GHC/Cmm/Expr.hs

{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances #-}

module GHC.Cmm.Expr
    ( CmmExpr(..), cmmExprType, cmmExprWidth, cmmExprAlignment, maybeInvertCmmExpr
    , CmmReg(..), cmmRegType, cmmRegWidth
    , CmmLit(..), cmmLitType
    , LocalReg(..), localRegType
    , GlobalReg(..), isArgReg, globalRegType
    , spReg, hpReg, spLimReg, hpLimReg, nodeReg
    , currentTSOReg, currentNurseryReg, hpAllocReg, cccsReg
    , node, baseReg
    , VGcPtr(..)

    , DefinerOfRegs, UserOfRegs
    , foldRegsDefd, foldRegsUsed
    , foldLocalRegsDefd, foldLocalRegsUsed

    , RegSet, LocalRegSet, GlobalRegSet
    , emptyRegSet, elemRegSet, extendRegSet, deleteFromRegSet, mkRegSet
    , plusRegSet, minusRegSet, timesRegSet, sizeRegSet, nullRegSet
    , regSetToList

    , Area(..)
    , module GHC.Cmm.MachOp
    , module GHC.Cmm.Type
    )
where

import GHC.Prelude

import GHC.Platform
import GHC.Cmm.BlockId
import GHC.Cmm.CLabel
import GHC.Cmm.MachOp
import GHC.Cmm.Type
import GHC.Driver.Session
import GHC.Utils.Outputable (panic)
import GHC.Types.Unique

import Data.Set (Set)
import qualified Data.Set as Set

import GHC.Types.Basic (Alignment, mkAlignment, alignmentOf)

-----------------------------------------------------------------------------
--              CmmExpr
-- An expression.  Expressions have no side effects.
-----------------------------------------------------------------------------

data CmmExpr
  = CmmLit CmmLit               -- Literal
  | CmmLoad !CmmExpr !CmmType   -- Read memory location
  | CmmReg !CmmReg              -- Contents of register
  | CmmMachOp MachOp [CmmExpr]  -- Machine operation (+, -, *, etc.)
  | CmmStackSlot Area {-# UNPACK #-} !Int
                                -- addressing expression of a stack slot
                                -- See Note [CmmStackSlot aliasing]
  | CmmRegOff !CmmReg Int
        -- CmmRegOff reg i
        --        ** is shorthand only, meaning **
        -- CmmMachOp (MO_Add rep) [x, CmmLit (CmmInt (fromIntegral i) rep)]
        --      where rep = typeWidth (cmmRegType reg)

instance Eq CmmExpr where       -- Equality ignores the types
  CmmLit l1          == CmmLit l2          = l1==l2
  CmmLoad e1 _       == CmmLoad e2 _       = e1==e2
  CmmReg r1          == CmmReg r2          = r1==r2
  CmmRegOff r1 i1    == CmmRegOff r2 i2    = r1==r2 && i1==i2
  CmmMachOp op1 es1  == CmmMachOp op2 es2  = op1==op2 && es1==es2
  CmmStackSlot a1 i1 == CmmStackSlot a2 i2 = a1==a2 && i1==i2
  _e1                == _e2                = False

data CmmReg
  = CmmLocal  {-# UNPACK #-} !LocalReg
  | CmmGlobal GlobalReg
  deriving( Eq, Ord )

-- | A stack area is either the stack slot where a variable is spilled
-- or the stack space where function arguments and results are passed.
data Area
  = Old            -- See Note [Old Area]
  | Young {-# UNPACK #-} !BlockId  -- Invariant: must be a continuation BlockId
                   -- See Note [Continuation BlockId] in GHC.Cmm.Node.
  deriving (Eq, Ord)

