avcodec/gdv: Optimize and factorize scaling loops

Fixes: Timeout
Fixes: 11067/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_GDV_fuzzer-5686623711264768

Before change: Executed clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_GDV_fuzzer-5686623711264768 in 34386 ms
After  change: Executed clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_GDV_fuzzer-5686623711264768 in 24327 ms

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/gdv.c

@@ -72,9 +72,64 @@ static av_cold int gdv_decode_init(AVCodecContext *avctx)
     return 0;
 }
 
+static void scaleup(uint8_t *dst, const uint8_t *src, int w)
+{
+    int x;
+    for (x = 0; x < w - 7; x+=8) {
+        dst[x + 0] =
+        dst[x + 1] = src[(x>>1) + 0];
+        dst[x + 2] =
+        dst[x + 3] = src[(x>>1) + 1];
+        dst[x + 4] =
+        dst[x + 5] = src[(x>>1) + 2];
+        dst[x + 6] =
+        dst[x + 7] = src[(x>>1) + 3];
+    }
+    for (; x < w; x++) {
+        dst[x] = src[(x>>1)];
+    }
+}
+
+static void scaleup_rev(uint8_t *dst, const uint8_t *src, int w)
+{
+    int x;
+
+    for (x = w - 1; (x+1) & 7; x--) {
+        dst[x] = src[(x>>1)];
+    }
+    for (x -= 7; x >= 0; x -= 8) {
+        dst[x + 6] =
+        dst[x + 7] = src[(x>>1) + 3];
+        dst[x + 4] =
+        dst[x + 5] = src[(x>>1) + 2];
+        dst[x + 2] =
+        dst[x + 3] = src[(x>>1) + 1];
+        dst[x + 0] =
+        dst[x + 1] = src[(x>>1) + 0];
+    }
+}
+
+static void scaledown(uint8_t *dst, const uint8_t *src, int w)
+{
+    int x;
+    for (x = 0; x < w - 7; x+=8) {
+        dst[x + 0] = src[2*x + 0];
+        dst[x + 1] = src[2*x + 2];
+        dst[x + 2] = src[2*x + 4];
+        dst[x + 3] = src[2*x + 6];
+        dst[x + 4] = src[2*x + 8];
+        dst[x + 5] = src[2*x +10];
+        dst[x + 6] = src[2*x +12];
+        dst[x + 7] = src[2*x +14];
+    }
+    for (; x < w; x++) {
+        dst[x] = src[2*x];
+    }
+}
+
 static void rescale(GDVContext *gdv, uint8_t *dst, int w, int h, int scale_v, int scale_h)
 {
-    int j, y, x;
+    int j, y;
 
     if ((gdv->scale_v == scale_v) && (gdv->scale_h == scale_h)) {
         return;
@@ -86,14 +141,7 @@ static void rescale(GDVContext *gdv, uint8_t *dst, int w, int h, int scale_v, in
             uint8_t *dst1 = dst + PREAMBLE_SIZE + y * w;
             uint8_t *src1 = dst + PREAMBLE_SIZE + (y>>!!gdv->scale_h) * (w>>1);
 
-            for (x = w - 1; x >= 0 && !(x&1); x--) {
-                dst1[x] = src1[(x>>1)];
-            }
-
-            for (x--; x >= 0; x-=2) {
-                dst1[x  ] =
-                dst1[x+1] = src1[(x>>1)];
-            }
+            scaleup_rev(dst1, src1, w);
         }
     } else if (gdv->scale_h) {
         for (j = 0; j < h; j++) {
@@ -108,9 +156,7 @@ static void rescale(GDVContext *gdv, uint8_t *dst, int w, int h, int scale_v, in
         for (y = 0; y < (h>>1); y++) {
             uint8_t *dst1 = dst + PREAMBLE_SIZE + y * (w>>1);
             uint8_t *src1 = dst + PREAMBLE_SIZE + y*2 * w;
-            for (x = 0; x < (w>>1); x++) {
-                dst1[x] = src1[x*2];
-            }
+            scaledown(dst1, src1, w>>1);
         }
     } else if (scale_h) {
         for (y = 0; y < (h>>1); y++) {
@@ -121,9 +167,7 @@ static void rescale(GDVContext *gdv, uint8_t *dst, int w, int h, int scale_v, in
     } else if (scale_v) {
         for (y = 0; y < h; y++) {
             uint8_t *dst1 = dst + PREAMBLE_SIZE + y * w;
-            for (x = 0; x < (w>>1); x++) {
-                dst1[x] = dst1[x*2];
-            }
+            scaledown(dst1, dst1, w>>1);
         }
     }
 
@@ -479,19 +523,16 @@ static int gdv_decode_frame(AVCodecContext *avctx, void *data,
         }
     } else {
         int sidx = PREAMBLE_SIZE, didx = 0;
-        int y, x;
+        int y;
 
         for (y = 0; y < avctx->height; y++) {
             if (!gdv->scale_v) {
                 memcpy(dst + didx, gdv->frame + sidx, avctx->width);
             } else {
-                for (x = 0; x < avctx->width - 1; x+=2) {
-                    dst[didx + x    ] =
-                    dst[didx + x + 1] = gdv->frame[sidx + (x>>1)];
-                }
-                for (; x < avctx->width; x++) {
-                    dst[didx + x] = gdv->frame[sidx + (x>>1)];
-                }
+                uint8_t *dst2 = dst + didx;
+                uint8_t *src2 = gdv->frame + sidx;
+
+                scaleup(dst2, src2, avctx->width);
             }
             if (!gdv->scale_h || ((y & 1) == 1)) {
                 sidx += !gdv->scale_v ? avctx->width : avctx->width/2;