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annotate docs/BitSets.rst @ 107:a03ddd01be7e

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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents afa8332a0e37
children
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1 =======
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2 Bitsets
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3 =======
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4
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5 This is a mechanism that allows IR modules to co-operatively build pointer
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6 sets corresponding to addresses within a given set of globals. One example
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7 of a use case for this is to allow a C++ program to efficiently verify (at
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8 each call site) that a vtable pointer is in the set of valid vtable pointers
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9 for the type of the class or its derived classes.
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10
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11 To use the mechanism, a client creates a global metadata node named
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12 ``llvm.bitsets``. Each element is a metadata node with three elements:
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13
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14 1. a metadata object representing an identifier for the bitset
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15 2. either a global variable or a function
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16 3. a byte offset into the global (generally zero for functions)
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17
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18 Each bitset must exclusively contain either global variables or functions.
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19
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20 .. admonition:: Limitation
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21
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22 The current implementation only supports functions as members of bitsets on
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23 the x86-32 and x86-64 architectures.
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24
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25 This will cause a link-time optimization pass to generate bitsets from the
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26 memory addresses referenced from the elements of the bitset metadata. The
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27 pass will lay out referenced global variables consecutively, so their
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28 definitions must be available at LTO time.
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29
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30 A bit set containing functions is transformed into a jump table, which
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31 is a block of code consisting of one branch instruction for each of the
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32 functions in the bit set that branches to the target function, and redirect
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33 any taken function addresses to the corresponding jump table entry. In the
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34 object file's symbol table, the jump table entries take the identities of
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35 the original functions, so that addresses taken outside the module will pass
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36 any verification done inside the module.
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37
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38 Jump tables may call external functions, so their definitions need not
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39 be available at LTO time. Note that if an externally defined function is a
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40 member of a bitset, there is no guarantee that its identity within the module
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41 will be the same as its identity outside of the module, as the former will
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42 be the jump table entry if a jump table is necessary.
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43
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44 The `GlobalLayoutBuilder`_ class is responsible for laying out the globals
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45 efficiently to minimize the sizes of the underlying bitsets. An intrinsic,
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46 :ref:`llvm.bitset.test <bitset.test>`, generates code to test whether a
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47 given pointer is a member of a bitset.
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48
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49 :Example:
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50
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51 ::
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52
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53 target datalayout = "e-p:32:32"
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54
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55 @a = internal global i32 0
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56 @b = internal global i32 0
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57 @c = internal global i32 0
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58 @d = internal global [2 x i32] [i32 0, i32 0]
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59
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60 define void @e() {
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61 ret void
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62 }
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63
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64 define void @f() {
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65 ret void
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66 }
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67
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68 declare void @g()
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69
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70 !llvm.bitsets = !{!0, !1, !2, !3, !4, !5, !6}
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71
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72 !0 = !{!"bitset1", i32* @a, i32 0}
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73 !1 = !{!"bitset1", i32* @b, i32 0}
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74 !2 = !{!"bitset2", i32* @b, i32 0}
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75 !3 = !{!"bitset2", i32* @c, i32 0}
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76 !4 = !{!"bitset2", i32* @d, i32 4}
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77 !5 = !{!"bitset3", void ()* @e, i32 0}
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78 !6 = !{!"bitset3", void ()* @g, i32 0}
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79
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80 declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
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81
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82 define i1 @foo(i32* %p) {
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83 %pi8 = bitcast i32* %p to i8*
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84 %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset1")
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85 ret i1 %x
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86 }
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87
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88 define i1 @bar(i32* %p) {
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89 %pi8 = bitcast i32* %p to i8*
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90 %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset2")
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91 ret i1 %x
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92 }
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93
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94 define i1 @baz(void ()* %p) {
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95 %pi8 = bitcast void ()* %p to i8*
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96 %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset3")
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97 ret i1 %x
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98 }
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99
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100 define void @main() {
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101 %a1 = call i1 @foo(i32* @a) ; returns 1
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102 %b1 = call i1 @foo(i32* @b) ; returns 1
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103 %c1 = call i1 @foo(i32* @c) ; returns 0
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104 %a2 = call i1 @bar(i32* @a) ; returns 0
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105 %b2 = call i1 @bar(i32* @b) ; returns 1
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106 %c2 = call i1 @bar(i32* @c) ; returns 1
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107 %d02 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 0)) ; returns 0
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108 %d12 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 1)) ; returns 1
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109 %e = call i1 @baz(void ()* @e) ; returns 1
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110 %f = call i1 @baz(void ()* @f) ; returns 0
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111 %g = call i1 @baz(void ()* @g) ; returns 1
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112 ret void
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113 }
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114
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115 .. _GlobalLayoutBuilder: http://llvm.org/klaus/llvm/blob/master/include/llvm/Transforms/IPO/LowerBitSets.h