Different behaviour with -O in concurrent program

[simonpj]

Mitar (mmitar@…) reports I am attaching a program which locks (throws an exception <<loop>>) when compiled like:

ghc --make -threaded -main-is Test.main Test.hs -o test-no-opt -package mtl

and doesn't loop when compiled with

ghc --make -threaded -O2 -main-is Test.main Test.hs -o test-opt -package mtl

This should probably not happen.

I am using 6.12.3 on Linux x86_64.

SimonPJ reproduced this with the HEAD:

simonpj@cam-04-unx:~/tmp$ ./test-opt
42
42
42
42
-- Ctrl-C out
simonpj@cam-04-unx:~/tmp$ ./test-no-opt
42
test-no-opt: <<loop>>
/bin/bash: line 0: echo: write error: Broken pipe
/bin/bash: line 0: echo: write error: Broken pipe
/bin/bash: line 0: echo: write error: Broken pipe
-- Ctrl-C out

It's not 100% clear that it's a bug, but Simon M will look at it.

[simonpj]

Test case

[simonmar]

The problem is here:

     let rest' = assert ((fromIntegral $ C.length rest') <= capacity) $ C.pack rest

rest' is _|_, which accounts for the <<loop>>, and the assert is only executed when optimisation is off, which accounts for the different behaviour when compiled with optimisation on.