On Mon, 14 Jan 2019, Geert Uytterhoeven wrote:
> On Fri, Dec 21, 2018 at 7:21 AM Finn Thain <[email protected]> wrote:
> > On Thu, 20 Dec 2018, John Paul Adrian Glaubitz wrote:
> > > On 12/20/18 12:16 AM, Finn Thain wrote:
> > > > Andreas said the error happened when the new compiler expanded a
> > > > __bultin_strcmp call to a strcmp call.
> > > > https://lore.kernel.org/lkml/[email protected]/
> > > >
> > > > The new compiler seems to assume that a strcmp symbol exists in
> > > > the final link. I don't see how that kind of assumption is valid
> > > > here.
> > > >
> > > > So I think the real bug is the lack of -ffreestanding. That
> > > > omission allows the compiler to assume that libc is available in
> > > > the final link. (At least, I imagine that's what the compiler
> > > > authors had in mind.) Anyway, the use of -ffreestanding certainly
> > > > avoids this optimization.
>
> Yeah, during the kernel history, several other architectures started
> using -ffreestanding, to avoid similar replacements behind our back. So
> there is definitely prior art for starting to use that.
>
These commits are interesting.
72fbfb260197 [MIPS] Fix optimization for size build.
fec468b0c9e0 [PATCH] uml: add -ffreestanding to CFLAGS
6edfba1b33c7 [PATCH] x86_64: Don't define string functions to builtin
b2444d34a0e7 [XTENSA] Add freestanding option to CFLAGS
I get the impression that people were reluctant to use -ffreestanding...
> >
> > This patch seems to work fine but may have implications for both 680x0
> > and Coldfire. It will make some optimizations unavailable but I
> > haven't tried to measure that effect. It could be a hard sell if it
> > harms performance.
>
> With gcc 7.3.0, it increases the kernel image size for atari_defconfig
> by ca. 300 bytes (which is an 0.01% increase). But -ffreestanding allows
> to switch to gcc 8.2.0, which reduces the same kernel by ca 7.5 KiB
> again.
>
If there is a performance drop caused by -ffreestanding, perhaps some
optimizations be done using semantic patches instead. I could imagine that
most sensible optimizations involving libc calls would also make sense
when written as high-level program transformations.
> Finn, can you please submit your patch with a proper SoB?
OK. -ffreestanding implies -fno-builtin, so we may need more of these:
#define foo(z) __builtin_foo(z)
...depending on the optimizations available in gcc 8.
--
> Thanks!
>
>
> Gr{oetje,eeting}s,
>
> Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 --
> [email protected]
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like
> that.
> -- Linus Torvalds
>
