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stb_image: Avoid left-shifts of signed values 

It's implementation-specified behavior. Writing this code and then
relying on compiler strength reduction to turn it back into shifts
feels extremely silly but it is what it is.

Fixes issue #1097.
This commit is contained in:
Fabian Giesen 2021-07-02 21:09:11 -07:00
parent 43b32c7bab
commit bb09317445
1 changed files with 3 additions and 3 deletions
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6
stb_image.h
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@ -2219,7 +2219,7 @@ static int stbi__jpeg_decode_block_prog_dc(stbi__jpeg *j, short data[64], stbi__
dc = j->img_comp[b].dc_pred + diff; dc = j->img_comp[b].dc_pred + diff;
j->img_comp[b].dc_pred = dc; j->img_comp[b].dc_pred = dc;
data[0] = (short) (dc << j->succ_low); data[0] = (short) (dc * (1 << j->succ_low));
} else { } else {
// refinement scan for DC coefficient // refinement scan for DC coefficient
if (stbi__jpeg_get_bit(j)) if (stbi__jpeg_get_bit(j))
@ -2256,7 +2256,7 @@ static int stbi__jpeg_decode_block_prog_ac(stbi__jpeg *j, short data[64], stbi__
j->code_buffer <<= s; j->code_buffer <<= s;
j->code_bits -= s; j->code_bits -= s;
zig = stbi__jpeg_dezigzag[k++]; zig = stbi__jpeg_dezigzag[k++];
data[zig] = (short) ((r >> 8) << shift); data[zig] = (short) ((r >> 8) * (1 << shift));
} else { } else {
int rs = stbi__jpeg_huff_decode(j, hac); int rs = stbi__jpeg_huff_decode(j, hac);
if (rs < 0) return stbi__err("bad huffman code","Corrupt JPEG"); if (rs < 0) return stbi__err("bad huffman code","Corrupt JPEG");
@ -2274,7 +2274,7 @@ static int stbi__jpeg_decode_block_prog_ac(stbi__jpeg *j, short data[64], stbi__
} else { } else {
k += r; k += r;
zig = stbi__jpeg_dezigzag[k++]; zig = stbi__jpeg_dezigzag[k++];
data[zig] = (short) (stbi__extend_receive(j,s) << shift); data[zig] = (short) (stbi__extend_receive(j,s) * (1 << shift));
} }
} }
} while (k <= j->spec_end); } while (k <= j->spec_end);