HADD(3clc)

💖 HADD, RHADD 💖

okay bestie, welcome to HADD and RHADD from the OpenCL Manual — two little integer functions that are genuinely lowkey iconic once you get what they do 🫶

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✨ NAME ✨

hadd, _rhadd — returns (x + y) >> 1, or (x + y + 1) >> 1 💅

gentype hadd(gentype x, gentype y);

gentype rhadd(gentype x, gentype y);

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🌸 DESCRIPTION 🌸

hadd returns (x+y) >> 1. the intermediate sum does not modulo overflow, periodt 💀

rhadd returns (x+y+1) >> 1. and the intermediate sum here? also not modulo overflowing. no cap.

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🎀 NOTES 🎀

okay so this is the part where it gets good. frequently, vector operations need n + 1 bits temporarily to calculate a result. the rhadd instruction hands you that extra bit without needing to upsample and downsample first. bestie, this can be a profound performance win. it's giving free real estate ✨

built-in integer functions take scalar or vector arguments. the vector versions of the integer functions operate component-wise, and the description is per component. iykyk.

we use the generic type name gentype to indicate that the function can take char, char{2|3|4|8|16}, uchar, uchar{2|3|4|8|16}, short, short{2|3|4|8|16}, ushort, ushort{2|3|4|8|16}, int, int{2|3|4|8|16}, uint, uint{2|3|4|8|16}, long, long{2|3|4|8|16}, ulong, or ulong{2|3|4|8|16} as the type for the arguments. 🧚

we also use the generic type name ugentype to refer to unsigned versions of gentype. for example, if gentype is char4, ugentype is uchar4. slay.

and then there's sgentype, the generic type name for when the function can take a scalar data type, i.e. char, uchar, short, ushort, int, uint, long, or ulong as the type for the arguments. for built-in integer functions that take gentype and sgentype arguments, the gentype argument must be a vector or scalar version of the sgentype argument. for example, if sgentype is uchar, gentype must be uchar or uchar{2|3|4|8|16}. for vector versions, sgentype is implicitly widened to gentype as described in section 6.3.a of the OpenCL specification. 👑

for any specific use of a function, the actual type has to be the same for all arguments and the return type, unless otherwise specified. no mixing and matching, bestie 🙈

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📜 SPECIFICATION 📜

OpenCL Specification[1] ⭐

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🦋 SEE ALSO 🦋

integerFunctions(3clc)

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👑 AUTHORS 👑

The Khronos Group 💜

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💌 COPYRIGHT 💌

Copyright © 2007-2011 The Khronos Group Inc.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and/or associated documentation files (the "Materials"), to deal in the Materials without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Materials, and to permit persons to whom the Materials are furnished to do so, subject to the condition that this copyright notice and permission notice shall be included in all copies or substantial portions of the Materials.

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🍓 NOTES 🍓

1. OpenCL Specification page 256, section 6.12.3 - Integer Functions

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and that's the whole vibe check, bestie — this page came straight from The Khronos Group, dated 01/14/2021, and it's literally the HADD, RHADD(3clc) page. go forth and shift those bits 🌈💖