{- Note [Old Area]
~~~~~~~~~~~~~~~~~~
There is a single call area 'Old', allocated at the extreme old
end of the stack frame (ie just younger than the return address)
which holds:
  * incoming (overflow) parameters,
  * outgoing (overflow) parameter to tail calls,
  * outgoing (overflow) result values
  * the update frame (if any)

Its size is the max of all these requirements.  On entry, the stack
pointer will point to the youngest incoming parameter, which is not
necessarily at the young end of the Old area.

End of note -}


{- Note [CmmStackSlot aliasing]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When do two CmmStackSlots alias?

 - T[old+N] aliases with U[young(L)+M] for all T, U, L, N and M
 - T[old+N] aliases with U[old+M] only if the areas actually overlap

Or more informally, different Areas may overlap with each other.

An alternative semantics, that we previously had, was that different
Areas do not overlap.  The problem that lead to redefining the
semantics of stack areas is described below.

e.g. if we had

    x = Sp[old + 8]
    y = Sp[old + 16]

    Sp[young(L) + 8]  = L
    Sp[young(L) + 16] = y
    Sp[young(L) + 24] = x
    call f() returns to L

if areas semantically do not overlap, then we might optimise this to

    Sp[young(L) + 8]  = L
    Sp[young(L) + 16] = Sp[old + 8]
    Sp[young(L) + 24] = Sp[old + 16]
    call f() returns to L

and now young(L) cannot be allocated at the same place as old, and we
are doomed to use more stack.

  - old+8  conflicts with young(L)+8
  - old+16 conflicts with young(L)+16 and young(L)+8

so young(L)+8 == old+24 and we get

    Sp[-8]  = L
    Sp[-16] = Sp[8]
    Sp[-24] = Sp[0]
    Sp -= 24
    call f() returns to L

However, if areas are defined to be "possibly overlapping" in the
semantics, then we cannot commute any loads/stores of old with
young(L), and we will be able to re-use both old+8 and old+16 for
young(L).

    x = Sp[8]
    y = Sp[0]

    Sp[8] = L
    Sp[0] = y
    Sp[-8] = x
    Sp = Sp - 8
    call f() returns to L

Now, the assignments of y go away,

    x = Sp[8]
    Sp[8] = L
    Sp[-8] = x
    Sp = Sp - 8
    call f() returns to L
-}

data CmmLit
  = CmmInt !Integer  Width
        -- Interpretation: the 2's complement representation of the value
        -- is truncated to the specified size.  This is easier than trying
        -- to keep the value within range, because we don't know whether
        -- it will be used as a signed or unsigned value (the CmmType doesn't
        -- distinguish between signed & unsigned).
  | CmmFloat  Rational Width
  | CmmVec [CmmLit]                     -- Vector literal
  | CmmLabel    CLabel                  -- Address of label
  | CmmLabelOff CLabel Int              -- Address of label + byte offset

        -- Due to limitations in the C backend, the following
        -- MUST ONLY be used inside the info table indicated by label2
        -- (label2 must be the info label), and label1 must be an
        -- SRT, a slow entrypoint or a large bitmap (see the Mangler)
        -- Don't use it at all unless tablesNextToCode.
        -- It is also used inside the NCG during when generating
        -- position-independent code.
  | CmmLabelDiffOff CLabel CLabel Int Width -- label1 - label2 + offset
        -- In an expression, the width just has the effect of MO_SS_Conv
        -- from wordWidth to the desired width.
        --
        -- In a static literal, the supported Widths depend on the
        -- architecture: wordWidth is supported on all
        -- architectures. Additionally W32 is supported on x86_64 when
        -- using the small memory model.

  | CmmBlock {-# UNPACK #-} !BlockId     -- Code label
        -- Invariant: must be a continuation BlockId
        -- See Note [Continuation BlockId] in GHC.Cmm.Node.

  | CmmHighStackMark -- A late-bound constant that stands for the max
                     -- #bytes of stack space used during a procedure.
