Make png filter functions compatible with libpng

src/codec/SkPngFilters.cpp

Index: src/codec/SkPngFilters.cpp
diff --git a/src/codec/SkPngFilters.cpp b/src/codec/SkPngFilters.cpp
index 472123fa15e6c6010eda52f1b37efd49ab06068b..2777939357556271feae34d129e4083f9c37ccd0 100644
--- a/src/codec/SkPngFilters.cpp
+++ b/src/codec/SkPngFilters.cpp
@@ -16,25 +16,43 @@
 
 #if defined(__SSE2__)
 
-    template <int bpp>
-    static __m128i load(const void* p) {
-        static_assert(bpp <= 4, "");
-
+    static __m128i load3(const void* p) {
         uint32_t packed;
-        memcpy(&packed, p, bpp);
+        memcpy(&packed, p, 3);
         return _mm_cvtsi32_si128(packed);
     }
 
-    template <int bpp>
-    static void store(void* p, __m128i v) {
-        static_assert(bpp <= 4, "");
+    static __m128i load4(const void* p) {
+        return _mm_cvtsi32_si128(*(const int*)p);
+    }
 
+    static void store3(void* p, __m128i v) {
         uint32_t packed = _mm_cvtsi128_si32(v);
-        memcpy(p, &packed, bpp);
+        memcpy(p, &packed, 3);
+    }
+
+    static void store4(void* p, __m128i v) {
+        *(int*)p = _mm_cvtsi128_si32(v);
+    }
+
+    void sk_sub3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
+        // The Sub filter predicts each pixel as the previous pixel, a.
+        // There is no pixel to the left of the first pixel.  It's encoded directly.
+        // That works with our main loop if we just say that left pixel was zero.
+        __m128i a, d = _mm_setzero_si128();
+
+        int rb = row_info->rowbytes;
+        while (rb > 0) {
+            a = d; d = load3(row);
+            d = _mm_add_epi8(d, a);
+            store3(row, d);
+
+            row += 3;
+            rb  -= 3;
+        }
     }
 
-    template <int bpp>
-    static void sk_sub_sse2(png_row_infop row_info, uint8_t* row, const uint8_t*) {
+    void sk_sub4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
         // The Sub filter predicts each pixel as the previous pixel, a.
         // There is no pixel to the left of the first pixel.  It's encoded directly.
         // That works with our main loop if we just say that left pixel was zero.
@@ -42,17 +60,44 @@
 
         int rb = row_info->rowbytes;
         while (rb > 0) {
-            a = d; d = load<bpp>(row);
+            a = d; d = load4(row);
             d = _mm_add_epi8(d, a);
-            store<bpp>(row, d);
+            store4(row, d);
+
+            row += 4;
+            rb  -= 4;
+        }
+    }
+
+    void sk_avg3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
+        // The Avg filter predicts each pixel as the (truncated) average of a and b.
+        // There's no pixel to the left of the first pixel.  Luckily, it's
+        // predicted to be half of the pixel above it.  So again, this works
+        // perfectly with our loop if we make sure a starts at zero.
+        const __m128i zero = _mm_setzero_si128();
+        __m128i    b;
+        __m128i a, d = zero;
+
+        int rb = row_info->rowbytes;
+        while (rb > 0) {
+                   b = load3(prev);
+            a = d; d = load3(row );
 
-            row += bpp;
-            rb  -= bpp;
+            // PNG requires a truncating average here, so sadly we can't just use _mm_avg_epu8...
+            __m128i avg = _mm_avg_epu8(a,b);
+            // ...but we can fix it up by subtracting off 1 if it rounded up.
+            avg = _mm_sub_epi8(avg, _mm_and_si128(_mm_xor_si128(a,b), _mm_set1_epi8(1)));
+
+            d = _mm_add_epi8(d, avg);
+            store3(row, d);
+
+            prev += 3;
+            row  += 3;
+            rb   -= 3;
         }
     }
 
-    template <int bpp>
-    void sk_avg_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
+    void sk_avg4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
         // The Avg filter predicts each pixel as the (truncated) average of a and b.
         // There's no pixel to the left of the first pixel.  Luckily, it's
         // predicted to be half of the pixel above it.  So again, this works
@@ -63,8 +108,8 @@
 
         int rb = row_info->rowbytes;
         while (rb > 0) {
-                   b = load<bpp>(prev);
-            a = d; d = load<bpp>(row );
+                   b = load4(prev);
+            a = d; d = load4(row );
 
             // PNG requires a truncating average here, so sadly we can't just use _mm_avg_epu8...
             __m128i avg = _mm_avg_epu8(a,b);
@@ -72,11 +117,11 @@
             avg = _mm_sub_epi8(avg, _mm_and_si128(_mm_xor_si128(a,b), _mm_set1_epi8(1)));
 
             d = _mm_add_epi8(d, avg);
-            store<bpp>(row, d);
+            store4(row, d);
 
-            prev += bpp;
-            row  += bpp;
-            rb   -= bpp;
+            prev += 4;
+            row  += 4;
+            rb   -= 4;
         }
     }
 
