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DmdAnal: Improve handling of precise exceptions

Sebastian Graf requested to merge wip/dmdanal-precise-exn-simpl into master

This patch does two things: Fix possible unsoundness in what was called the "IO hack" and implement part 2.1 of the "fixing precise exceptions" plan in https://gitlab.haskell.org/ghc/ghc/wikis/fixing-precise-exceptions, which, in combination with !2956 (closed), supersedes !3014 (closed) and !2525 (closed).

IO hack

The "IO hack" (which is a fallback to preserve precise exceptions semantics and thus soundness, rather than some smart thing that increases precision) is called exprMayThrowPreciseException now. I came up with two testcases exemplifying possible unsoundness (if twisted enough) in the old approach:

  • T13380d: Demonstrating unsoundness of the "IO hack" when resorting to manual state token threading and direct use of primops. More details below.
  • T13380e: Demonstrating unsoundness of the "IO hack" when we have Nested CPR. Not currently relevant, as we don't have Nested CPR yet.

Basically, the IO hack assumed that precise exceptions can only be thrown from a case scrutinee of type (# State# RealWorld, _ #). I couldn't come up with a program using the IO abstraction that violates this assumption. But it's easy to do so via manual state token threading and direct use of primops, see T13380d. Also similar code might be generated by Nested CPR in the (hopefully not too) distant future, see T13380e. Hence, we now have a more careful test in forcesRealWorld that passes T13380{d,e} (and will hopefully be robust to Nested CPR).

Dead code elimination and precise exceptions

In #13380 (closed) and #17676 (closed) we saw that we didn't preserve precise exception semantics in demand analysis. We fixed that with minimal changes in !2956 (closed), but that was terribly unprincipled.

That unprincipledness resulted in a loss of precision, which is tracked by these new test cases:

  • T13380b: Regression in dead code elimination, because !2956 (closed) was too syntactic about raiseIO#
  • T13380c: No need to apply the "IO hack" when the IO action may not throw a precise exception (and the existing IO hack doesn't detect that)

Fixing both issues in !3014 (closed) turned out to be too complicated and had the potential to regress in the future. Hence we decided to only fix T13380b and augment the Divergence lattice with a new middle-layer element, ExnOrDiv, which means either Diverges (, throws an imprecise exception) or throws a precise exception.

See the wiki page on Step 2.1 for more implementational details: https://gitlab.haskell.org/ghc/ghc/wikis/fixing-precise-exceptions#dead-code-elimination-for-raiseio-with-isdeadenddiv-introducing-exnordiv-step-21

Edited by Sebastian Graf

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