forked from OSchip/llvm-project
71 lines
2.6 KiB
ReStructuredText
71 lines
2.6 KiB
ReStructuredText
=======
|
|
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
|