    -- | Free regs map for PowerPC
    module RegAlloc.Linear.PPC.FreeRegs
    where

    import PPC.Regs
    import RegClass
    import Reg

    import Outputable
    import Platform

    import Data.Word
    import Data.Bits
    -- import Data.List

    -- The PowerPC has 32 integer and 32 floating point registers.
    -- This is 32bit PowerPC, so Word64 is inefficient - two Word32s are much
    -- better.
    -- Note that when getFreeRegs scans for free registers, it starts at register
    -- 31 and counts down. This is a hack for the PowerPC - the higher-numbered
    -- registers are callee-saves, while the lower regs are caller-saves, so it
    -- makes sense to start at the high end.
    -- Apart from that, the code does nothing PowerPC-specific, so feel free to
    -- add your favourite platform to the #if (if you have 64 registers but only
    -- 32-bit words).

    data FreeRegs = FreeRegs !Word32 !Word32
                  deriving( Show )  -- The Show is used in an ASSERT

    noFreeRegs :: FreeRegs
    noFreeRegs = FreeRegs 0 0

    releaseReg :: RealReg -> FreeRegs -> FreeRegs
    releaseReg (RealRegSingle r) (FreeRegs g f)
        | r > 31    = FreeRegs g (f .|. (1 `shiftL` (r - 32)))
        | otherwise = FreeRegs (g .|. (1 `shiftL` r)) f

    releaseReg _ _
            = panic "RegAlloc.Linear.PPC.releaseReg: bad reg"

    initFreeRegs :: Platform -> FreeRegs
    initFreeRegs platform = foldr releaseReg noFreeRegs (allocatableRegs platform)

    getFreeRegs :: RegClass -> FreeRegs -> [RealReg]        -- lazily
    getFreeRegs cls (FreeRegs g f)
        | RcDouble <- cls = go f (0x80000000) 63
        | RcInteger <- cls = go g (0x80000000) 31
        | otherwise = pprPanic "RegAllocLinear.getFreeRegs: Bad register class" (ppr cls)
        where
            go _ 0 _ = []
            go x m i | x .&. m /= 0 = RealRegSingle i : (go x (m `shiftR` 1) $! i-1)
                     | otherwise    = go x (m `shiftR` 1) $! i-1

    allocateReg :: RealReg -> FreeRegs -> FreeRegs
    allocateReg (RealRegSingle r) (FreeRegs g f)
        | r > 31    = FreeRegs g (f .&. complement (1 `shiftL` (r - 32)))
        | otherwise = FreeRegs (g .&. complement (1 `shiftL` r)) f

    allocateReg _ _
            = panic "RegAlloc.Linear.PPC.allocateReg: bad reg"