SSE2 implementation of ImportRowShrink

some limitations: only for RGBA output,
and if reduction factor is not too small (dst_width > src_width / 128)

20-25% faster, ~4-6% global improvement total decoding.

Change-Id: I95366ddaa4a38e0a96bed754dfe790126f7bb84a

src/dsp/rescaler_sse2.c

@@ -25,6 +25,63 @@
 #define ROUNDER (WEBP_RESCALER_ONE >> 1)
 #define MULT_FIX(x, y) (((uint64_t)(x) * (y) + ROUNDER) >> WEBP_RESCALER_RFIX)
 
+static void RescalerImportRowShrinkSSE2(WebPRescaler* const wrk,
+                                        const uint8_t* src) {
+  const int x_sub = wrk->x_sub;
+  int accum = 0;
+  const __m128i zero = _mm_setzero_si128();
+  const __m128i mult0 = _mm_set1_epi16(x_sub);
+  const __m128i mult1 = _mm_set1_epi32(wrk->fx_scale);
+  const __m128i rounder = _mm_set_epi32(0, ROUNDER, 0, ROUNDER);
+  __m128i sum = zero;
+  rescaler_t* frow = wrk->frow;
+  const rescaler_t* const frow_end = wrk->frow + 4 * wrk->dst_width;
+
+  if (wrk->num_channels != 4) {
+    return WebPRescalerImportRowShrinkC(wrk, src);
+  }
+  if (wrk->x_add > (x_sub << 7)) {
+    return WebPRescalerImportRowShrinkC(wrk, src);
+  }
+  assert(!WebPRescalerInputDone(wrk));
+  assert(!wrk->x_expand);
+
+  for (; frow < frow_end; frow += 4) {
+    __m128i base = zero;
+    accum += wrk->x_add;
+    while (accum > 0) {
+      const __m128i A = _mm_cvtsi32_si128(*(int*)src);
+      src += 4;
+      base = _mm_unpacklo_epi8(A, zero);
+      // To avoid overflow, we need: base * x_add / x_sub < 32768
+      // => x_add < x_sub << 7. That's a 1/128 reduction ratio limit.
+      sum = _mm_add_epi16(sum, base);
+      accum -= x_sub;
+    }
+    {    // Emit next horizontal pixel.
+      const __m128i mult = _mm_set1_epi16(-accum);
+      const __m128i frac0 = _mm_mullo_epi16(base, mult);  // 16b x 16b -> 32b
+      const __m128i frac1 = _mm_mulhi_epu16(base, mult);
+      const __m128i frac = _mm_unpacklo_epi16(frac0, frac1);  // frac is 32b
+      const __m128i A0 = _mm_mullo_epi16(sum, mult0);
+      const __m128i A1 = _mm_mulhi_epu16(sum, mult0);
+      const __m128i B0 = _mm_unpacklo_epi16(A0, A1);      // sum * x_sub
+      const __m128i frow_out = _mm_sub_epi32(B0, frac);   // sum * x_sub - frac
+      const __m128i D0 = _mm_srli_epi64(frac, 32);
+      const __m128i D1 = _mm_mul_epu32(frac, mult1);      // 32b x 16b -> 64b
+      const __m128i D2 = _mm_mul_epu32(D0, mult1);
+      const __m128i E1 = _mm_add_epi64(D1, rounder);
+      const __m128i E2 = _mm_add_epi64(D2, rounder);
+      const __m128i F1 = _mm_shuffle_epi32(E1, 1 | (3 << 2));
+      const __m128i F2 = _mm_shuffle_epi32(E2, 1 | (3 << 2));
+      const __m128i G = _mm_unpacklo_epi32(F1, F2);
+      sum = _mm_packs_epi32(G, zero);
+      _mm_storeu_si128((__m128i*)frow, frow_out);
+    }
+  }
+  assert(accum == 0);
+}
+
 //------------------------------------------------------------------------------
 // Row export
 
@@ -222,6 +279,7 @@ static void RescalerExportRowShrinkSSE2(WebPRescaler* const wrk) {
 extern void WebPRescalerDspInitSSE2(void);
 
 WEBP_TSAN_IGNORE_FUNCTION void WebPRescalerDspInitSSE2(void) {
+  WebPRescalerImportRowShrink = RescalerImportRowShrinkSSE2;
   WebPRescalerExportRowExpand = RescalerExportRowExpandSSE2;
   WebPRescalerExportRowShrink = RescalerExportRowShrinkSSE2;
 }