GHC/CmmToAsm/Reg/Utils.hs

    module GHC.CmmToAsm.Reg.Utils
        ( toRegMap, toVRegMap )
    where

    {- Note [UniqFM and the register allocator]
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

       Before UniqFM had a key type the register allocator
       wasn't picky about key types, using VirtualReg, Reg
       and Unique at various use sites for the same map.

       This is safe.
       * The Unique values come from registers at various
         points where we lose a reference to the original
         register value, but the unique is still valid.

       * VirtualReg is a subset of the registers in Reg's type.
         Making a value of VirtualReg into a Reg in fact doesn't
         change its unique. This is because Reg consists of virtual
         regs and real regs, whose unique values do not overlap.

       * Since the code was written in the assumption that keys are
         not typed it's hard to reverse this assumption now. So we get
         some gnarly but correct code where we often pass around Uniques
         and switch between using Uniques, VirtualReg and RealReg as keys
         of the same map. These issues were always there. But with the
         now-typed keys they become visible. It's a classic case of not all
         correct programs type checking.

       We reduce some of the burden by providing a way to cast

            UniqFM VirtualReg a

       to

            UniqFM Reg a

        in this module. This is safe as Reg is the sum of VirtualReg and
        RealReg. With each kind of register keeping the same unique when
        treated as Reg.

       TODO: If you take offense to this I encourage you to refactor this
       code. I'm sure we can do with less casting of keys and direct use
       of uniques. It might also be reasonable to just use a IntMap directly
       instead of dealing with UniqFM at all.

    -}
    import GHC.Types.Unique.FM
    import GHC.Platform.Reg

    -- These should hopefully be zero cost.

    toRegMap :: UniqFM VirtualReg elt -> UniqFM Reg elt
    toRegMap = unsafeCastUFMKey

    toVRegMap :: UniqFM Reg elt -> UniqFM VirtualReg elt
    toVRegMap = unsafeCastUFMKey