question about coercions between primitive types in STG level

[Simon Peyton Jones]

If memory serves, there are primops for converting between unboxed values of different widths.

Certainly converting between a float and a non-float will require an instruction on some architectures, since they use different register sets.

Re (2) I have no idea.  You'll need to get more information... pprTrace or something.

Simon

|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ömer
|  Sinan Agacan
|  Sent: 06 December 2015 18:25
|  To: ghc-devs <ghc-devs at haskell.org>
|  Subject: question about coercions between primitive types in STG level
|  
|  Hi all,
|  
|  In my compiler pass(D1559, see ElimUbxSums.hs) I'm doing some unsafe
|  coercions at the STG level. It works fine for lifted types, but for
|  unlifted ones I'm having some problems. What I'm trying to do is given
|  a number of primitive types I'm finding the one with biggest size, and
|  then generating a constructor that takes this biggest primitive type
|  as argument.
|  
|  The problem is that this is not working very well - GHC is generating
|  illegal instructions that try to load a F32 value to a register
|  allocated for I64, using movss instruction.
|  
|  CoreLint is catching this error and printing this:
|  
|      Cmm lint error:
|        in basic block c1hF
|          in assignment:
|            _g16W::I64 = 4.5 :: W32;   // CmmAssign
|            Reg ty: I64
|            Rhs ty: F32
|  
|  So I have two questions about this:
|  
|  1. Is there a way to safely do this? What are my options here? What
|  I'm trying
|     to do is to use a single data constructor field for different
|  primitive
|     types.  The field is guaranteed to be as big as necessary.
|  
|  2. In the Cmm code shown above, the type annotation is showing `W32`
|  but in the
|     error message it says `F32`. I'm confused about this, is this error
|  message
|     given because the sizes don't match? (64bits vs 32bits) Why the
|  type
|     annotation says W32 while the value has type F32?
|  
|  Thanks..
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