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diff --git a/docs/BitSets.rst b/docs/BitSets.rst new file mode 100644 index 0000000..c6ffdbd --- /dev/null +++ b/docs/BitSets.rst @@ -0,0 +1,70 @@ +======= +Bitsets +======= + +This is a mechanism that allows IR modules to co-operatively build pointer +sets corresponding to addresses within a given set of globals. One example +of a use case for this is to allow a C++ program to efficiently verify (at +each call site) that a vtable pointer is in the set of valid vtable pointers +for the type of the class or its derived classes. + +To use the mechanism, a client creates a global metadata node named +``llvm.bitsets``. Each element is a metadata node with three elements: +the first is a metadata string containing an identifier for the bitset, +the second is a global variable and the third is a byte offset into the +global variable. + +This will cause a link-time optimization pass to generate bitsets from the +memory addresses referenced from the elements of the bitset metadata. The pass +will lay out the referenced globals consecutively, so their definitions must +be available at LTO time. The `GlobalLayoutBuilder`_ class is responsible for +laying out the globals efficiently to minimize the sizes of the underlying +bitsets. An intrinsic, :ref:`llvm.bitset.test <bitset.test>`, generates code +to test whether a given pointer is a member of a bitset. + +:Example: + +:: + + target datalayout = "e-p:32:32" + + @a = internal global i32 0 + @b = internal global i32 0 + @c = internal global i32 0 + @d = internal global [2 x i32] [i32 0, i32 0] + + !llvm.bitsets = !{!0, !1, !2, !3, !4} + + !0 = !{!"bitset1", i32* @a, i32 0} + !1 = !{!"bitset1", i32* @b, i32 0} + !2 = !{!"bitset2", i32* @b, i32 0} + !3 = !{!"bitset2", i32* @c, i32 0} + !4 = !{!"bitset2", i32* @d, i32 4} + + declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone + + define i1 @foo(i32* %p) { + %pi8 = bitcast i32* %p to i8* + %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset1") + ret i1 %x + } + + define i1 @bar(i32* %p) { + %pi8 = bitcast i32* %p to i8* + %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset2") + ret i1 %x + } + + define void @main() { + %a1 = call i1 @foo(i32* @a) ; returns 1 + %b1 = call i1 @foo(i32* @b) ; returns 1 + %c1 = call i1 @foo(i32* @c) ; returns 0 + %a2 = call i1 @bar(i32* @a) ; returns 0 + %b2 = call i1 @bar(i32* @b) ; returns 1 + %c2 = call i1 @bar(i32* @c) ; returns 1 + %d02 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 0)) ; returns 0 + %d12 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 1)) ; returns 1 + ret void + } + +.. _GlobalLayoutBuilder: http://llvm.org/klaus/llvm/blob/master/include/llvm/Transforms/IPO/LowerBitSets.h |