GHC 8.0.1 Panic for current Aeson 1.0.2.1

    [1 of 1] Compiling Main             ( Dates.hs, .stack-work/dist/x86_64-linux/Cabal-1.24.0.0/build/aeson-benchmark-dates/aeson-benchmark-dates-tmp/Main.o )
    ghc: panic! (the 'impossible' happened)
      (GHC version 8.0.1 for x86_64-unknown-linux):
    	Simplifier ticks exhausted
      When trying UnfoldingDone $s>$<
      To increase the limit, use -fsimpl-tick-factor=N (default 100)
      If you need to do this, let GHC HQ know, and what factor you needed
      To see detailed counts use -ddump-simpl-stats
      Total ticks: 153200

I am not certain if this is a duplicate of:

• #10083 (closed)

• #12789 (closed)

• #11263

• #5539 (closed)

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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changed weight to 5

added Tbug Trac import labels

Did you try increasing the tick limit as the message suggests? How high did you have to turn it up?

FWIW, aeson has resolved this upstream by increasing the simpl-tick-factor to 200 for that particular benchmark (see this issue, and this commit).

I slightly reduced the test case here, but I do ultimately think that overly aggressive INLINE-ing is the culprit here.

I think Ryan is right that the INLINE pragmas in Data.Aeson.Encoding.Builder are too aggressive.

In this example, utcTime contains a call to timeOfDay which is a huge function marked with an INLINE pragma. This causes the unoptimised unfolding for timeOfDay to be inlined into the unfolding for utcTime which is then inlined 12 times in the example Ryan posted. So there is a lot of duplicated work because of how the inline pragmas cause unfoldings not to be optimised.

It seems like this is likely an issue that will need to be fixed upstream.

added upstream label

closed

Way ahead of you :)

The timeOfDay issue (or to be more precise, the timeOfDay64 issue) was fixed upstream in aeson HEAD. As a result, I'll go ahead and close this.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed upstream label

added Pnormal label