                     -- During the stack-layout pass, CmmHighStackMark
                     -- is replaced by a CmmInt for the actual number
                     -- of bytes used
  deriving Eq

cmmExprType :: Platform -> CmmExpr -> CmmType
cmmExprType platform = \case
   (CmmLit lit)        -> cmmLitType platform lit
   (CmmLoad _ rep)     -> rep
   (CmmReg reg)        -> cmmRegType platform reg
   (CmmMachOp op args) -> machOpResultType platform op (map (cmmExprType platform) args)
   (CmmRegOff reg _)   -> cmmRegType platform reg
   (CmmStackSlot _ _)  -> bWord platform -- an address
   -- Careful though: what is stored at the stack slot may be bigger than
   -- an address

cmmLitType :: Platform -> CmmLit -> CmmType
cmmLitType platform = \case
   (CmmInt _ width)     -> cmmBits  width
   (CmmFloat _ width)   -> cmmFloat width
   (CmmVec [])          -> panic "cmmLitType: CmmVec []"
   (CmmVec (l:ls))      -> let ty = cmmLitType platform l
                          in if all (`cmmEqType` ty) (map (cmmLitType platform) ls)
                               then cmmVec (1+length ls) ty
                               else panic "cmmLitType: CmmVec"
   (CmmLabel lbl)       -> cmmLabelType platform lbl
   (CmmLabelOff lbl _)  -> cmmLabelType platform lbl
   (CmmLabelDiffOff _ _ _ width) -> cmmBits width
   (CmmBlock _)         -> bWord platform
   (CmmHighStackMark)   -> bWord platform

cmmLabelType :: Platform -> CLabel -> CmmType
cmmLabelType platform lbl
 | isGcPtrLabel lbl = gcWord platform
 | otherwise        = bWord platform

cmmExprWidth :: Platform -> CmmExpr -> Width
cmmExprWidth platform e = typeWidth (cmmExprType platform e)

-- | Returns an alignment in bytes of a CmmExpr when it's a statically
-- known integer constant, otherwise returns an alignment of 1 byte.
-- The caller is responsible for using with a sensible CmmExpr
-- argument.
cmmExprAlignment :: CmmExpr -> Alignment
cmmExprAlignment (CmmLit (CmmInt intOff _)) = alignmentOf (fromInteger intOff)
cmmExprAlignment _                          = mkAlignment 1
--------
--- Negation for conditional branches

maybeInvertCmmExpr :: CmmExpr -> Maybe CmmExpr
maybeInvertCmmExpr (CmmMachOp op args) = do op' <- maybeInvertComparison op
                                            return (CmmMachOp op' args)
maybeInvertCmmExpr _ = Nothing

-----------------------------------------------------------------------------
--              Local registers
-----------------------------------------------------------------------------

data LocalReg
  = LocalReg {-# UNPACK #-} !Unique CmmType
    -- ^ Parameters:
    --   1. Identifier
    --   2. Type

instance Eq LocalReg where
  (LocalReg u1 _) == (LocalReg u2 _) = u1 == u2

-- This is non-deterministic but we do not currently support deterministic
-- code-generation. See Note [Unique Determinism and code generation]
-- See Note [No Ord for Unique]
instance Ord LocalReg where
  compare (LocalReg u1 _) (LocalReg u2 _) = nonDetCmpUnique u1 u2

instance Uniquable LocalReg where
  getUnique (LocalReg uniq _) = uniq

cmmRegType :: Platform -> CmmReg -> CmmType
cmmRegType _        (CmmLocal  reg) = localRegType reg
cmmRegType platform (CmmGlobal reg) = globalRegType platform reg

cmmRegWidth :: Platform -> CmmReg -> Width
cmmRegWidth platform = typeWidth . cmmRegType platform

localRegType :: LocalReg -> CmmType
localRegType (LocalReg _ rep) = rep

-----------------------------------------------------------------------------
--    Register-use information for expressions and other types
-----------------------------------------------------------------------------

-- | Sets of registers

-- These are used for dataflow facts, and a common operation is taking
-- the union of two RegSets and then asking whether the union is the
-- same as one of the inputs.  UniqSet isn't good here, because
-- sizeUniqSet is O(n) whereas Set.size is O(1), so we use ordinary
