umovestr: speed up check for NUL byte in the fetched word

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/util.c b/util.c
index 34c1b6260b4fcffb016507cc9d2de0ef9d7b05e0..00846a28112897a7e477b456992711c8888a60b8 100644 (file)
--- a/util.c
+++ b/util.c
@@ -843,7 +843,10 @@ umoven(struct tcb *tcp, long addr, int len, char *laddr)
                }
                m = MIN(sizeof(long) - n, len);
                memcpy(laddr, &u.x[n], m);
-               addr += sizeof(long), laddr += m, len -= m, nread += m;
+               addr += sizeof(long);
+               laddr += m;
+               nread += m;
+               len -= m;
        }
        while (len) {
                errno = 0;
@@ -869,7 +872,10 @@ umoven(struct tcb *tcp, long addr, int len, char *laddr)
                }
                m = MIN(sizeof(long), len);
                memcpy(laddr, u.x, m);
-               addr += sizeof(long), laddr += m, len -= m, nread += m;
+               addr += sizeof(long);
+               laddr += m;
+               nread += m;
+               len -= m;
        }
 
        return 0;
@@ -890,10 +896,20 @@ umoven(struct tcb *tcp, long addr, int len, char *laddr)
 int
 umovestr(struct tcb *tcp, long addr, int len, char *laddr)
 {
+#if SIZEOF_LONG == 4
+       const unsigned long x01010101 = 0x01010101ul;
+       const unsigned long x80808080 = 0x80808080ul;
+#elif SIZEOF_LONG == 8
+       const unsigned long x01010101 = 0x0101010101010101ul;
+       const unsigned long x80808080 = 0x8080808080808080ul;
+#else
+# error SIZEOF_LONG > 8
+#endif
+
        int pid = tcp->pid;
-       int i, n, m, nread;
+       int n, m, nread;
        union {
-               long val;
+               unsigned long val;
                char x[sizeof(long)];
        } u;
 
@@ -989,8 +1005,12 @@ umovestr(struct tcb *tcp, long addr, int len, char *laddr)
                while (n & (sizeof(long) - 1))
                        if (u.x[n++] == '\0')
                                return 1;
-               addr += sizeof(long), laddr += m, len -= m, nread += m;
+               addr += sizeof(long);
+               laddr += m;
+               nread += m;
+               len -= m;
        }
+
        while (len) {
                errno = 0;
                u.val = ptrace(PTRACE_PEEKDATA, pid, (char *)addr, 0);
@@ -1015,10 +1035,13 @@ umovestr(struct tcb *tcp, long addr, int len, char *laddr)
                }
                m = MIN(sizeof(long), len);
                memcpy(laddr, u.x, m);
-               for (i = 0; i < sizeof(long); i++)
-                       if (u.x[i] == '\0')
-                               return 1;
-               addr += sizeof(long), laddr += m, len -= m, nread += m;
+               /* "If a NUL char exists in this word" */
+               if ((u.val - x01010101) & ~u.val & x80808080)
+                       return 1;
+               addr += sizeof(long);
+               laddr += m;
+               nread += m;
+               len -= m;
        }
        return 0;
 }