[LLVMdev] Enhancing BasicAliasAnalysis for Rust

[John Kåre Alsaker]

On Tue, Jul 29, 2014 at 6:14 PM, Sean Silva <chisophugis at gmail.com> wrote:

> Did you mean to phrase this as a proposed approach and ask for feasibility?
>
Yes.


> Or are you more interested in the question "Rust has these language
> semantics, how can I get LLVM to best exploit this?"? LLVM may have an
> existing mechanism which would serve your needs.
>
I am interested in that as well. Rust has the very nice property where
references never alias, with the exception of references to UnsafeCell.


>
> Do you have some concrete examples of IR produced by the Rust frontend
> that you are seeing LLVM not optimize as well as it could?
>
An example my proposed approach would solve if the Rust frontend was
modified to output !inheritalias metadata:

!0 = metadata !{}

define void @foo(i8** noalias nocapture readonly %g) {
  %1 = load i8** %g, align 8, !inheritalias !0
  %2 = load i8* %1, align 1
  tail call void @bar(i8 signext %2)
  %3 = load i8** %g, align 8, !inheritalias !0
  %4 = load i8* %3, align 1
  tail call void @bar(i8 signext %4)
  ret void
}

declare void @bar(i8 signext)

With this metadata LLVM could infer that %1 and %3 doesn't alias with
anything since %g doesn't and optimize @foo to:

define void @foo(i8** noalias nocapture readonly %g) {
  %1 = load i8** %g, align 8, !inheritalias !0
  %2 = load i8* %1, align 1
  tail call void @bar(i8 signext %2)
  tail call void @bar(i8 signext %2)
  ret void
}



> -- Sean Silva
>
>
> On Mon, Jul 28, 2014 at 6:35 PM, John Kåre Alsaker <
> john.mailinglists at gmail.com> wrote:
>
>> Since Rust references usually never aliases it would be nice if we could
>> exploit this for code generation. One idea I had to improve alias analysis
>> is to insert metadata on pointer loads. !inheritalias could be added to
>> load instructions to indicate that if we know all the aliases of the
>> pointer we load from, then we also know all the aliases of the pointer
>> value loaded. Given that pointer loads are common and things are likely
>> marked with `noalias` in Rust, this seems like useful metadata. It could
>> also for example apply to loading C++'s unique_ptr fields.
>>
>> I'm wondering what the feasibility of extending BasicAliasAnalysis to
>> utilize the proposed metadata would be.
>>
>> _______________________________________________
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>>
>>
>
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