1=======
2Bitsets
3=======
4
5This is a mechanism that allows IR modules to co-operatively build pointer
6sets corresponding to addresses within a given set of globals. One example
7of a use case for this is to allow a C++ program to efficiently verify (at
8each call site) that a vtable pointer is in the set of valid vtable pointers
9for the type of the class or its derived classes.
10
11To use the mechanism, a client creates a global metadata node named
12``llvm.bitsets``.  Each element is a metadata node with three elements:
13the first is a metadata string containing an identifier for the bitset,
14the second is a global variable and the third is a byte offset into the
15global variable.
16
17This will cause a link-time optimization pass to generate bitsets from the
18memory addresses referenced from the elements of the bitset metadata. The pass
19will lay out the referenced globals consecutively, so their definitions must
20be available at LTO time. The `GlobalLayoutBuilder`_ class is responsible for
21laying out the globals efficiently to minimize the sizes of the underlying
22bitsets. An intrinsic, :ref:`llvm.bitset.test <bitset.test>`, generates code
23to test whether a given pointer is a member of a bitset.
24
25:Example:
26
27::
28
29    target datalayout = "e-p:32:32"
30
31    @a = internal global i32 0
32    @b = internal global i32 0
33    @c = internal global i32 0
34    @d = internal global [2 x i32] [i32 0, i32 0]
35
36    !llvm.bitsets = !{!0, !1, !2, !3, !4}
37
38    !0 = !{!"bitset1", i32* @a, i32 0}
39    !1 = !{!"bitset1", i32* @b, i32 0}
40    !2 = !{!"bitset2", i32* @b, i32 0}
41    !3 = !{!"bitset2", i32* @c, i32 0}
42    !4 = !{!"bitset2", i32* @d, i32 4}
43
44    declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
45
46    define i1 @foo(i32* %p) {
47      %pi8 = bitcast i32* %p to i8*
48      %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset1")
49      ret i1 %x
50    }
51
52    define i1 @bar(i32* %p) {
53      %pi8 = bitcast i32* %p to i8*
54      %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset2")
55      ret i1 %x
56    }
57
58    define void @main() {
59      %a1 = call i1 @foo(i32* @a) ; returns 1
60      %b1 = call i1 @foo(i32* @b) ; returns 1
61      %c1 = call i1 @foo(i32* @c) ; returns 0
62      %a2 = call i1 @bar(i32* @a) ; returns 0
63      %b2 = call i1 @bar(i32* @b) ; returns 1
64      %c2 = call i1 @bar(i32* @c) ; returns 1
65      %d02 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 0)) ; returns 0
66      %d12 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 1)) ; returns 1
67      ret void
68    }
69
70.. _GlobalLayoutBuilder: http://llvm.org/klaus/llvm/blob/master/include/llvm/Transforms/IPO/LowerBitSets.h
71