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The builder is based on a layout algorithm that tries to keep members of small bit sets together. The new layout compresses Chromium's bit sets to around 15% of their original size. Differential Revision: http://reviews.llvm.org/D7796 llvm-svn: 230394
71 lines
2.6 KiB
ReStructuredText
71 lines
2.6 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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the first is a metadata string containing an identifier for the bitset,
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the second is a global variable and the third is a byte offset into the
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global variable.
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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 pass
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will lay out the referenced globals consecutively, so their definitions must
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be available at LTO time. The `GlobalLayoutBuilder`_ class is responsible for
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laying out the globals efficiently to minimize the sizes of the underlying
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bitsets. An intrinsic, :ref:`llvm.bitset.test <bitset.test>`, generates code
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to test whether a 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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!llvm.bitsets = !{!0, !1, !2, !3, !4}
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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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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 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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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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