avcodec/vc1: Introduce fast path for unescaping bitstream buffer

Includes a checkasm test.

Signed-off-by: Ben Avison <bavison@riscosopen.org>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vc1dec.c

@@ -491,7 +491,7 @@ static av_cold int vc1_decode_init(AVCodecContext *avctx)
             size = next - start - 4;
             if (size <= 0)
                 continue;
-            buf2_size = vc1_unescape_buffer(start + 4, size, buf2);
+            buf2_size = v->vc1dsp.vc1_unescape_buffer(start + 4, size, buf2);
             init_get_bits(&gb, buf2, buf2_size * 8);
             switch (AV_RB32(start)) {
             case VC1_CODE_SEQHDR:
@@ -681,7 +681,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                 case VC1_CODE_FRAME:
                     if (avctx->hwaccel)
                         buf_start = start;
-                    buf_size2 = vc1_unescape_buffer(start + 4, size, buf2);
+                    buf_size2 = v->vc1dsp.vc1_unescape_buffer(start + 4, size, buf2);
                     break;
                 case VC1_CODE_FIELD: {
                     int buf_size3;
@@ -698,8 +698,8 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                         ret = AVERROR(ENOMEM);
                         goto err;
                     }
-                    buf_size3 = vc1_unescape_buffer(start + 4, size,
+                    buf_size3 = v->vc1dsp.vc1_unescape_buffer(start + 4, size,
-                                                    slices[n_slices].buf);
+                                                              slices[n_slices].buf);
                     init_get_bits(&slices[n_slices].gb, slices[n_slices].buf,
                                   buf_size3 << 3);
                     slices[n_slices].mby_start = avctx->coded_height + 31 >> 5;
@@ -710,7 +710,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                     break;
                 }
                 case VC1_CODE_ENTRYPOINT: /* it should be before frame data */
-                    buf_size2 = vc1_unescape_buffer(start + 4, size, buf2);
+                    buf_size2 = v->vc1dsp.vc1_unescape_buffer(start + 4, size, buf2);
                     init_get_bits(&s->gb, buf2, buf_size2 * 8);
                     ff_vc1_decode_entry_point(avctx, v, &s->gb);
                     break;
@@ -727,8 +727,8 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                         ret = AVERROR(ENOMEM);
                         goto err;
                     }
-                    buf_size3 = vc1_unescape_buffer(start + 4, size,
+                    buf_size3 = v->vc1dsp.vc1_unescape_buffer(start + 4, size,
-                                                    slices[n_slices].buf);
+                                                              slices[n_slices].buf);
                     init_get_bits(&slices[n_slices].gb, slices[n_slices].buf,
                                   buf_size3 << 3);
                     slices[n_slices].mby_start = get_bits(&slices[n_slices].gb, 9);
@@ -762,7 +762,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                     ret = AVERROR(ENOMEM);
                     goto err;
                 }
-                buf_size3 = vc1_unescape_buffer(divider + 4, buf + buf_size - divider - 4, slices[n_slices].buf);
+                buf_size3 = v->vc1dsp.vc1_unescape_buffer(divider + 4, buf + buf_size - divider - 4, slices[n_slices].buf);
                 init_get_bits(&slices[n_slices].gb, slices[n_slices].buf,
                               buf_size3 << 3);
                 slices[n_slices].mby_start = s->mb_height + 1 >> 1;
@@ -771,9 +771,9 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                 n_slices1 = n_slices - 1;
                 n_slices++;
             }
-            buf_size2 = vc1_unescape_buffer(buf, divider - buf, buf2);
+            buf_size2 = v->vc1dsp.vc1_unescape_buffer(buf, divider - buf, buf2);
         } else {
-            buf_size2 = vc1_unescape_buffer(buf, buf_size, buf2);
+            buf_size2 = v->vc1dsp.vc1_unescape_buffer(buf, buf_size, buf2);
         }
         init_get_bits(&s->gb, buf2, buf_size2*8);
     } else{

libavcodec/vc1dsp.c

@@ -34,6 +34,7 @@
 #include "rnd_avg.h"
 #include "vc1dsp.h"
 #include "startcode.h"
+#include "vc1_common.h"
 
