[cfe-dev] RFC: range attribute

[Matthieu Monrocq]

On Wed, Aug 22, 2012 at 10:23 PM, Xi Wang <xi.wang at gmail.com> wrote:

> Hi,
>
> User-supplied range information would be useful for both static analyzers
> and optimizers.  For static analyzers, the information can be used to
> reduce false positives if the developer knows the physical limit of some
> variables.  For the backend, the information can provide more precise value
> ranges for optimizations.
>
> How about a new range attribute that allows developers to annotate range
> information?
>
> Here's a prototype implementation that adds the range attribute to clang
> and emits range metadata.
>
>   https://github.com/xiw/clang/compare/range
>
> Here's an example.
>
>   struct A {
>       int x;
>       int y __attribute__((range(100, 200)));
>   };
>
>   int z __attribute__((range(250, 500)));
>
>   int foo(struct A *a)
>   {
>       return a->y + z;
>   }
>
> // CHECK: load i32* %y, align 4, !range !0
> // CHECK: load i32* @z, align 4, !range !1
>
> // CHECK: !0 = metadata !{i32 100, i32 201}
> // CHECK: !1 = metadata !{i32 250, i32 501}
>
> One bummer is that !range can only be attached to load instructions, and
> thus the patch doesn't support range attributes on function return values
> (or parameters).  Maybe we can generate some module metadata instead?
>
> - xi
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> cfe-dev at cs.uiuc.edu
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>

Richard Smith spoke about detecting integer overflow/underflow in his own
RFC for catching as much undefined behavior as possible; I believe this
would interact with this proposal since getting out of the range should
probably be seen as overlfowing/underflowing.

I added him so he does not miss this.

-- Matthieu.
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