Make til::color's COLORREF conversion more optimal #7619
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Clang (10) has no trouble optimizing the COLORREF conversion operator, below, to a simple 32-bit load with mask (!) even though it's a series of bit shifts across mutliple struct members. MSVC (19.24) doesn't make the same optimization decision, and it emits three 8-bit loads and some shifting. In any case, the optimimization only applies at -O2 (clang) and above. In this commit, we leverage the spec-legality of using unions for type conversions and the overlap of four uint8_ts and a uint32_t to make the conversion very obvious to both compilers. x86_64 msvc | O0 | O1 | O2 ------------|----|----|-------------------- shifts | 12 | 11 | 11 (fully inlined) union | 5 | 1 | 1 (fully inlined) x86_64 clang | O0 | O1 | O2 + O3 -------------|----|----|-------------------- shifts | 14 | 5 | 1 (fully inlined) union | 9 | 3 | 1 (fully inlined)
DHowett
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Poke around at the implementation here: |
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You work on the weirdest things in your free time 😄 |
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@zadjii-msft HA! haha. You know, you're right about that. |
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@DHowett This is awesome! Thank you for doing this. |
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@j4james Thanks for being a champion for perf here! 😄 |
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Clang (10) has no trouble optimizing the COLORREF conversion operator to a simple 32-bit load with mask (!) even though it's a series of bit shifts across multiple struct members. MSVC (19.24) doesn't make the same optimization decision, and it emits three 8-bit loads and some shifting. In any case, the optimization only applies at -O2 (clang) and above. In this commit, we leverage the spec-legality of using unions for type conversions and the overlap of four uint8_ts and a uint32_t to make the conversion very obvious to both compilers. x86_64 msvc | O0 | O1 | O2 ------------|----|----|-------------------- shifts | 12 | 11 | 11 (fully inlined) union | 5 | 1 | 1 (fully inlined) x86_64 clang | O0 | O1 | O2 + O3 -------------|----|----|-------------------- shifts | 14 | 5 | 1 (fully inlined) union | 9 | 3 | 1 (fully inlined) j4james brought up some concerns about til::color's minor wastefulness in #7578 (comment). This is a clear, simple transformation that saves us a few instructions in a relatively common case, so I'm accepting a micro-optimization even though we don't have data showing this to be a hot spot. (cherry picked from commit c17f448)
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Clang (10) has no trouble optimizing the COLORREF conversion operator to
a simple 32-bit load with mask (!) even though it's a series of bit
shifts across multiple struct members.
MSVC (19.24) doesn't make the same optimization decision, and it emits
three 8-bit loads and some shifting.
In any case, the optimization only applies at -O2 (clang) and above.
In this commit, we leverage the spec-legality of using unions for type
conversions and the overlap of four uint8_ts and a uint32_t to make the
conversion very obvious to both compilers.
j4james brought up some concerns about til::color's minor wastefulness
in #7578 (comment).
This is a clear, simple transformation that saves us a few instructions
in a relatively common case, so I'm accepting a micro-optimization even
though we don't have data showing this to be a hot spot.