 /* Apply overlap transform to horizontal edge */
 static void vc1_v_overlap_c(uint8_t *src, ptrdiff_t stride)
@@ -1030,6 +1031,7 @@ av_cold void ff_vc1dsp_init(VC1DSPContext *dsp)
 #endif /* CONFIG_WMV3IMAGE_DECODER || CONFIG_VC1IMAGE_DECODER */
 
     dsp->startcode_find_candidate = ff_startcode_find_candidate_c;
+    dsp->vc1_unescape_buffer      = vc1_unescape_buffer;
 
     if (ARCH_AARCH64)
         ff_vc1dsp_init_aarch64(dsp);

libavcodec/vc1dsp.h

@@ -80,6 +80,9 @@ typedef struct VC1DSPContext {
      * one or more further zero bytes and a one byte.
      */
     int (*startcode_find_candidate)(const uint8_t *buf, int size);
+
+    /* Copy a buffer, removing startcode emulation escape bytes as we go */
+    int (*vc1_unescape_buffer)(const uint8_t *src, int size, uint8_t *dst);
 } VC1DSPContext;
 
 void ff_vc1dsp_init(VC1DSPContext* c);

tests/checkasm/vc1dsp.c

@@ -374,6 +374,70 @@ static void check_loop_filter(void)
     }
 }
 
+#define TEST_UNESCAPE                                                                               \
+    do {                                                                                            \
+        for (int count = 100; count > 0; --count) {                                                 \
+            escaped_offset = rnd() & 7;                                                             \
+            unescaped_offset = rnd() & 7;                                                           \
+            escaped_len = (1u << (rnd() % 8) + 3) - (rnd() & 7);                                    \
+            RANDOMIZE_BUFFER8(unescaped, UNESCAPE_BUF_SIZE);                                        \
+            len0 = call_ref(escaped0 + escaped_offset, escaped_len, unescaped0 + unescaped_offset); \
+            len1 = call_new(escaped1 + escaped_offset, escaped_len, unescaped1 + unescaped_offset); \
+            if (len0 != len1 || memcmp(unescaped0, unescaped1, UNESCAPE_BUF_SIZE))                  \
+                fail();                                                                             \
+        }                                                                                           \
+    } while (0)
+
+static void check_unescape(void)
+{
+    /* This appears to be a typical length of buffer in use */
+#define LOG2_UNESCAPE_BUF_SIZE 17
+#define UNESCAPE_BUF_SIZE (1u<<LOG2_UNESCAPE_BUF_SIZE)
+    LOCAL_ALIGNED_8(uint8_t, escaped0, [UNESCAPE_BUF_SIZE]);
+    LOCAL_ALIGNED_8(uint8_t, escaped1, [UNESCAPE_BUF_SIZE]);
+    LOCAL_ALIGNED_8(uint8_t, unescaped0, [UNESCAPE_BUF_SIZE]);
+    LOCAL_ALIGNED_8(uint8_t, unescaped1, [UNESCAPE_BUF_SIZE]);
+
+    VC1DSPContext h;
+
+    ff_vc1dsp_init(&h);
+
+    if (check_func(h.vc1_unescape_buffer, "vc1dsp.vc1_unescape_buffer")) {
+        int len0, len1, escaped_offset, unescaped_offset, escaped_len;
+        declare_func_emms(AV_CPU_FLAG_MMX, int, const uint8_t *, int, uint8_t *);
+
+        /* Test data which consists of escapes sequences packed as tightly as possible */
+        for (int x = 0; x < UNESCAPE_BUF_SIZE; ++x)
+            escaped1[x] = escaped0[x] = 3 * (x % 3 == 0);
+        TEST_UNESCAPE;
+
+        /* Test random data */
+        RANDOMIZE_BUFFER8(escaped, UNESCAPE_BUF_SIZE);
+        TEST_UNESCAPE;
+
+        /* Test data with escape sequences at random intervals */
+        for (int x = 0; x <= UNESCAPE_BUF_SIZE - 4;) {
+            int gap, gap_msb;
+            escaped1[x+0] = escaped0[x+0] = 0;
+            escaped1[x+1] = escaped0[x+1] = 0;
+            escaped1[x+2] = escaped0[x+2] = 3;
+            escaped1[x+3] = escaped0[x+3] = rnd() & 3;
+            gap_msb = 2u << (rnd() % 8);
+            gap = (rnd() &~ -gap_msb) | gap_msb;
+            x += gap;
+        }
+        TEST_UNESCAPE;
+
+        /* Test data which is known to contain no escape sequences */
+        memset(escaped0, 0xFF, UNESCAPE_BUF_SIZE);
+        memset(escaped1, 0xFF, UNESCAPE_BUF_SIZE);
+        TEST_UNESCAPE;
+
+        /* Benchmark the no-escape-sequences case */
+        bench_new(escaped1, UNESCAPE_BUF_SIZE, unescaped1);
+    }
+}
+
 void checkasm_check_vc1dsp(void)
 {
     check_inv_trans_inplace();
@@ -382,4 +446,7 @@ void checkasm_check_vc1dsp(void)
 
     check_loop_filter();
     report("loop_filter");
+
+    check_unescape();
+    report("unescape_buffer");
 }