mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-08-21 03:29:40 +00:00
5a81e14385
This patch introduces a new mechanism that allows IR modules to co-operatively build pointer sets corresponding to addresses within a given set of globals. One particular 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 class or its derived classes. One way of doing this is for a toolchain component to build, for each class, a bit set that maps to the memory region allocated for the vtables, such that each 1 bit in the bit set maps to a valid vtable for that class, and lay out the vtables next to each other, to minimize the total size of the bit sets. The patch introduces a metadata format for representing pointer sets, an '@llvm.bitset.test' intrinsic and an LTO lowering pass that lays out the globals and builds the bitsets, and documents the new feature. Differential Revision: http://reviews.llvm.org/D7288 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230054 91177308-0d34-0410-b5e6-96231b3b80d8
67 lines
2.3 KiB
ReStructuredText
67 lines
2.3 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. 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
|
|
}
|