(original) (raw)

There are no existing attributes for this in upstream, but you can hook isAllocLikeFunction to get the effect you're looking for.

Philip

On 11/17/2017 05:17 AM, Siddharth Bhat via llvm-dev wrote:

Hello all,

I have a custom allocator, and would like to teach LLVM about its semantics. In particular, I would like LLVM to kill allocations that are "dead", similar to dead new in C++.

Consider this example:

; ModuleID = ''
source\_filename = "Module"

; Function Attrs: inaccessiblememonly noinline norecurse nounwind
declare i8\* @alloc(i64) local\_unnamed\_addr #0

; Function Attrs: inaccessiblememonly noinline norecurse nounwind
declare void @useClosure(i8\*) local\_unnamed\_addr #1

; Function Attrs: alwaysinline norecurse nounwind
define void @main() #1 {
entry:
%closure.raw = tail call noalias i8\* @alloc(i64 8)
%fn\_slot = bitcast i8\* %closure.raw to void ()\*\*
store void ()\* @"case\_ackerman(atom-3 atom-10)\_alts", void ()\*\* %fn\_slot, align 8, !invariant.group !0
tail call void @useClosure(i8\* %closure.raw)
;========
; dead
%closure.raw.i = tail call noalias i8\* @alloc(i64 24)
%fn\_slot.i = bitcast i8\* %closure.raw.i to void ()\*\*
store void ()\* @"case\_aint()\_alts", void ()\*\* %fn\_slot.i, align 8, !invariant.group !0
;========
ret void
}

; Function Attrs: alwaysinline
declare void @"case\_aint()\_alts"() #2

; Function Attrs: alwaysinline
declare void @"case\_ackerman(atom-3 atom-10)\_alts"() #2

attributes #0 = { inaccessiblememonly noinline norecurse nounwind writeonly }
attributes #1 = { alwaysinline norecurse nounwind }
attributes #2 = { alwaysinline }

!0 = distinct !{!0, !"closure\_invariant\_group"}
\-----

In my view, %closure.raw.i is a dead allocation, because the memory returned by it is not used anywhere. How do I communicate this fact to LLVM?

I tried seeing how clang does it, but I'm not sure - it doesn't seem to have any attribute that would help, except perhaps builtin. Does LLVM know about the special semantics of new?

Cheers,

\~Siddharth.

--

Sending this from my phone, please excuse any typos!

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