truncate = float2Int rule is incorrect

This is only on powerpc...

$ cat Test.hs
module Main where

f = read "2147483648"  :: Float -- Hack to prevent inlining
main = print (truncate f :: Int)
b@brian:~$ ghc -no-recomp -O Test.hs   && ./a.out  
2147483647
b@brian:~$ 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.

Trac metadata

Trac field	Value
Version	6.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Multiple
Architecture