Subject: Re: [PATCH 2/5] kbuild: allow archs to select build for link dead code/data elimination

From: Alan Modra <[email-masked]>
Date: Mon, 8 Aug 2016 09:19:47 +0930

On Sun, Aug 07, 2016 at 10:26:19PM +0200, Arnd Bergmann wrote:
> On Sunday, August 7, 2016 7:27:39 PM CEST Alan Modra wrote:
> > 
> > If it can, then Nicholas' patch should be:
> > 
> >         *(.text.hot .text.hot.*) *(.text.unlikely .text.unlikely.*) *(.text .text.*)
> > 
> > If you can't put .text.fixup too far away then you may as well just use
> > 
> >         *(.text .text.*)
> 
> I tried this version:
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index b1f8828e9eac..fc210dacac9a 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -438,7 +438,9 @@
>   * during second ld run in second ld pass when generating System.map */
>  #define TEXT_TEXT							\
>  		ALIGN_FUNCTION();					\
> -		*(.text.hot .text .text.fixup .text.unlikely .text.*)	\
> +		*(.text.hot .text.hot.*)				\
> +		*(.text.unlikely .text.fixup .text.unlikely.*)		\
> +		*(.text .text.*)					\
>  		*(.ref.text)						\
>  	MEM_KEEP(init.text)						\
>  	MEM_KEEP(exit.text)						\
> 
> but that failed to link an allyesconfig kernel because of references
> from .fixup to .text.*. Trying your version now:

Well then, that proves you can't put .text.fixup too far aways from
the associated input section.

> *(.text.hot .text.hot.*) *(.text.unlikely .text.unlikely.*) *(.text .text.*)

Which means this is guaranteed to fail when you test it properly using
gcc's profiling options, in order to generate .text.hot* and/or
.text.unlikely* sections.

It seems to me the right thing to do would be to change kernel asm to
generate .text.foo.fixup for any .text.foo section.  A gas feature
available with binutils-2.26 enabled by --sectname-subst might help
with implementing that.

-- 
Alan Modra
Australia Development Lab, IBM