-flate-dmd-anal triggers "Entered absent arg"

If I compile my program with -flate-dmd-anal, it crashes at run-time with Oops! Entered absent arg w_sJ66e M V1 Double

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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changed weight to 5

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Could you attach your program (preferably after minimizing it as much as possible)?

Yes indeed! You should never get "Entered absent arg", so there's an outright bug there. But we can't fix it unless we can reproduce it.

Simon

See Main.hs here: https://gist.github.com/yongqli/5dc94aeaeda2e24c6637

You need the following packages: linear, lens, vector, vector-th-unbox, strict

Running with ghc Main.hs -O2 -flate-dmd-anal -fforce-recomp; ./Main will give

Main: Oops! Entered absent arg w_sGY6 MVN V1 Double

It takes about 3 days's worth of work to produce a test case like this. Is there a better option than deleting a piece of code and then recompiling? I essentially have to "unwrite" my whole program. Is there a fuzzer-like tool that could conceivably be used?

Good news. I think this is fixed. At least I can't reproduce your crash with HEAD.

I think that one of these two fixes has solved it:

• #10218 (closed)
• #10148 (closed)

Both are merged to the 7.10 branch and so will appear in 7.10.2.

It would be good if you were able to test that the 7.10 branch is indeed fixed in your case. I will try to do the same.

Simon

PS: for "entered absent arg w_sJ66e" bugs, compile with "-ddump-simpl" and grep for that string. That will show exactly which thunk in which function is being entered, and might save time compared to delete/recompile.

Oh VERY interesting! I have reproduced the problem with 7.10, and it is YET ANOTHER bug in the demand analyser. Here is the offending code

dmdAnal' sigs dmd (App fun (Coercion co))
  = (fun_ty, App fun' (Coercion co))
  where
    (fun_ty, fun') = dmdAnal sigs dmd fun

The incoming demand dmd is not modified before passing it on to fun.

But that is wrong wrong wrong. That gets the demands from the function out of step, so the wrong demand goes to the wrong argument. It's a miracle that this has not been causing chaos for ages. Horror. Coercions are value arguments and so they must be treated as such.

I think the solution is simply to delete these lines so that coercion arguments are handled uniformly; but I want to tread carefully.

Thank you for extracting this example. More anon.

changed milestone to %7.10.2

assigned to @simonpj

mentioned in commit d5773a49

mentioned in commit 5c7e4db5

changed weight to 10

yongqli, your test case nailed it -- thank you! It was so helpful to be able to reproduce it, and it's not an easy case to make happen, so I might well have failed to find it, or assumed it was already fixed, without your help.

Austin: please merge the above two to 7.10.

I'm not adding a regression test because it's hard to provoke; and the fix (which consisted only of deleting code) is pretty robust.

Simon

Trac metadata

Trac field	Value
Priority	normal → highest

added 1 deleted label

closed

Merged to ghc-7.10 (via 354f5063).

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

Note that the commit to the ghc-7.10 branch has a typo. I've opened D881 to fix this.

added runtime crash label

added Phighest label