Re: [PATCH v2] lib/string.c: implement stpcpy

Nick Desaulniers
2020-08-17T18:37:22Z

On Mon, Aug 17, 2020 at 10:14 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> On Sun, Aug 16, 2020 at 8:02 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Sun, Aug 16, 2020 at 07:22:35AM +0200, Sedat Dilek wrote:
> > > On Sun, Aug 16, 2020 at 2:19 AM 'Fangrui Song' via Clang Built Linux
> > > <clang-built-linux@googlegroups.com> wrote:
> > > >
> > > > Adding a definition without a declaration for stpcpy looks good.
> > > > Clang LTO will work.
> > > >
> > > > (If the kernel does not want to provide these routines,
> > > > is http://git.kernel.org/linus/6edfba1b33c701108717f4e036320fc39abe1912
> > > > probably wrong? (why remove -ffreestanding from the main Makefile) )
> > > >
> > >
> > > We had some many issues in arch/x86 where *FLAGS were removed or used
> > > differently and had to re-add them :-(.
> > >
> > > So if -ffreestanding is used in arch/x86 and was! used in top-level
> > > Makefile - it makes sense to re-add it back?
> > > ( I cannot speak for archs other than x86. )
> > >
> > > - Sedat -
> >
> > -ffreestanding disables _all_ builtins and libcall optimizations, which
> > is probably not desirable. If we added it back, we'd need to also go

I agree.

> > back to #define various string functions to the __builtin versions.
> >
> > Though I don't understand the original issue, with -ffreestanding,
> > sprintf shouldn't have been turned into strcpy in the first place.

Huh? The original issue for this thread is because `-ffreestanding`
*isn't* being used for most targets (oh boy, actually mixed usage by
ARCH. Looks like MIPS, m68k, superH, xtensa, and 32b x86 use it?); and
I'm not suggesting it be used.

> > 32-bit still has -ffreestanding -- I wonder if that's actually necessary
> > any more?

Fair question. Someone will have to go chase git history, since
0a6ef376d4ba covers it up. If anyone has any tricks to do so quickly;
I'd love to know. I generally checkout the commit prior, then use vim
fugitive to get git blame.

> > Why does -fno-builtin-stpcpy not work with clang LTO? Isn't that a
> > compiler bug?

Yes; Sami found a recent patch that looks to me like it may have
recently solved that bug.
https://reviews.llvm.org/D71193 which landed Dec 12 2019. The bug
report was based on
https://github.com/ClangBuiltLinux/linux/issues/416#issuecomment-472231304
(Issue reported March 8 2019). And I do recall being able to
reproduce the bug when I sent
commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")

Now that that is fixed as reported by Sami below, I don't mind sending
a revert for 5f074f3e192f that adds -fno-builtin-bcmp, because the
current implementation of bcmp isn't useful.

That said, this libcall optimization/transformation (sprintf->stpcpy)
does look useful to me. Kees, do you have thoughts on me providing
the implementation without exposing it in a header vs using
-fno-builtin-stpcpy? (I would need to add the missing EXPORT_SYMBOL,
as pointed out by 0day bot and on the github thread). I don't care
either way; I'd just like your input before sending a V+1. Maybe
better to just not implement it and never implement it?

>
> I just confirmed that adding -fno-builtin-stpcpy to KBUILD_CFLAGS does
> work with LTO as well.
>
> Sami

--
Thanks,
~Nick Desaulniers