ふと思ったmozillaのSSE2最適化

/modules/libpr0n/decoders/png/nsPNGDecoder.cpp

695 void
696 row_callback(png_structp png_ptr, png_bytep new_row,
697              png_uint_32 row_num, int pass)
698 {
 :
 :
793     case gfxIFormats::RGB_A1:
794       {
795         for (PRUint32 x=iwidth; x>0; --x) {
796           *cptr32++ = GFX_PACKED_PIXEL(line[3]?0xFF:0x00, line[0], line[1], line[2]);
797           if (line[3] == 0)
798             rowHasNoAlpha = PR_FALSE;
799           line += 4;
800         }
801       }
802       break;
803     case gfxIFormats::RGB_A8:
804       {
805         for (PRUint32 x=width; x>0; --x) {
806           *cptr32++ = GFX_PACKED_PIXEL(line[3], line[0], line[1], line[2]);
807           if (line[3] != 0xff)
808             rowHasNoAlpha = PR_FALSE;
809           line += 4;
810         }
811       }

ここで使ってる GFX_PACKED_PIXEL のマクロってどうなっているかと思ったら、

/gfx/thebes/public/gfxColor.h

118 /**
119  * Fast approximate division by 255. It has the property that
120  * for all 0 <= n <= 255*255, FAST_DIVIDE_BY_255(n) == n/255.
121  * But it only uses two adds and two shifts instead of an 
122  * integer division (which is expensive on many processors).
123  *
124  * equivalent to ((v)/255)
125  */
126 #define GFX_DIVIDE_BY_255(v)  \
127      (((((unsigned)(v)) << 8) + ((unsigned)(v)) + 255) >> 16)
128 
129 /**
130  * Fast premultiply macro
131  *
132  * equivalent to (((c)*(a))/255)
133  */
134 #define GFX_PREMULTIPLY(c,a) GFX_DIVIDE_BY_255((c)*(a))
135 
136 /** 
137  * Macro to pack the 4 8-bit channels (A,R,G,B) 
138  * into a 32-bit packed premultiplied pixel.
139  *
140  * The checks for 0 alpha or max alpha ensure that the
141  * compiler selects the quicked calculation when alpha is constant.
142  */
143 #define GFX_PACKED_PIXEL(a,r,g,b)                                       \
144     ((a) == 0x00) ? 0x00000000 :                                        \
145     ((a) == 0xFF) ? ((0xFF << 24) | ((r) << 16) | ((g) << 8) | (b))     \
146                   : ((a) << 24) |                                       \
147                     (GFX_PREMULTIPLY(r,a) << 16) |                      \
148                     (GFX_PREMULTIPLY(g,a) << 8) |                       \
149                     (GFX_PREMULTIPLY(b,a))

だから、アルファブレンディングしたPNGを持ってくると遅すぎじゃないかな？

こんな感じのパックドコードをSSE2で実装すれば約2倍くらいは速くなるような。

xmm0080 = _mm_set1_epi16(0x0080);
xmm0101 = _mm_set1_epi16(0x0101);
xmmAlpha = _mm_set_epi32(0x00ff0000, 0x00000000, 0x00ff0000, 0x00000000);

data = _mm_loadu_si128((__m128i*)src);
dataLo = _mm_unpacklo_epi8 (data, _mm_setzero_si128 ());
dataHi = _mm_unpackhi_epi8 (data, _mm_setzero_si128 ());

alphaLo = _mm_shufflelo_epi16 (dataLo, _MM_SHUFFLE(3, 3, 3, 3));
alphaHi = _mm_shufflelo_epi16 (dataHi, _MM_SHUFFLE(3, 3, 3, 3));
alphaLo = _mm_shufflehi_epi16 (alphaLo, _MM_SHUFFLE(3, 3, 3, 3));
alphaHi = _mm_shufflehi_epi16 (alphaHi, _MM_SHUFFLE(3, 3, 3, 3));

alphaLo = _mm_or_si128(alphaLo, xmmAlpha);
alphaHi = _mm_or_si128(alphaHi, xmmAlpha);

dataLo = _mm_shufflelo_epi16 (dataLo, _MM_SHUFFLE(3, 0, 1, 2));
dataLo = _mm_shufflehi_epi16 (dataLo, _MM_SHUFFLE(3, 0, 1, 2));
dataHi = _mm_shufflelo_epi16 (dataHi, _MM_SHUFFLE(3, 0, 1, 2));
dataHi = _mm_shufflehi_epi16 (dataHi, _MM_SHUFFLE(3, 0, 1, 2));

dataLo = _mm_mullo_epi16(dataLo, alphaLo);
dataHi = _mm_mullo_epi16(dataHi, alphaHi);

dataLo = _mm_adds_epu16(dataLo, xmm0080);
dataHi = _mm_adds_epu16(dataHi, xmm0080);

dataLo = _mm_mulhi_epu16(dataLo, xmm0101);
dataHi = _mm_mulhi_epu16(dataHi, xmm0101);

data = _mm_packus_epi16 (dataLo, dataHi);
_mm_storeu_si128((__m128i*)dst, data);

今度テストしてみよう。