[C++][Compute] "Scatter" vector functions #44393
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pitrou
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Jan 15, 2025
…ion` and `scatter` (#44394) ### Rationale for this change For background please see #44393. When implementing the "scatter" function requested in #44393, I found it also useful to make it a public vector API. After a painful thinking, I decided to name it "permute". And when implementing permute, I found it fairly easy to implement it by first computing the "reverse indices" of the positions, and then invoking the existing "take", where I think "reverse_indices" itself can also be a useful public vector API. Thus the PR categorized them as "placement functions". ### What changes are included in this PR? Implement vector selection API `inverse_permutation` and `scatter`, where `scatter(values, indices)` is implemented as `take(values, inverse_permutation(indices))`. ### Are these changes tested? UT included. ### Are there any user-facing changes? Yes, new public APIs added. Documents updated. * GitHub Issue: #44393 Lead-authored-by: Ruoxi Sun <[email protected]> Co-authored-by: Rossi Sun <[email protected]> Co-authored-by: Antoine Pitrou <[email protected]> Signed-off-by: Antoine Pitrou <[email protected]>
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Issue resolved by pull request 44394 |
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Describe the enhancement requested
We discussed the solution for #41094 , the conclusion is that the "special form" is the way. Comment #41094 (comment) gives a thorough description of how special forms work.
Here I summarize a bit: a special form "mask-ably" evaluates some of its subexpressions based on some masks obtained from its other subexpressions. For example consider
if cond then expr1 else expr2, the result ofcondis the mask, which controls which rows goes toexpr1and which goes toexpr2. Another example is logicaland/or, each of its subexpressions is part of the mask to evaluate the rest subexpressions (boolean short-circuit).One way to implement special forms is that every expression selectively executes its kernel by respecting a selection vector (which rows this kernel should execute on) or a equally boolean mask. But unfortunately this isn't practical because we can't afford to change every (scalar) compute functions to support selection vector/mask all at once. So we must take an adaptive way, allowing functions to be selection vector/mask agnostic. To do so, a special form should 1) takes rows specific to each branch; 2) invoke the function of each branch on each group of these rows; 3) combine the results of all the branches by scattering each row to its original position in the input.
So far we have vector function
filter/taketo do 1), but there isn't a handy utility to do 3).Component(s)
C++
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