[llvm-dev] analysis based on nonnull attribute

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Sanjay Patel via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Wednesday, December 14, 2016 3:47:03 PM
> Subject: [llvm-dev] analysis based on nonnull attribute

> Does the nonnull parameter attribute give us information about
> subsequent uses of that value outside of the function that has the
> attribute?

Yes. We're guaranteeing that we never pass a null value for the argument, so that information can be used to optimize the caller as well. 

> Example:

> define i1 @bar(i32* nonnull %x) { ; %x must be non-null in this
> function
> %y = load i32, i32* %x
> %z = icmp ugt i32 %y, 23
> ret i1 %z
> }

> define i1 @foo(i32* %x) {
> %d = call i1 @bar(i32* %x)
> %null_check = icmp eq i32* %x, null ; check if null after call that
> guarantees non-null?
> br i1 %null_check, label %t, label %f
> t:
> ret i1 1
> f:
> ret i1 %d
> }

> $ opt -inline nonnull.ll -S
> ...
> define i1 @foo(i32* %x) {
> %y.i = load i32, i32* %x ; inlined and non-null knowledge is lost?

It should be replaced by !nonnull metadata on the load. We might not be doing that today, however. 

-Hal 

> %z.i = icmp ugt i32 %y.i, 23
> %null_check = icmp eq i32* %x, null
> br i1 %null_check, label %t, label %f

> t:
> ret i1 true

> f:
> ret i1 %z.i
> }

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-- 

Hal Finkel 
Lead, Compiler Technology and Programming Languages 
Leadership Computing Facility 
Argonne National Laboratory 
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