util/bufferiszero: Improve scalar variant

Split less-than and greater-than 256 cases.
Use unaligned accesses for head and tail.
Avoid using out-of-bounds pointers in loop boundary conditions.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

util/bufferiszero.c

@@ -28,40 +28,57 @@
 
 static bool (*buffer_is_zero_accel)(const void *, size_t);
 
-static bool buffer_is_zero_integer(const void *buf, size_t len)
+static bool buffer_is_zero_int_lt256(const void *buf, size_t len)
 {
-    if (unlikely(len < 8)) {
+    uint64_t t;
-        /* For a very small buffer, simply accumulate all the bytes.  */
+    const uint64_t *p, *e;
-        const unsigned char *p = buf;
-        const unsigned char *e = buf + len;
-        unsigned char t = 0;
 
-        do {
+    /*
+     * Use unaligned memory access functions to handle
+     * the beginning and end of the buffer.
+     */
+    if (unlikely(len <= 8)) {
+        return (ldl_he_p(buf) | ldl_he_p(buf + len - 4)) == 0;
+    }
+
+    t = ldq_he_p(buf) | ldq_he_p(buf + len - 8);
+    p = QEMU_ALIGN_PTR_DOWN(buf + 8, 8);
+    e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 8);
+
+    /* Read 0 to 31 aligned words from the middle. */
+    while (p < e) {
         t |= *p++;
-        } while (p < e);
+    }
-
     return t == 0;
-    } else {
+}
-        /* Otherwise, use the unaligned memory access functions to
-           handle the beginning and end of the buffer, with a couple
-           of loops handling the middle aligned section.  */
-        uint64_t t = ldq_he_p(buf);
-        const uint64_t *p = (uint64_t *)(((uintptr_t)buf + 8) & -8);
-        const uint64_t *e = (uint64_t *)(((uintptr_t)buf + len) & -8);
 
-        for (; p + 8 <= e; p += 8) {
+static bool buffer_is_zero_int_ge256(const void *buf, size_t len)
+{
+    /*
+     * Use unaligned memory access functions to handle
+     * the beginning and end of the buffer.
+     */
+    uint64_t t = ldq_he_p(buf) | ldq_he_p(buf + len - 8);
+    const uint64_t *p = QEMU_ALIGN_PTR_DOWN(buf + 8, 8);
+    const uint64_t *e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 8);
+
+    /* Collect a partial block at the tail end. */
+    t |= e[-7] | e[-6] | e[-5] | e[-4] | e[-3] | e[-2] | e[-1];
+
+    /*
+     * Loop over 64 byte blocks.
+     * With the head and tail removed, e - p >= 30,
+     * so the loop must iterate at least 3 times.
+     */
+    do {
         if (t) {
             return false;
         }
         t = p[0] | p[1] | p[2] | p[3] | p[4] | p[5] | p[6] | p[7];
-        }
+        p += 8;
-        while (p < e) {
+    } while (p < e - 7);
-            t |= *p++;
-        }
-        t |= ldq_he_p(buf + len - 8);
 
     return t == 0;
-    }
 }
 
 #if defined(CONFIG_AVX2_OPT) || defined(__SSE2__)
@@ -173,7 +190,7 @@ select_accel_cpuinfo(unsigned info)
         { CPUINFO_AVX2,    buffer_zero_avx2 },
 #endif
         { CPUINFO_SSE2,    buffer_zero_sse2 },
-        { CPUINFO_ALWAYS,  buffer_is_zero_integer },
+        { CPUINFO_ALWAYS,  buffer_is_zero_int_ge256 },
     };
 
     for (unsigned i = 0; i < ARRAY_SIZE(all); ++i) {
@@ -211,7 +228,7 @@ bool test_buffer_is_zero_next_accel(void)
     return false;
 }
 
-#define INIT_ACCEL buffer_is_zero_integer
+#define INIT_ACCEL buffer_is_zero_int_ge256
 #endif
 
 static bool (*buffer_is_zero_accel)(const void *, size_t) = INIT_ACCEL;
@@ -232,7 +249,7 @@ bool buffer_is_zero_ool(const void *buf, size_t len)
     if (likely(len >= 256)) {
         return buffer_is_zero_accel(buf, len);
     }
-    return buffer_is_zero_integer(buf, len);
+    return buffer_is_zero_int_lt256(buf, len);
 }
 
 bool buffer_is_zero_ge256(const void *buf, size_t len)