gh-104615: don't make unsafe swaps in apply_static_swaps by carljm · Pull Request #104620 · python/cpython

This can happen with code like a, a, b = x, y, z as in some of the added tests. This compiles to LOAD_FAST x; LOAD_FAST y; LOAD_FAST z; SWAP 3; STORE_FAST a; STORE_FAST a; STORE_FAST b. Before this PR, apply_static_swaps would optimize that (ignoring the loads) to STORE_FAST b; STORE_FAST a; STORE_FAST a (swapping the first and third store), which is invalid because it reorders the two stores to a.

k - j == n - 1 here, where n is the oparg to the SWAP. So for a SWAP 2, j and k will be adjacent, but for SWAP with oparg > 2, there will be intervening instructions. And STORE_FAST is a SWAPPABLE instruction, so those intervening instructions can be STORE_FAST.