Memory usage exploding for complex pattern matching

[Victor Miraldo (UU)]

Hello,

@Ben

> I tried 8.4 and indeed the renamer/typechecker takes dramatically less
> memory (yay!), leaving desugar as the largest allocator (allocating
> around 3GB) while compiling the minimal example. Furthermore, disabling
> the pattern match checker (via -fmax-pmcheck-iterations=0) dramatically
> reduces desugarer allocations (to around 60MByte). It looks like you are
> right.

I have just tried compiling my code with 8.4.2 and using
-fmax-pmcheck-iterations=0,
I gave GHC 12GB of ram and it still ran out (through `ulimit
-v$((1024*1024*12))`).

I saw many warnings about the pattern match checker exceeding the iterations
I specified:

  warning:
    Pattern match checker exceeded (0) iterations in
    a case alternative. (Use -fmax-pmcheck-iterations=n
    to set the maximun number of iterations to n)

Which makes me wonder, is the option really stopping the checker or
just displaying something when it exceeds the given number?

> Unfortunately, it's not clear what can be done to work around this short
> of
> disabling the match checker for the entire module, as suggested by
> mpickering. Ideally we would have some notion of local warning flags
> (e.g. #602), allowing us to disable this (rather expensive) warning for
> generated code. Sadly, we aren't there yet. Perhaps you would like to
> propose such a mechanism?

You guys are much more indicated to propose mechanisms to fix this.
If you think this is a sensible mechanism to implement, sure! I'd be
happy to propose it.

@Simon

> Let's make a Trac ticket for this (since there is a nice small reproducer).  Or, if it seems a dup, attach the reproducer to some existing ticket.

Should I be the one creating it or adding the reproducer?
The closest I've seen is: https://ghc.haskell.org/trac/ghc/ticket/11195

> Maybe we should revert the (recent) patch that makes TH code behave like source code, until this pattern-match check business is solved.

Let me know when this happens, I'll be happy to test my code again!

Thanks!
Cheers,
Victor