8000 Array slice optimizations. by psivaraj · Pull Request #2539 · google/xls · GitHub
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Array slice optimizations. #2539

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Array slice optimizations. #2539

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@psivaraj psivaraj commented Jul 5, 2025 •
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Addresses #453.

Extends ArraySimplificationPass to add array_slice optimizations, including:

Nested slice flattening:

array_slice(array_slice(A, start1, width1), start2, width2) 
    → array_slice(A, start1 + start2, width2)

Identity optimization:

array_slice(array, start=0, width=full_size) 
    → array (works with any array-typed node)

Constant start optimization:

array_slice(array(a,b,c,d), start=1, width=2) 
    → array(b,c) (extracts operands directly)

Select transformation to reduce mux width:

array_slice(select(p, [A0, A1]), start, width) 
    → select(p, [array_slice(A0, start, width), array_slice(A1, start, width)])

Added tests to cover all cases, and tried to consolidate shared logic with array_index optimizations.

I've also tried to preserve XLS array clamping behavior (out-of-bounds access returns the last element) throughout these optimizations, but would appreciate feedback on whether this is implemented correctly.

aside: It might very well be worth exploring the idea in #453 where we just convert array slices into array index operations, but I'm leaving that for the future. This should at least be a steady improvement.

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@psivaraj psivaraj force-pushed the psivaraj/06-29-25-array-simpl branch 2 times, most recently from 8382b67 to 0374402 Compare July 6, 2025 20:51
@psivaraj
Array slice optimizations.
c801ed6 
- Flatten sequential array slices.
- Reduce mux width optimization.
- Optimize known slice widths
@psivaraj psivaraj force-pushed the psivaraj/06-29-25-array-simpl branch from 0374402 to c801ed6 Compare July 6, 2025 20:53 5D8C
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