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SubjectRe: [RFC 1/8] x86: objtool: use asm macro for better compiler decisions
On Tue, May 15, 2018 at 02:53:52PM -0700, Nadav Amit wrote:
> Josh Triplett <josh@joshtriplett.org> wrote:
>
> > On Tue, May 15, 2018 at 07:11:08AM -0700, Nadav Amit wrote:
> >> GCC considers the number of statements in inlined assembly blocks,
> >> according to new-lines and semicolons, as an indication to the cost of
> >> the block in time and space. This data is distorted by the kernel code,
> >> which puts information in alternative sections. As a result, the
> >> compiler may perform incorrect inlining and branch optimizations.
> >>
> >> In the case of objtool, this distortion is extreme, since anyhow the
> >> annotations of objtool are discarded during linkage.
> >>
> >> The solution is to set an assembly macro and call it from the inlinedv
> >> assembly block. As a result GCC considers the inline assembly block as
> >> a single instruction.
> >>
> >> This patch slightly increases the kernel size.
> >>
> >> text data bss dec hex filename
> >> 18126699 10066728 2936832 31130259 1db0293 ./vmlinux before
> >> 18126824 10067268 2936832 31130924 1db052c ./vmlinux after (+665)
> >
> > With what kernel config? What's the text size change for an allnoconfig?
>
> I use my custom config and to include the drivers in the image. Other
> configs were not too appropriate: defconfig does not enable paravirt and
> allyesconfig is too heavy.
>
> There is no effect with allnoconfig on size:
>
> text data bss dec hex filename
> 1018441 230148 1215604 2464193 2599c1 ./vmlinux
> 1018441 230148 1215604 2464193 2599c1 ./vmlinux
>
> I think it is expected since CONFIG_STACK_VALIDATION is off. No?

Thanks for checking; fine by me then. :)

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