#5837: Context reduction stack overflow can take very long
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Reporter: dreixel | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 7.6.1
Component: Compiler (Type checker) | Version: 7.4.1-rc2
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonpj):
OK, yes, this is much better. With HEAD:
{{{
simonpj@cam-05-unx:~/tmp$ ~/5builds/HEAD/inplace/bin/ghc-stage1 -c -ddump-
cs-trace -fcontext-stack=5 T5837.hs
1>[0] Top react: Fun/Top (given)
2>[0] Top react: Fun/Top (given)
3>[1] Top react: Fun/Top (given)
4>[0] Top react: Fun/Top (given)
5>[1] Top react: Fun/Top (given)
6>[2] Top react: Fun/Top (given)
7>[0] Top react: Fun/Top (given)
8>[1] Top react: Fun/Top (given)
9>[2] Top react: Fun/Top (given)
10>[3] Top react: Fun/Top (given)
11>[0] Top react: Fun/Top (given)
12>[1] Top react: Fun/Top (given)
13>[2] Top react: Fun/Top (given)
14>[3] Top react: Fun/Top (given)
15>[4] Top react: Fun/Top (given)
16>[0] Top react: Fun/Top (given)
17>[1] Top react: Fun/Top (given)
18>[2] Top react: Fun/Top (given)
19>[3] Top react: Fun/Top (given)
20>[4] Top react: Fun/Top (given)
21>[5] Top react: Fun/Top (given)
T5837.hs:8:6:
Context reduction stack overflow; size = 6
Use -fcontext-stack=N to increase stack size to N
cobox :: TF (TF (TF (TF (TF a))))
~ (TF (TF (TF (TF (TF (TF a))))), TF (TF (TF (TF (TF (TF
Int))))))
}}}
Notice that the context stack is set to 5, but the trace goes
{{{
0
0, 1
0, 1, 2
0, 1, 2, 3
0, 1, 2, 3, 4
etc
}}}
This quadratic behaviour is bad with a depth of 200!
But with the new `ghc-new-flavor` branch we see
{{{
simonpj@cam-05-unx:~/tmp$ ~/5builds/ghc-solver/inplace/bin/ghc-stage1 -c
-ddump-cs-trace -fcontext-stack=5 T5837.hs
1>[0] Top react: Fun/Top
2>[1] Top react: Fun/Top
3>[2] Top react: Fun/Top
4>[3] Top react: Fun/Top
5>[4] Top react: Fun/Top
6>[5] Top react: Fun/Top
T5837.hs:8:6:
Context reduction stack overflow; size = 6
Use -fcontext-stack=N to increase stack size to N
(TF (TF (TF (TF (TF (TF a))))), TF (TF (TF (TF (TF (TF Int))))))
~ TF (TF (TF (TF (TF a))))
}}}
Which is fine.
But there is still something wrong! If you try the above you'll see the
output coming out fairly slowly. And indeed adding `+RTS -s` and trying
with different context stack sizes gives
{{{
-fcontext-stack=N Bytes allocated
100 366M
200 1,700M
300 5,041M
}}}
Badly non-linear. This is with a standard stage2 build, so no DEBUG flags
on.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5837#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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