-- Sets.

type RegSet r     = Set r
type LocalRegSet  = RegSet LocalReg
type GlobalRegSet = RegSet GlobalReg

emptyRegSet             :: RegSet r
nullRegSet              :: RegSet r -> Bool
elemRegSet              :: Ord r => r -> RegSet r -> Bool
extendRegSet            :: Ord r => RegSet r -> r -> RegSet r
deleteFromRegSet        :: Ord r => RegSet r -> r -> RegSet r
mkRegSet                :: Ord r => [r] -> RegSet r
minusRegSet, plusRegSet, timesRegSet :: Ord r => RegSet r -> RegSet r -> RegSet r
sizeRegSet              :: RegSet r -> Int
regSetToList            :: RegSet r -> [r]

emptyRegSet      = Set.empty
nullRegSet       = Set.null
elemRegSet       = Set.member
extendRegSet     = flip Set.insert
deleteFromRegSet = flip Set.delete
mkRegSet         = Set.fromList
minusRegSet      = Set.difference
plusRegSet       = Set.union
timesRegSet      = Set.intersection
sizeRegSet       = Set.size
regSetToList     = Set.toList

class Ord r => UserOfRegs r a where
  foldRegsUsed :: DynFlags -> (b -> r -> b) -> b -> a -> b

foldLocalRegsUsed :: UserOfRegs LocalReg a
                  => DynFlags -> (b -> LocalReg -> b) -> b -> a -> b
foldLocalRegsUsed = foldRegsUsed

class Ord r => DefinerOfRegs r a where
  foldRegsDefd :: DynFlags -> (b -> r -> b) -> b -> a -> b

foldLocalRegsDefd :: DefinerOfRegs LocalReg a
                  => DynFlags -> (b -> LocalReg -> b) -> b -> a -> b
foldLocalRegsDefd = foldRegsDefd

instance UserOfRegs LocalReg CmmReg where
    foldRegsUsed _ f z (CmmLocal reg) = f z reg
    foldRegsUsed _ _ z (CmmGlobal _)  = z

instance DefinerOfRegs LocalReg CmmReg where
    foldRegsDefd _ f z (CmmLocal reg) = f z reg
    foldRegsDefd _ _ z (CmmGlobal _)  = z

instance UserOfRegs GlobalReg CmmReg where
    foldRegsUsed _ _ z (CmmLocal _)    = z
    foldRegsUsed _ f z (CmmGlobal reg) = f z reg

instance DefinerOfRegs GlobalReg CmmReg where
    foldRegsDefd _ _ z (CmmLocal _)    = z
    foldRegsDefd _ f z (CmmGlobal reg) = f z reg

instance Ord r => UserOfRegs r r where
    foldRegsUsed _ f z r = f z r

instance Ord r => DefinerOfRegs r r where
    foldRegsDefd _ f z r = f z r

instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r CmmExpr where
  -- The (Ord r) in the context is necessary here
  -- See Note [Recursive superclasses] in GHC.Tc.TyCl.Instance
  foldRegsUsed dflags f !z e = expr z e
    where expr z (CmmLit _)          = z
          expr z (CmmLoad addr _)    = foldRegsUsed dflags f z addr
          expr z (CmmReg r)          = foldRegsUsed dflags f z r
          expr z (CmmMachOp _ exprs) = foldRegsUsed dflags f z exprs
          expr z (CmmRegOff r _)     = foldRegsUsed dflags f z r
          expr z (CmmStackSlot _ _)  = z

instance UserOfRegs r a => UserOfRegs r [a] where
  foldRegsUsed dflags f set as = foldl' (foldRegsUsed dflags f) set as
  {-# INLINABLE foldRegsUsed #-}

instance DefinerOfRegs r a => DefinerOfRegs r [a] where
  foldRegsDefd dflags f set as = foldl' (foldRegsDefd dflags f) set as
  {-# INLINABLE foldRegsDefd #-}

-----------------------------------------------------------------------------
--              Global STG registers
-----------------------------------------------------------------------------

data VGcPtr = VGcPtr | VNonGcPtr deriving( Eq, Show )

-----------------------------------------------------------------------------
--              Global STG registers
-----------------------------------------------------------------------------
{-
Note [Overlapping global registers]

The backend might not faithfully implement the abstraction of the STG
machine with independent registers for different values of type
GlobalReg. Specifically, certain pairs of registers (r1, r2) may
overlap in the sense that a store to r1 invalidates the value in r2,
and vice versa.

Currently this occurs only on the x86_64 architecture where FloatReg n
and DoubleReg n are assigned the same microarchitectural register, in
