llvm-project

8ac9461e - [SLP] Reject 2-element vectorization when vector inst count exceeds scalar

[SLP] Reject 2-element vectorization when vector inst count exceeds scalar The LLVM cost model uses integer-valued throughput costs which cannot represent fractional costs. For 2-element vectors, this rounding can make vectorization appear profitable when it actually produces more instructions than the scalar code — the overhead from shuffles, inserts, extracts, and buildvectors is underestimated. Add an instruction-count safety check in getTreeCost that estimates the number of vector instructions (including gathers, shuffles, and extracts) and compares against the number of scalar instructions. If the vector code would produce more instructions, reject the tree regardless of what the cost model says. This catches cases where fractional cost rounding hides real overhead. The check is gated behind -slp-inst-count-check (default: on) and only applies to 2-element root trees where rounding errors matter most. Reviewers: hiraditya, bababuck, RKSimon Pull Request: https://github.com/llvm/llvm-project/pull/190414