x86: Fix bug in strchrnul-evex512 [BZ #32078]

Issue was we were expecting not matches with CHAR before the start of
the string in the page cross case.

The check code in the page cross case:
```
    and    $0xffffffffffffffc0,%rax
    vmovdqa64 (%rax),%zmm17
    vpcmpneqb %zmm17,%zmm16,%k1
    vptestmb %zmm17,%zmm17,%k0{%k1}
    kmovq  %k0,%rax
    inc    %rax
    shr    %cl,%rax
    je     L(continue)
```

expects that all characters that neither match null nor CHAR will be
1s in `rax` prior to the `inc`. Then the `inc` will overflow all of
the 1s where no relevant match was found.

This is incorrect in the page-cross case, as the
`vmovdqa64 (%rax),%zmm17` loads from before the start of the input
string.

If there are matches with CHAR before the start of the string, `rax`
won't properly overflow.

The fix is quite simple. Just replace:

```
    inc    %rax
    shr    %cl,%rax
```
With:
```
    sar    %cl,%rax
    inc    %rax
```

The arithmetic shift will clear any matches prior to the start of the
string while maintaining the signbit so the 1s can properly overflow
to zero in the case of no matches.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
(cherry picked from commit 7da08862471dfec6fdae731c2a5f351ad485c71f)

diff --git a/string/test-strchr.c b/string/test-strchr.c
index 933fc0bbbada98910015f18e25270aa38233bdbb..2bfcf901fa38d2f9c2ea60a192265cce1ff64cda 100644 (file)
--- a/string/test-strchr.c
+++ b/string/test-strchr.c
@@ -248,6 +248,69 @@ check1 (void)
     check_result (impl, s, c, exp_result);
 }
 
+static void
+check2 (void)
+{
+  CHAR *s = (CHAR *) (buf1 + getpagesize () - 4 * sizeof (CHAR));
+  CHAR *s_begin = (CHAR *) (buf1 + getpagesize () - 64);
+#ifndef USE_FOR_STRCHRNUL
+  CHAR *exp_result = NULL;
+#else
+  CHAR *exp_result = s + 1;
+#endif
+  CHAR val = 0x12;
+  for (; s_begin != s; ++s_begin)
+    *s_begin = val;
+
+  s[0] = val + 1;
+  s[1] = 0;
+  s[2] = val + 1;
+  s[3] = val + 1;
+
+  {
+    FOR_EACH_IMPL (impl, 0)
+      check_result (impl, s, val, exp_result);
+  }
+  s[3] = val;
+  {
+    FOR_EACH_IMPL (impl, 0)
+      check_result (impl, s, val, exp_result);
+  }
+  exp_result = s;
+  s[0] = val;
+  {
+    FOR_EACH_IMPL (impl, 0)
+      check_result (impl, s, val, exp_result);
+  }
+
+  s[3] = val + 1;
+  {
+    FOR_EACH_IMPL (impl, 0)
+      check_result (impl, s, val, exp_result);
+  }
+
+  s[0] = val + 1;
+  s[1] = val + 1;
+  s[2] = val + 1;
+  s[3] = val + 1;
+  s[4] = val;
+  exp_result = s + 4;
+  {
+    FOR_EACH_IMPL (impl, 0)
+      check_result (impl, s, val, exp_result);
+  }
+  s[4] = 0;
+#ifndef USE_FOR_STRCHRNUL
+  exp_result = NULL;
+#else
+  exp_result = s + 4;
+#endif
+  {
+    FOR_EACH_IMPL (impl, 0)
+      check_result (impl, s, val, exp_result);
+  }
+}
+
 int
 test_main (void)
 {
@@ -256,7 +319,7 @@ test_main (void)
   test_init ();
 
   check1 ();
-
+  check2 ();
   printf ("%20s", "");
   FOR_EACH_IMPL (impl, 0)
     printf ("\t%s", impl->name);

diff --git a/sysdeps/x86_64/multiarch/strchr-evex-base.S b/sysdeps/x86_64/multiarch/strchr-evex-base.S
index 7209435caf26f0f3e488cfbb630a5aa077570f28..da6d0eafbb41fd703e029d5dc0acf26de2dde2bb 100644 (file)
--- a/sysdeps/x86_64/multiarch/strchr-evex-base.S
+++ b/sysdeps/x86_64/multiarch/strchr-evex-base.S
@@ -124,13 +124,13 @@ L(page_cross):
        VPCMPNE %VMM(1), %VMM(0), %k1
        VPTEST  %VMM(1), %VMM(1), %k0{%k1}
        KMOV    %k0, %VRAX
-# ifdef USE_AS_WCSCHR
+       sar     %cl, %VRAX
+#ifdef USE_AS_WCSCHR
        sub     $VEC_MATCH_MASK, %VRAX
-# else
+#else
        inc     %VRAX
-# endif
+#endif
        /* Ignore number of character for alignment adjustment.  */
-       shr     %cl, %VRAX
        jz      L(align_more)
 
        bsf     %VRAX, %VRAX