| ======= |
| 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 |