Re: [PATCH v1 2/2] x86: Remove unnecessary overflow check from wcsnlen-sse4_1.S

Re: [PATCH v1 2/2] x86: Remove unnecessary overflow check from wcsnlen-sse4_1.S

[Sunil Pandey]

On Wed, Jun 23, 2021 at 5:42 PM H.J. Lu via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Wed, Jun 23, 2021 at 5:00 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > No bug. The way wcsnlen will check if near the end of maxlen
> > is the following macro:
> >
> >         mov     %r11, %rsi;     \
> >         subq    %rax, %rsi;     \
> >         andq    $-64, %rax;     \
> >         testq   $-64, %rsi;     \
> >         je      L(strnlen_ret)
> >
> > Which words independently of s + maxlen overflowing. So the
> > second overflow check is unnecissary for correctness and
> > just extra overhead in the common no overflow case.
> >
> > test-strlen.c, test-wcslen.c, test-strnlen.c and test-wcsnlen.c are
> > all passing
> >
> > Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>
> > ---
> > Sorry I didn't notice this earlier before my last commit. As
> > of submitting this patch
> >
> >         a775a7a3eb1e85b54af0b4ee5ff4dcf66772a1fb
> >
> > Is HEAD of master to maybe rebase so commit history isnt messy?
> >
> >
> >  sysdeps/x86_64/multiarch/strlen-vec.S | 7 -------
> >  1 file changed, 7 deletions(-)
> >
> > diff --git a/sysdeps/x86_64/multiarch/strlen-vec.S b/sysdeps/x86_64/multiarch/strlen-vec.S
> > index 439e486a43..b7657282bd 100644
> > --- a/sysdeps/x86_64/multiarch/strlen-vec.S
> > +++ b/sysdeps/x86_64/multiarch/strlen-vec.S
> > @@ -71,19 +71,12 @@ L(n_nonzero):
> >     suffice.  */
> >         mov     %RSI_LP, %R10_LP
> >         sar     $62, %R10_LP
> > -       test    %R10_LP, %R10_LP
> >         jnz     __wcslen_sse4_1
> >         sal     $2, %RSI_LP
> >  # endif
> >
> > -
> >  /* Initialize long lived registers.  */
> > -
> >         add     %RDI_LP, %RSI_LP
> > -# ifdef AS_WCSLEN
> > -/* Check for overflow again from s + maxlen * sizeof(wchar_t).  */
> > -       jbe     __wcslen_sse4_1
> > -# endif
> >         mov     %RSI_LP, %R10_LP
> >         and     $-64, %R10_LP
> >         mov     %RSI_LP, %R11_LP
> > --
> > 2.25.1
> >
>
> LGTM.
>
> Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
>
> Thanks.
>
> --
> H.J.

I would like to backport this patch to release branches.
Any comments or objections?

--Sunil