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116 lines
4.1 KiB
ReStructuredText
116 lines
4.1 KiB
ReStructuredText
=======
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Bitsets
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=======
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This is a mechanism that allows IR modules to co-operatively build pointer
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sets corresponding to addresses within a given set of globals. One example
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of a use case for this is to allow a C++ program to efficiently verify (at
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each call site) that a vtable pointer is in the set of valid vtable pointers
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for the type of the class or its derived classes.
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To use the mechanism, a client creates a global metadata node named
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``llvm.bitsets``. Each element is a metadata node with three elements:
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1. a metadata object representing an identifier for the bitset
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2. either a global variable or a function
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3. a byte offset into the global (generally zero for functions)
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Each bitset must exclusively contain either global variables or functions.
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.. admonition:: Limitation
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The current implementation only supports functions as members of bitsets on
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the x86-32 and x86-64 architectures.
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This will cause a link-time optimization pass to generate bitsets from the
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memory addresses referenced from the elements of the bitset metadata. The
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pass will lay out referenced global variables consecutively, so their
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definitions must be available at LTO time.
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A bit set containing functions is transformed into a jump table, which
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is a block of code consisting of one branch instruction for each of the
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functions in the bit set that branches to the target function, and redirect
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any taken function addresses to the corresponding jump table entry. In the
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object file's symbol table, the jump table entries take the identities of
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the original functions, so that addresses taken outside the module will pass
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any verification done inside the module.
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Jump tables may call external functions, so their definitions need not
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be available at LTO time. Note that if an externally defined function is a
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member of a bitset, there is no guarantee that its identity within the module
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will be the same as its identity outside of the module, as the former will
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be the jump table entry if a jump table is necessary.
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The `GlobalLayoutBuilder`_ class is responsible for laying out the globals
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efficiently to minimize the sizes of the underlying bitsets. An intrinsic,
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:ref:`llvm.bitset.test <bitset.test>`, generates code to test whether a
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given pointer is a member of a bitset.
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:Example:
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::
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target datalayout = "e-p:32:32"
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@a = internal global i32 0
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@b = internal global i32 0
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@c = internal global i32 0
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@d = internal global [2 x i32] [i32 0, i32 0]
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define void @e() {
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ret void
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}
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define void @f() {
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ret void
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}
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declare void @g()
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!llvm.bitsets = !{!0, !1, !2, !3, !4, !5, !6}
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!0 = !{!"bitset1", i32* @a, i32 0}
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!1 = !{!"bitset1", i32* @b, i32 0}
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!2 = !{!"bitset2", i32* @b, i32 0}
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!3 = !{!"bitset2", i32* @c, i32 0}
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!4 = !{!"bitset2", i32* @d, i32 4}
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!5 = !{!"bitset3", void ()* @e, i32 0}
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!6 = !{!"bitset3", void ()* @g, i32 0}
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declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
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define i1 @foo(i32* %p) {
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%pi8 = bitcast i32* %p to i8*
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%x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset1")
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ret i1 %x
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}
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define i1 @bar(i32* %p) {
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%pi8 = bitcast i32* %p to i8*
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%x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset2")
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ret i1 %x
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}
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define i1 @baz(void ()* %p) {
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%pi8 = bitcast void ()* %p to i8*
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%x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset3")
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ret i1 %x
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}
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define void @main() {
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%a1 = call i1 @foo(i32* @a) ; returns 1
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%b1 = call i1 @foo(i32* @b) ; returns 1
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%c1 = call i1 @foo(i32* @c) ; returns 0
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%a2 = call i1 @bar(i32* @a) ; returns 0
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%b2 = call i1 @bar(i32* @b) ; returns 1
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%c2 = call i1 @bar(i32* @c) ; returns 1
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%d02 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 0)) ; returns 0
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%d12 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 1)) ; returns 1
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%e = call i1 @baz(void ()* @e) ; returns 1
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%f = call i1 @baz(void ()* @f) ; returns 0
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%g = call i1 @baz(void ()* @g) ; returns 1
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ret void
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}
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.. _GlobalLayoutBuilder: http://llvm.org/klaus/llvm/blob/master/include/llvm/Transforms/IPO/LowerBitSets.h
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