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[IR] Introduce a dereferenceable_or_null(N) attribute.

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Summary:
If a pointer is marked as dereferenceable_or_null(N), LLVM assumes it
is either `null` or `dereferenceable(N)` or both.  This change only
introduces the attribute and adds a token test case for the `llvm-as`
/ `llvm-dis`.  It does not hook up other parts of the optimizer to
actually exploit the attribute -- those changes will come later.

For pointers in address space 0, `dereferenceable(N)` is now exactly
equivalent to `dereferenceable_or_null(N)` && `nonnull`.  For other
address spaces, `dereferenceable(N)` is potentially weaker than
`dereferenceable_or_null(N)` && `nonnull` (since we could have a null
`dereferenceable(N)` pointer).

The motivating case for this change is Java (and other managed
languages), where pointers are either `null` or dereferenceable up to
some usually known-at-compile-time constant offset.

Reviewers: rafael, hfinkel

Reviewed By: hfinkel

Subscribers: nicholas, llvm-commits

Differential Revision: http://reviews.llvm.org/D8650

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235132 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das
2015-04-16 20:29:50 +00:00
parent 3054c3e982
commit 5ff5907996
17 changed files with 181 additions and 31 deletions
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6
test/Bitcode/attributes.ll
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@@ -245,6 +245,12 @@ define void @f41(i8* align 32, double* align 64) {
ret void
}
; CHECK: define dereferenceable_or_null(8) i8* @f42(i8* dereferenceable_or_null(8) %foo)
define dereferenceable_or_null(8) i8* @f42(i8* dereferenceable_or_null(8) %foo) {
entry:
ret i8* %foo
}
; CHECK: attributes #0 = { noreturn }
; CHECK: attributes #1 = { nounwind }
; CHECK: attributes #2 = { readnone }