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Preserve range facts for unpromoted span lengths - #136865
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Mark nonnegative span-length field and indirection VNs as checked bounds, matching promoted locals. Generate nonnegative assertions for local field loads so existing symbolic range analysis can eliminate redundant Slice checks. Annotate in the local-field and indirection VN handlers so unrelated nodes do not pay for the extra test. Keep ordinary local-variable assertion handling on its original fast path, and skip nonnegative assertion generation during local propagation. Recognize the loads structurally with IntegralRange::ForNode, as the existing indirection assertion already did. The VN-based half of GenTree::IsNeverNegative cannot succeed for these load shapes in practice and costs a VN walk on every int-typed load, which on 32-bit targets includes pointer-sized loads. Replaying the windows x64 libraries PMI collection with and without CSE of address-exposed locals produced no differences between the two forms across 331,945 contexts, while both change the same 14 methods relative to main. Look up the value number's function once in IsVNNeverNegative. The array length test, the hardware intrinsic test and the function switch each fetched it separately. Add an unpromoted-span regression covering the signed guard and writes that invalidate the guarded length.
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Promoted span-length locals are marked as nonnegative and their VNs as checked bounds. Equivalent field loads don’t receive all the same annotations, leaving redundant Slice checks when the span remains in memory or is passed implicitly by reference.
Mark recognized span-length field and indirection VNs as checked bounds, and generate nonnegative assertions for local-field loads. This lets the existing range analysis eliminate the redundant unsigned check after a signed guard.
Add coverage with struct promotion disabled, including intervening writes that must prevent elimination.
Complements recently merged changes in: #136615