runtime/gc/model.h

   1 /* Copyright (C) 2005-2007 Henry Cejtin, Matthew Fluet, Suresh
   2  *    Jagannathan, and Stephen Weeks.
   3  *
   4  * MLton is released under a BSD-style license.
   5  * See the file MLton-LICENSE for details.
   6  */
   7
   8 #if (defined (MLTON_GC_INTERNAL_TYPES))
   9
  10 /*
  11 Consider the following schemes for representing object pointers and
  12 mapping them to 64-bit native pointers.
  13
  14 A. 32 bits, with bottom two bits zero.
  15 B. 32 bits, with bottom bit zero, shift left by one.
  16 C. 32 bits, with bottom bit zero, shift left by two.
  17 D. 32 bits, shift left by two.
  18 E. 32 bits, shift left by three.
  19 F. 40 bits, with bottom two bits zero.
  20 G. 64 bits, with bottom two bits zero.
  21
  22 These schemes vary in the number of bits to represent a pointer in an
  23 object, the time to load a pointer from memory into a register, the
  24 amount of addressable memory, and the object alignment.
  25
  26         bits    time    mem(G)  align
  27 A       32      fast      4     4
  28 B       32      slow      8     4
  29 C       32      slow     16     8
  30 D       32      slow     16     4
  31 E       32      slow     32     8
  32 F       40      slow    256     4
  33 G       64      fast     4G     4
  34
  35 Each of the (A-F) has a variant (AX-FX) in which pointers are added to
  36 some constant base address.  This gives access to any region in the
  37 virtual address space instead of just the low addresses.
  38
  39 The following diagram demonstrates what portion of the native pointer
  40 to which the object pointer corresponds.
  41
  42 64                              32                              0
  43 |                               |                               |
  44 -----------------------------------------------------------------
  45
  46                                  ==============================00   A
  47
  48                                 ===============================0    B
  49
  50                                ===============================0     C
  51
  52                                ================================     D
  53
  54                              =================================      E
  55
  56                          ======================================00   F
  57
  58  ==============================================================00   G
  59
  60 Algorithmically, we can compute the native pointer (P) from the object
  61 pointer (O) (with bitsize Z), given a shift (S) and a base (B):
  62
  63 P = %add64(%shl64(%zxZ_64(O),S),B)
  64
  65 Likewise, we can compute the object pointer (O) from the native
  66 pointer (P), given a shift (S) and a base (B):
  67
  68 O = %lobits64_Z(%shr64(%sub64(P,B),S))
  69
  70 Hence, each of the schemes may be characterized by the size Z of the
  71 object pointer, the shift S, and whether or not the base B is zero.
  72 Noting that
  73  %zx64_64(x) = x
  74  %shl64(x, 0) = x
  75  %add64(x, 0) = x
  76  %lobits64_64(x) = x
  77  %shr64(x, 0) = x
  78  %sub64(x, 0) = x
  79 it is easy to compute the number of ALU operations required by each
  80 scheme:
  81
  82 A  :: Z = 32, S == 0, B == 0   ops = 1
  83 AX :: Z = 32, S == 0, B != 0   ops = 2
  84 B  :: Z = 32, S == 1, B == 0   ops = 2
  85 BX :: Z = 32, S == 1, B != 0   ops = 3
  86 C  :: Z = 32, S == 2, B == 0   ops = 2
  87 CX :: Z = 32, S == 2, B != 0   ops = 3
  88 D  :: Z = 32, S == 2, B == 0   ops = 2
  89 DX :: Z = 32, S == 2, B != 0   ops = 3
  90 E  :: Z = 32, S == 3, B == 0   ops = 2
  91 EX :: Z = 32, S == 3, B != 0   ops = 3
  92 F  :: Z = 40, S == 0, B == 0   ops = 1 (#)
  93 FX :: Z = 40, S == 0, B != 0   ops = 2 (#)
  94 G  :: Z = 64, S == 0, B == 0   ops = 0
  95
  96 #: In schemes F and FX, the conversion from object pointer to native
  97 pointer requires logical-shift-right, rather than zero-extend, since
  98 the object pointer would be fetched from memory as a 64-bit value.
  99 The cost may actually be higher, as storing an object pointer in
 100 memory requires some care so as not to overwrite neighboring data.
 101
 102 It is not clear that any of the thirteen schemes dominates another.
 103 Here are some thoughts.
 104
 105 (A) This is is what we have now, but is still useful on 64-bit
 106 machines where the bottom 4G may be less cluttered than on a 32-bit
 107 machine.
 108
 109 (AX) seems like a nice cost/benefit tradeoff for a program that only
 110 needs 4G of memory, since the base can be used to find a contiguous 4G
 111 somewhere in the address space.
 112
 113 (B) and (C) are similar, the tradeoff being to increase object
 114 alignment requirements in order to allow more memory.  Importantly,
 115 pointers having a bottom zero bit means that we can still set the
 116 bottom bit to one to represent small values in sum types.
 117
 118 (D) and (E) are problematic because they leave no room to represent
 119 small objects in sum types with pointers.  I think that really rules
 120 them out.
 121
 122 (F) costs some in object alignment because a sequence of pointers in
 123 an object may have to be padded to meet 4-byte alignment.  Loading a
 124 pointer from memory into a register may be slightly faster than in
 125 (B) or (C) because we don't have to shift, but I wonder if that
 126 matters.
 127
 128 (G) costs the most in space, but has the fastest load time for
 129 pointers of the schemes that allow access to 4G of memory.
 130
 131 A reasonable tradeoff in implementation complexity vs allowing our
 132 users enough flexibility might be to provide:
 133
 134         A, AX, B, BX, C, CX, G
 135
 136 After some experiments on those, we might be able to find a more
 137 manageable set for users.
 138 */
 139
 140 #if defined (GC_MODEL_NATIVE32)
 141 #define GC_MODEL_OBJPTR_SIZE 32
 142 #define GC_MODEL_OBJPTR_SHIFT 0
 143 #define GC_MODEL_OBJPTR_BASE 0
 144 #define GC_MODEL_HEADER_SIZE 32
 145 #define GC_MODEL_ARRLEN_SIZE 32
 146 #elif defined (GC_MODEL_NATIVE64)
 147 #define GC_MODEL_OBJPTR_SIZE 64
 148 #define GC_MODEL_OBJPTR_SHIFT 0
 149 #define GC_MODEL_OBJPTR_BASE 0
 150 #define GC_MODEL_HEADER_SIZE 64
 151 #define GC_MODEL_ARRLEN_SIZE 64
 152 #else
 153 #error GC_MODEL_* undefined
 154 #endif
 155
 156 #define GC_MODEL_MINALIGN_SHIFT max(2, GC_MODEL_OBJPTR_SHIFT + 1)
 157 #define GC_MODEL_MINALIGN TWOPOWER(GC_MODEL_MINALIGN_SHIFT)
 158
 159 #endif /* (defined (MLTON_GC_INTERNAL_TYPES)) */