@@ -88,23 +133,22 @@
         // Read this all as, return x<0 ? -x : x.
         // To negate two's complement, you flip all the bits then add 1.
         __m128i is_negative = _mm_cmplt_epi16(x, _mm_setzero_si128());
-        x = _mm_xor_si128(x, is_negative);                      // Flip negative lanes.
-        x = _mm_add_epi16(x, _mm_srli_epi16(is_negative, 15));  // +1 to negative lanes, else +0.
+        x = _mm_xor_si128(x, is_negative);                     // Flip negative lanes.
+        x = _mm_add_epi16(x, _mm_srli_epi16(is_negative, 15)); // +1 to negative lanes, else +0.
         return x;
     #endif
     }
 
     // Bytewise c ? t : e.
     static __m128i if_then_else(__m128i c, __m128i t, __m128i e) {
-    #if 0 && defined(__SSE4_1__)  // Make sure we have a bot testing this before enabling.
+    #if defined(__SSE4_1__)
         return _mm_blendv_epi8(e,t,c);
     #else
         return _mm_or_si128(_mm_and_si128(c, t), _mm_andnot_si128(c, e));
     #endif
     }
 
-    template <int bpp>
-    void sk_paeth_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
+    void sk_paeth3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
         // Paeth tries to predict pixel d using the pixel to the left of it, a,
         // and two pixels from the previous row, b and c:
         //   prev: c b
@@ -121,9 +165,8 @@
         int rb = row_info->rowbytes;
         while (rb > 0) {
             // It's easiest to do this math (particularly, deal with pc) with 16-bit intermediates.
-            c = b; b = _mm_unpacklo_epi8(load<bpp>(prev), zero);
-            a = d; d = _mm_unpacklo_epi8(load<bpp>(row ), zero);
-
+            c = b; b = _mm_unpacklo_epi8(load3(prev), zero);
+            a = d; d = _mm_unpacklo_epi8(load3(row ), zero);
             __m128i pa = _mm_sub_epi16(b,c),   // (p-a) == (a+b-c - a) == (b-c)
                     pb = _mm_sub_epi16(a,c),   // (p-b) == (a+b-c - b) == (a-c)
                     pc = _mm_add_epi16(pa,pb); // (p-c) == (a+b-c - c) == (a+b-c-c) == (b-c)+(a-c)
@@ -140,33 +183,55 @@
                                                                            c));
 
             d = _mm_add_epi8(d, nearest);  // Note `_epi8`: we need addition to wrap modulo 255.
-            store<bpp>(row, _mm_packus_epi16(d,d));
+            store3(row, _mm_packus_epi16(d,d));
 
-            prev += bpp;
-            row  += bpp;
-            rb   -= bpp;
+            prev += 3;
+            row  += 3;
+            rb   -= 3;
         }
     }
 
-    void sk_sub3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
-        sk_sub_sse2<3>(row_info, row, prev);
-    }
-    void sk_sub4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
-        sk_sub_sse2<4>(row_info, row, prev);
-    }
+    void sk_paeth4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
+        // Paeth tries to predict pixel d using the pixel to the left of it, a,
+        // and two pixels from the previous row, b and c:
+        //   prev: c b
+        //   row:  a d
+        // The Paeth function predicts d to be whichever of a, b, or c is nearest to p=a+b-c.
 
-    void sk_avg3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
-        sk_avg_sse2<3>(row_info, row, prev);
-    }
-    void sk_avg4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
-        sk_avg_sse2<4>(row_info, row, prev);
-    }
+        // The first pixel has no left context, and so uses an Up filter, p = b.
+        // This works naturally with our main loop's p = a+b-c if we force a and c to zero.
+        // Here we zero b and d, which become c and a respectively at the start of the loop.
+        const __m128i zero = _mm_setzero_si128();
+        __m128i c, b = zero,
+                a, d = zero;
 
-    void sk_paeth3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
-        sk_paeth_sse2<3>(row_info, row, prev);
-    }
-    void sk_paeth4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
-        sk_paeth_sse2<4>(row_info, row, prev);
+        int rb = row_info->rowbytes;
+        while (rb > 0) {
+            // It's easiest to do this math (particularly, deal with pc) with 16-bit intermediates.
+            c = b; b = _mm_unpacklo_epi8(load4(prev), zero);
+            a = d; d = _mm_unpacklo_epi8(load4(row ), zero);
+            __m128i pa = _mm_sub_epi16(b,c),   // (p-a) == (a+b-c - a) == (b-c)
+                    pb = _mm_sub_epi16(a,c),   // (p-b) == (a+b-c - b) == (a-c)
+                    pc = _mm_add_epi16(pa,pb); // (p-c) == (a+b-c - c) == (a+b-c-c) == (b-c)+(a-c)
+
+            pa = abs_i16(pa);  // |p-a|
+            pb = abs_i16(pb);  // |p-b|
+            pc = abs_i16(pc);  // |p-c|
+
+            __m128i smallest = _mm_min_epi16(pc, _mm_min_epi16(pa, pb));
+
+            // Paeth breaks ties favoring a over b over c.
+            __m128i nearest  = if_then_else(_mm_cmpeq_epi16(smallest, pa), a,
+                               if_then_else(_mm_cmpeq_epi16(smallest, pb), b,
+                                                                           c));
+
+            d = _mm_add_epi8(d, nearest);  // Note `_epi8`: we need addition to wrap modulo 255.
+            store4(row, _mm_packus_epi16(d,d));
+
+            prev += 4;
+            row  += 4;
+            rb   -= 4;
+        }
     }
 
 #endif