grep: don't check extensively for invalid prefix bytes unless -P

Problem reported by Jim Meyering in: http://bugs.gnu.org/18454#56
* src/grep.c (grep): After the first buffer is checked, leave the
file-type checker in TEXTBIN_UNKNOWN state only when -P is used.
Only the -P matcher has performance problems with checking binary
data that make it worthwhile to check every prefix input byte so
the -P matcher's TEXTBIN_UNKNOWN optimizations can come into play.
Other matchers can simply check the data directly, and using
TEXTBIN_UNKNOWN with them slows 'grep' down for no benefit.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/grep.c b/src/grep.c
index 1e5d0fa4..046f17f1 100644
--- a/src/grep.c
+++ b/src/grep.c
@@ -1298,6 +1298,8 @@ grep (int fd, struct stat const *st)
           nul_zapper = eol;
           skip_nuls = skip_empty_lines;
         }
+      else if (execute != Pexecute)
+        textbin = TEXTBIN_TEXT;
     }
 
   for (;;)