#1254: truncate = int2Float rule is incorrect
-----------------------------------+----------------------------------------
Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: lowest | Milestone: _|_
Component: libraries/base | Version: 6.7
Severity: minor | Keywords:
Difficulty: Easy (1 hr) | Testcase:
Architecture: powerpc | Os: Multiple
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This is only on powerpc...
{{{
$ cat Test.hs
module Main where
f = read "2147483648" :: Float -- Hack to prevent inlining
main = print (truncate f :: Int)
[EMAIL PROTECTED]:~$ ghc -no-recomp -O Test.hs && ./a.out
2147483647
[EMAIL PROTECTED]:~$ ghc -no-recomp -O Test.hs -frules-off && ./a.out
-2147483648
}}}
In GHC.Float we have
{{{
{-# RULES "truncate/Float->Int" truncate = float2Int #-}
...
float2Int (F# x) = I# (float2Int# x)
}}}
But when there is oveflow float2Int# on PPC doesn't behave the same way as
fromIntegral . (truncate::Float -> Integer), it does on x86 though.
I'm not sure if this should be fixed in the libraries or in the codegen.
I'd probably argue that float2Int# should stay the same though, so
programmers have access to a fast float to int conversion when they know
overflow isn't an issue. Something like uncheckedFloat2Int# (like the
shift ops) with a float2Int# wrapper is probably right.
As a side note, the builtin rule in PrelRules for float2Int is also
incorrect as it goes via Integer. If float2Int# in the codegen is modified
this will become correct but if the uncheckedFloat2Int# route is taken
then that rule should probably be fixed.
I may fix this sometime, but for now I just wanted to document it.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1254>
GHC <http://www.haskell.org/ghc/>
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