Re: [RFC][ PATCH -tip v2 3/7] Kbuild: disable gcc crossjumping

Sam Ravnborg
2009-07-17T20:28:30Z

On Sat, Jun 27, 2009 at 04:13:04PM -0400, Steven Rostedt wrote:
>
> A change like this requires an ACK from Sam Ravnborg.
>
> -- Steve

Overdue review...

I do not know the gcc option so I cannot comment on theeffect of it.

>
> >
> > diff --git a/Makefile b/Makefile
> > index 2903e13..f73b139 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -524,6 +524,10 @@ else
> > KBUILD_CFLAGS += -O2
> > endif
> >
> > +ifdef CONFIG_DISABLE_CROSSJUMP
> > +KBUILD_CFLAGS += -fno-crossjumping
> > +endif
> > +
> > include $(srctree)/arch/$(SRCARCH)/Makefile

Why do we add this option _before_ we include the arch specific Makefile?
If we do not want the arch specific Makefile to undo this then move it lower.

Also please add a comment what the effect of the disabling is.
There is no reason I should fire up info gcc to find out
what -fno-crossjumping is about.

> >
> > ifneq ($(CONFIG_FRAME_WARN),0)
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 8da7467..f88e6b8 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -673,6 +673,13 @@ config FRAME_POINTER
> > larger and slower, but it gives very useful debugging information
> > in case of kernel bugs. (precise oopses/stacktraces/warnings)
> >
> > +config DISABLE_CROSSJUMP
> > + bool "Disable cross-function jump optimization"
> > + help
> > + This build option disables cross-function jump optimization
> > + (crossjumping) of gcc. Disabling crossjumping might increase
> > + kernel binary size a little.

So the above comment tells me that is I disable crossjumping I will
increase the kernel image a little.
That is by no means a good explanation.

Please explain the potential advantage of disabling crossjumping.

Sam