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stb_image: Faster stbi__extend_receive.

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Fabian Giesen 2014-12-14 00:38:17 -08:00
parent f547761c15
commit a1bd1f7f1f
1 changed files with 6 additions and 14 deletions
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20
stb_image.h
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@ -1155,31 +1155,23 @@ stbi_inline static int stbi__jpeg_huff_decode(stbi__jpeg *j, stbi__huffman *h)
return h->values[c]; return h->values[c];
} }
// bias[n] = (-1<<n) + 1
static int const stbi__jbias[16] = {0,-1,-3,-7,-15,-31,-63,-127,-255,-511,-1023,-2047,-4095,-8191,-16383,-32767};
// combined JPEG 'receive' and JPEG 'extend', since baseline // combined JPEG 'receive' and JPEG 'extend', since baseline
// always extends everything it receives. // always extends everything it receives.
stbi_inline static int stbi__extend_receive(stbi__jpeg *j, int n) stbi_inline static int stbi__extend_receive(stbi__jpeg *j, int n)
{ {
unsigned int m = 1 << (n-1);
unsigned int k; unsigned int k;
int sgn;
if (j->code_bits < n) stbi__grow_buffer_unsafe(j); if (j->code_bits < n) stbi__grow_buffer_unsafe(j);
#if 1 sgn = (stbi__int32)j->code_buffer >> 31; // sign bit is always in MSB
k = stbi_lrot(j->code_buffer, n); k = stbi_lrot(j->code_buffer, n);
j->code_buffer = k & ~stbi__bmask[n]; j->code_buffer = k & ~stbi__bmask[n];
k &= stbi__bmask[n]; k &= stbi__bmask[n];
j->code_bits -= n; j->code_bits -= n;
#else return k + (stbi__jbias[n] & ~sgn);
k = (j->code_buffer >> (32 - n)) & stbi__bmask[n];
j->code_bits -= n;
j->code_buffer <<= n;
#endif
// the following test is probably a random branch that won't
// predict well. I tried to table accelerate it but failed.
// maybe it's compiling as a conditional move?
if (k < m)
return (-1 << n) + k + 1;
else
return k;
} }
// given a value that's at position X in the zigzag stream, // given a value that's at position X in the zigzag stream,