order to allow functions to receive more Float# or Double# arguments
in registers (as opposed to on the stack).

There are no specific rules about which registers might overlap with
which other registers, but presumably it's safe to assume that nothing
will overlap with special registers like Sp or BaseReg.

Use GHC.Cmm.Utils.regsOverlap to determine whether two GlobalRegs overlap
on a particular platform. The instance Eq GlobalReg is syntactic
equality of STG registers and does not take overlap into
account. However it is still used in UserOfRegs/DefinerOfRegs and
there are likely still bugs there, beware!
-}

data GlobalReg
  -- Argument and return registers
  = VanillaReg                  -- pointers, unboxed ints and chars
        {-# UNPACK #-} !Int     -- its number
        VGcPtr

  | FloatReg            -- single-precision floating-point registers
        {-# UNPACK #-} !Int     -- its number

  | DoubleReg           -- double-precision floating-point registers
        {-# UNPACK #-} !Int     -- its number

  | LongReg             -- long int registers (64-bit, really)
        {-# UNPACK #-} !Int     -- its number

  | XmmReg                      -- 128-bit SIMD vector register
        {-# UNPACK #-} !Int     -- its number

  | YmmReg                      -- 256-bit SIMD vector register
        {-# UNPACK #-} !Int     -- its number

  | ZmmReg                      -- 512-bit SIMD vector register
        {-# UNPACK #-} !Int     -- its number

  -- STG registers
  | Sp                  -- Stack ptr; points to last occupied stack location.
  | SpLim               -- Stack limit
  | Hp                  -- Heap ptr; points to last occupied heap location.
  | HpLim               -- Heap limit register
  | CCCS                -- Current cost-centre stack
  | CurrentTSO          -- pointer to current thread's TSO
  | CurrentNursery      -- pointer to allocation area
  | HpAlloc             -- allocation count for heap check failure

                -- We keep the address of some commonly-called
                -- functions in the register table, to keep code
                -- size down:
  | EagerBlackholeInfo  -- stg_EAGER_BLACKHOLE_info
  | GCEnter1            -- stg_gc_enter_1
  | GCFun               -- stg_gc_fun

  -- Base offset for the register table, used for accessing registers
  -- which do not have real registers assigned to them.  This register
  -- will only appear after we have expanded GlobalReg into memory accesses
  -- (where necessary) in the native code generator.
  | BaseReg

  -- The register used by the platform for the C stack pointer. This is
  -- a break in the STG abstraction used exclusively to setup stack unwinding
  -- information.
  | MachSp

  -- The is a dummy register used to indicate to the stack unwinder where
  -- a routine would return to.
  | UnwindReturnReg

  -- Base Register for PIC (position-independent code) calculations
  -- Only used inside the native code generator. It's exact meaning differs
  -- from platform to platform (see module PositionIndependentCode).
  | PicBaseReg

  deriving( Show )

instance Eq GlobalReg where
   VanillaReg i _ == VanillaReg j _ = i==j -- Ignore type when seeking clashes
   FloatReg i == FloatReg j = i==j
   DoubleReg i == DoubleReg j = i==j
   LongReg i == LongReg j = i==j
   -- NOTE: XMM, YMM, ZMM registers actually are the same registers
   -- at least with respect to store at YMM i and then read from XMM i
   -- and similarly for ZMM etc.
   XmmReg i == XmmReg j = i==j
   YmmReg i == YmmReg j = i==j
   ZmmReg i == ZmmReg j = i==j
   Sp == Sp = True
   SpLim == SpLim = True
   Hp == Hp = True
   HpLim == HpLim = True
   CCCS == CCCS = True
   CurrentTSO == CurrentTSO = True
   CurrentNursery == CurrentNursery = True
   HpAlloc == HpAlloc = True
   EagerBlackholeInfo == EagerBlackholeInfo = True
   GCEnter1 == GCEnter1 = True
   GCFun == GCFun = True
