On 08/07/2010 17:56, Daniel Fischer wrote:
On Thursday 08 July 2010 18:15:44, Ian Lynagh wrote:
On Thu, Jul 08, 2010 at 04:49:00PM +0200, Daniel Fischer wrote:
unsafeCoerce# :: Word64# -> Double# ?
By the docs, that isn't supposed to work. Is it not supposed to work only because it's not value-preserving (unsafeCoerce# 1## /=## 1.0##) or are there more pitfalls?
It can fail to compile, even; see http://hackage.haskell.org/trac/ghc/ticket/2209
Yeah, tried that myself, with optimisations:
[2 of 2] Compiling Main ( testUCastD.hs, testUCastD.o ) ghc: panic! (the 'impossible' happened) (GHC version 6.12.3 for i386-unknown-linux): getRegister(x86) I64[R1 + 3]
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
But without optimisations, it compiles and seems to work (I won't rely on that, was just curious).
Should I report it or is the panic okay since unsafeCoerce# isn't supposed to work for those types?
The panic is to be expected. Basically this use of unsafeCoerce# can break some assumptions in the code generator, so it gets into a state where it was expecting an integer and it finds a float. I think -dcmm-lint will catch it. It's fixable; we have to add some explicit coercions, and extend the code generator to handle them. Cheers, Simon