[llvm-branch-commits] [CodeGen][NFC] Fix quadratic c-t for large jump tables (PR #144108)

[Alexis Engelke via llvm-branch-commits]

https://github.com/aengelke created https://github.com/llvm/llvm-project/pull/144108

Deleting a basic block removes all references from jump tables, which
is O(n). When freeing a MachineFunction, all basic blocks are deleted
before the jump tables, causing O(n^2) runtime. Fix this by deallocating
the jump table first.

Test case generator:

    import sys

    n = int(sys.argv[1])
    print("define void @f(i64 %c, ptr %p) {")
    print("  switch i64 %c, label %d [")
    for i in range(n):
        print(f"    i64 {i}, label %h{i}")
    print(f"  ]")
    for i in range(n):
        print(f'h{i}:')
        print(f'  store i64 {i*i}, ptr %p')
        print(f'  ret void')
    print('d:')
    print('  ret void')
    print('}')