Minor LodePNG optimizations #173
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Also correct typo and update build command to link SDL2 and not SDL
zvezdochiot
reviewed
Sep 18, 2022
| *r = *g = *b = in[i]; | ||
| if(mode->key_defined && *r == mode->key_r) *a = 0; | ||
| else *a = 255; | ||
| *a = 255 * !(mode->key_defined && *r == mode->key_r); |
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❓ Why use multiplication where it is unnecessary?:
*a = ((mode->key_defined) && (*r == mode->key_r)) ? 0u : 255u;Or use array?:
unsigned char wb[2] = {255, 0);
...
*a = wb[(int)((mode->key_defined) && (*r == mode->key_r))];(for const time).
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Each conversion from if-else statement to multiplication in this case removes one jump instruction, creating code that has a few less branches and is also slightly smaller
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I tested this out with godbolt.org, and it looks like the way you write it doesn't matter, the effect is equivalent and it gets compiled to the same thing:
So the original with if/else looks more readable I'd say
Encoding speeds up by ~1% when compiled with -O3 on Clang 15.0.0
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LodePNG code size is decreased by about 1.5kB when compiled with MSYS2 CLANG64 Clang 15.0.0
Decoding (RGB and non-alpha grey colortypes) and encoding (any colortype) should be about 1-2% faster
A warning is fixed in lodepng_benchmark.cpp so it can successfully be compiled with the recommended build command