Stack check not optimized out even if it could be

CmmLayoutStack tries to remove a stack check if a function uses no stack space. It knows that Sp < SpLim is always false, but not that Sp >= SpLim is always true. However, the latter can arise when GHC flips the comparison (which it does sometimes).

An example would be the worker function generated for go:

countDown :: Int -> Int
countDown = go 0
    where
        go acc n | n > 0 = go (acc + 1) (n - 1)
                 | otherwise = acc + n

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
Test case
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Architecture