santhanam <sharan.santhanam@xxxxxxxxx>]

Hello Simon,


On 05/14/2018 03:29 PM, Simon Kuenzer wrote:
> Hey Sharan,
>
> On 14.05.2018 14:17, Sharan Santhanam wrote:
> > Hello Simon,
> >
> > Please find the comments below:
> >
> > On 05/11/2018 04:24 PM, Simon Kuenzer wrote:
> > > Introduces a build option that enables link time optmizations (LTO).
> > > When enabled, GCC is doing code optimization also while linking.
> > > Although the overall building time is increased, more efficient
> > > Unikraft images can be created.
> > >
> > > Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
> > > ---
> > >   Config.uk   | 8 ++++++++
> > >   Makefile.uk | 6 ++++++
> > >   2 files changed, 14 insertions(+)
> > >
> > > diff --git a/Config.uk b/Config.uk
> > > index eea961e..21cec9b 100644
> > > --- a/Config.uk
> > > +++ b/Config.uk
> > > @@ -78,6 +78,14 @@ config OPTIMIZE_DEADELIM
> > >   comment "Hint: Enable dropping of unused code to further reduce 
> > > target image size"
> > >       depends on OPTIMIZE_SIZE && !OPTIMIZE_DEADELIM
> > > +config OPTIMIZE_LTO
> > > +    bool "Link time optimizations (LTO)"
> > > +    default n
> > > +    help
> > > +        Enables GCC to do code optimizations during linking. This 
> > > option
> > > +        will increase overall building time but creates more efficient
> > > +        Unikraft binaries.
> > > +
> > gcc support stable LTO option since version 4.7. We may have to add 
> > checks to verify if the LTO option can be enabled with the gcc version.
> Yes, this is true. For now we are not doing any checks about the used 
> GCC version. ARM's patch series is going to introducing a GCC version 
> checking subsystem. We could make use of this one as soon this series 
> is in. We can then force the user to use GCC >= 4.7 for compiling with 
> LTO.
Yes, makes sense. We can add that support once that patch is merged.

> > >   config DEBUG_SYMBOLS
> > >       bool "Debugging information"
> > >       default n
> > > diff --git a/Makefile.uk b/Makefile.uk
> > > index c540ada..1964563 100644
> > > --- a/Makefile.uk
> > > +++ b/Makefile.uk
> > > @@ -34,6 +34,12 @@ CFLAGS-$(OPTIMIZE_DEADELIM)        += 
> > > -fdata-sections -ffunction-sections
> > >   CXXFLAGS-$(OPTIMIZE_DEADELIM)      += -fdata-sections 
> > > -ffunction-sections
> > >   LDFLAGS-$(OPTIMIZE_DEADELIM)       += -Wl,--gc-sections
> > > +# LTO requires the compiler flags to be handed over also for linking
> > > +CFLAGS-$(OPTIMIZE_LTO)             += -flto
> > > +CXXFLAGS-$(OPTIMIZE_LTO)           += -flto
> > > +LIBLDFLAGS-$(OPTIMIZE_LTO)         += $(CFLAGS) $(CFLAGS-y)
> > > +LDFLAGS-$(OPTIMIZE_LTO)            += $(CFLAGS) $(CFLAGS-y)
> > > +
> > >   ifneq ($(DEBUG_SYMBOLS),y)
> > >   CFLAGS                             += -g0
> > >   CXXFLAGS                           += -g0
> > The rest of the patch is fine.
> >
> >
> > Thanks & Regards
> > Sharan Santhanam
> Thanks,
>
> Simon
Thanks & Regards
Sharan Santhanam


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