Backport faster T.35 payload copy from dav1d 1.2.1 (#1001)

src/getbits.rs

@@ -197,4 +197,11 @@ impl<'a> GetBits<'a> {
     pub const fn has_pending_bits(&self) -> bool {
         self.state != 0 || self.bits_left != 0
     }
+
+    pub fn get_bytes(&mut self, n: usize) -> &[u8] {
+        assert_eq!(self.bits_left, 0);
+        let i = self.index;
+        self.index += n;
+        &self.data[i..][..n]
+    }
 }

src/obu.c

@@ -1597,10 +1597,13 @@ int dav1d_parse_obus(Dav1dContext *const c, Dav1dData *const in) {
 
             itut_t35_metadata->country_code = country_code;
             itut_t35_metadata->country_code_extension_byte = country_code_extension_byte;
-            for (int i = 0; i < payload_size; i++)
-                itut_t35_metadata->payload[i] = dav1d_get_bits(&gb, 8);
             itut_t35_metadata->payload_size = payload_size;
 
+            // We know that we've read a whole number of bytes and that the
+            // payload is within the OBU boundaries, so just use memcpy()
+            assert(gb.bits_left == 0);
+            memcpy(itut_t35_metadata->payload, gb.ptr, payload_size);
+
             c->n_itut_t35++;
             break;
         }

src/obu.rs

@@ -2460,7 +2460,7 @@ unsafe fn parse_obus(
                     } else {
                         let country_code = country_code as u8;
                         let country_code_extension_byte = country_code_extension_byte as u8;
-                        let payload = (0..payload_size).map(|_| gb.get_bits(8) as u8).collect(); // TODO(kkysen) fallible allocation
+                        let payload = gb.get_bytes(payload_size as usize).into(); // TODO fallible allocation
                         let itut_t35 = Rav1dITUTT35 {
                             country_code,
                             country_code_extension_byte,