   BaseReg == BaseReg = True
   MachSp == MachSp = True
   UnwindReturnReg == UnwindReturnReg = True
   PicBaseReg == PicBaseReg = True
   _r1 == _r2 = False

instance Ord GlobalReg where
   compare (VanillaReg i _) (VanillaReg j _) = compare i j
     -- Ignore type when seeking clashes
   compare (FloatReg i)  (FloatReg  j) = compare i j
   compare (DoubleReg i) (DoubleReg j) = compare i j
   compare (LongReg i)   (LongReg   j) = compare i j
   compare (XmmReg i)    (XmmReg    j) = compare i j
   compare (YmmReg i)    (YmmReg    j) = compare i j
   compare (ZmmReg i)    (ZmmReg    j) = compare i j
   compare Sp Sp = EQ
   compare SpLim SpLim = EQ
   compare Hp Hp = EQ
   compare HpLim HpLim = EQ
   compare CCCS CCCS = EQ
   compare CurrentTSO CurrentTSO = EQ
   compare CurrentNursery CurrentNursery = EQ
   compare HpAlloc HpAlloc = EQ
   compare EagerBlackholeInfo EagerBlackholeInfo = EQ
   compare GCEnter1 GCEnter1 = EQ
   compare GCFun GCFun = EQ
   compare BaseReg BaseReg = EQ
   compare MachSp MachSp = EQ
   compare UnwindReturnReg UnwindReturnReg = EQ
   compare PicBaseReg PicBaseReg = EQ
   compare (VanillaReg _ _) _ = LT
   compare _ (VanillaReg _ _) = GT
   compare (FloatReg _) _     = LT
   compare _ (FloatReg _)     = GT
   compare (DoubleReg _) _    = LT
   compare _ (DoubleReg _)    = GT
   compare (LongReg _) _      = LT
   compare _ (LongReg _)      = GT
   compare (XmmReg _) _       = LT
   compare _ (XmmReg _)       = GT
   compare (YmmReg _) _       = LT
   compare _ (YmmReg _)       = GT
   compare (ZmmReg _) _       = LT
   compare _ (ZmmReg _)       = GT
   compare Sp _ = LT
   compare _ Sp = GT
   compare SpLim _ = LT
   compare _ SpLim = GT
   compare Hp _ = LT
   compare _ Hp = GT
   compare HpLim _ = LT
   compare _ HpLim = GT
   compare CCCS _ = LT
   compare _ CCCS = GT
   compare CurrentTSO _ = LT
   compare _ CurrentTSO = GT
   compare CurrentNursery _ = LT
   compare _ CurrentNursery = GT
   compare HpAlloc _ = LT
   compare _ HpAlloc = GT
   compare GCEnter1 _ = LT
   compare _ GCEnter1 = GT
   compare GCFun _ = LT
   compare _ GCFun = GT
   compare BaseReg _ = LT
   compare _ BaseReg = GT
   compare MachSp _ = LT
   compare _ MachSp = GT
   compare UnwindReturnReg _ = LT
   compare _ UnwindReturnReg = GT
   compare EagerBlackholeInfo _ = LT
   compare _ EagerBlackholeInfo = GT

-- convenient aliases
baseReg, spReg, hpReg, spLimReg, hpLimReg, nodeReg,
  currentTSOReg, currentNurseryReg, hpAllocReg, cccsReg  :: CmmReg
baseReg = CmmGlobal BaseReg
spReg = CmmGlobal Sp
hpReg = CmmGlobal Hp
hpLimReg = CmmGlobal HpLim
spLimReg = CmmGlobal SpLim
nodeReg = CmmGlobal node
currentTSOReg = CmmGlobal CurrentTSO
currentNurseryReg = CmmGlobal CurrentNursery
hpAllocReg = CmmGlobal HpAlloc
cccsReg = CmmGlobal CCCS

node :: GlobalReg
node = VanillaReg 1 VGcPtr

globalRegType :: Platform -> GlobalReg -> CmmType
globalRegType platform = \case
   (VanillaReg _ VGcPtr)    -> gcWord platform
   (VanillaReg _ VNonGcPtr) -> bWord platform
   (FloatReg _)             -> cmmFloat W32
   (DoubleReg _)            -> cmmFloat W64
   (LongReg _)              -> cmmBits W64
   -- TODO: improve the internal model of SIMD/vectorized registers
   -- the right design SHOULd improve handling of float and double code too.
   -- see remarks in "NOTE [SIMD Design for the future]"" in GHC.StgToCmm.Prim
   (XmmReg _) -> cmmVec 4 (cmmBits W32)
   (YmmReg _) -> cmmVec 8 (cmmBits W32)
   (ZmmReg _) -> cmmVec 16 (cmmBits W32)

   Hp         -> gcWord platform -- The initialiser for all
                                 -- dynamically allocated closures
   _          -> bWord platform

isArgReg :: GlobalReg -> Bool
isArgReg (VanillaReg {}) = True
isArgReg (FloatReg {})   = True
isArgReg (DoubleReg {})  = True
isArgReg (LongReg {})    = True
isArgReg (XmmReg {})     = True
isArgReg (YmmReg {})     = True
isArgReg (ZmmReg {})     = True
isArgReg _               = False