Re: [PATCH] x86-64: Use notl in EVEX strcmp [BZ #28646]

Re: [PATCH] x86-64: Use notl in EVEX strcmp [BZ #28646]

[Sunil Pandey]

On Fri, Dec 3, 2021 at 7:50 PM Noah Goldstein via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Fri, Dec 3, 2021 at 9:24 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > From: Noah Goldstein <goldstein.w.n@gmail.com>
> >
> > Must use notl %edi here as lower bits are for CHAR comparisons
> > potentially out of range thus can be 0 without indicating mismatch.
> > This fixes BZ #28646.
> >
> > Co-Authored-By: H.J. Lu <hjl.tools@gmail.com>
> > ---
> >  string/test-strcmp.c                   | 28 ++++++++++++++++++++++++++
> >  sysdeps/x86_64/multiarch/strcmp-evex.S | 14 +++++++------
> >  2 files changed, 36 insertions(+), 6 deletions(-)
> >
> > diff --git a/string/test-strcmp.c b/string/test-strcmp.c
> > index 77ae73110f..42178a260b 100644
> > --- a/string/test-strcmp.c
> > +++ b/string/test-strcmp.c
> > @@ -23,6 +23,7 @@
> >  # define TEST_NAME "strcmp"
> >  #endif
> >  #include "test-string.h"
> > +#include <support/test-driver.h>
> >
> >  #ifdef WIDE
> >  # include <wchar.h>
> > @@ -390,6 +391,32 @@ check2 (void)
> >         }
> >  }
> >
> > +static void
> > +check3 (void)
> > +{
> > +  size_t size = 0xd000 + 0x4000;
> > +  CHAR *s1, *s2;
> > +  CHAR *buffer1 = mmap (NULL, size, PROT_READ | PROT_WRITE,
> > +                       MAP_PRIVATE | MAP_ANON, -1, 0);
> > +  CHAR *buffer2 = mmap (NULL, size, PROT_READ | PROT_WRITE,
> > +                       MAP_PRIVATE | MAP_ANON, -1, 0);
> > +  if (buffer1 == MAP_FAILED || buffer1 == MAP_FAILED)
> > +    error (EXIT_UNSUPPORTED, errno, "mmap failed");
> > +
> > +  s1 = (CHAR *) (buffer1 + 0x8f8 / sizeof (CHAR));
> > +  s2 = (CHAR *) (buffer2 + 0xcff3 / sizeof (CHAR));
> > +
> > +  STRCPY(s1, L("/export/redhat/rpms/BUILD/java-1.8.0-openjdk-1.8.0.312.b07-2.fc35.x86_64/openjdk/langtools/src/share/classes/com/sun/tools/doclets/internal/toolkit/util/PathDocFileFactory.java"));
> > +  STRCPY(s2, L("/export/redhat/rpms/BUILD/java-1.8.0-openjdk-1.8.0.312.b07-2.fc35.x86_64/openjdk/langtools/src/share/classes/com/sun/tools/doclets/internal/toolkit/taglets/ThrowsTaglet.java"));
> > +
> > +  int exp_result = SIMPLE_STRCMP (s1, s2);
> > +  FOR_EACH_IMPL (impl, 0)
> > +    check_result (impl, s1, s2, exp_result);
> > +
> > +  munmap ((void *) buffer1, size);
> > +  munmap ((void *) buffer2, size);
> > +}
> > +
> >  int
> >  test_main (void)
> >  {
> > @@ -398,6 +425,7 @@ test_main (void)
> >    test_init ();
> >    check();
> >    check2 ();
> > +  check3 ();
> >
> >    printf ("%23s", "");
> >    FOR_EACH_IMPL (impl, 0)
> > diff --git a/sysdeps/x86_64/multiarch/strcmp-evex.S b/sysdeps/x86_64/multiarch/strcmp-evex.S
> > index 0bea318abd..b68e1f7a1e 100644
> > --- a/sysdeps/x86_64/multiarch/strcmp-evex.S
> > +++ b/sysdeps/x86_64/multiarch/strcmp-evex.S
> > @@ -646,12 +646,13 @@ L(loop_cross_page):
> >            in YMM3 and 32 bytes at VEC_SIZE(%rdx, %r10).  */
> >         VPCMP   $0, VEC_SIZE(%rdx, %r10), %YMM3, %k3{%k4}
> >         kmovd   %k3, %edi
> > +    /* Must use notl %edi here as lower bits are for CHAR
> > +          comparisons potentially out of range thus can be 0 without
> > +          indicating mismatch.  */
> > +       notl    %edi
> >  # ifdef USE_AS_WCSCMP
> >         /* Don't use subl since it is the upper 8 bits of EDI below.  */
> > -       notl    %edi
> >         andl    $0xff, %edi
> > -# else
> > -       incl    %edi
> >  # endif
> >
> >  # ifdef USE_AS_WCSCMP
> > @@ -733,12 +734,13 @@ L(loop_cross_page_2_vec):
> >            in YMM1 and 32 bytes at (VEC_SIZE * 3)(%rdx, %r10).  */
> >         VPCMP   $0, (VEC_SIZE * 3)(%rdx, %r10), %YMM1, %k3{%k4}
> >         kmovd   %k3, %edi
> > +       /* Must use notl %edi here as lower bits are for CHAR
> > +          comparisons potentially out of range thus can be 0 without
> > +          indicating mismatch.  */
> > +       notl    %edi
> >  # ifdef USE_AS_WCSCMP
> >         /* Don't use subl since it is the upper 8 bits of EDI below.  */
> > -       notl    %edi
> >         andl    $0xff, %edi
> > -# else
> > -       incl    %edi
> >  # endif
> >
> >  # ifdef USE_AS_WCSCMP
> > --
> > 2.33.1
> >
>
> LGTM

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

--Sunil