:steplocal is orders of magnitude slower than normal interpreter

Summary

It seems that calling :steplocal on a breakpoint to continue execution of an expression in the ghci debugger takes much much much much longer than executing that expression directly or using :continue rather than calling step local.

Even though :steplocal is responsible for stopping at the next breakpoint at the same level as the current one, this slowness makes it unusable.

Steps to reproduce

Fib.hs:

module Fib where

main = fib 30

fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

Run two distinct GHCi sessions on Fib.hs. On both, :set +s to output statistics

On the first,

ghci> main
832040
(1.40 secs, 921,592,600 bytes)

On the second,

ghci> :break main
Breakpoint 0 activated at Fib.hs:3:8-13
ghci> main
Stopped in Fib.main, Fib.hs:3:8-13
_result :: Integer = _
(0.00 secs,)
[Fib.hs:3:8-13] ghci> :steplocal
...
..
.

Haven't been able to observe it terminating with this setup.

If we do fib 20 rather than 30, then the normal execution takes 0.04s but the :steplocal one:

• Doesn't say how much time it took, despite having :set +s
• Takes roughly 6s measured with a stopwatch

I suspect steplocal is doing something asymptotically worse to figure out the "local" part.

Expected behavior

:steplocal ought to be fast!

Environment

• GHC version used: 9.10