[llvm-commits] PATCH: stronger tail recursion elimination with nocapture

[Duncan Sands]

Hi Nick,

On 04/10/12 06:59, Nick Lewycky wrote:
> Tail recursion elimination has to check whether allocas are escaping. It
> currently doesn't bother and just assumes that if mem2reg/sroa hasn't removed
> the alloca, then it's an escaping alloca. This patch teaches it to use
> CaptureTracking to find calls to functions where the alloca is passed in through
> an argument marked nocapture, and safely ignore those. Those calls can't
> themselves be marked tail, but they don't block other calls from being marked tail.

I didn't read very far, but...

> --- include/llvm/Analysis/CaptureTracking.h	(revision 165196)
> +++ include/llvm/Analysis/CaptureTracking.h	(working copy)
> @@ -46,12 +46,17 @@
>      /// capture) return false. To search it, return true.
>      ///
>      /// U->getUser() is always an Instruction.
> -    virtual bool shouldExplore(Use *U) = 0;
> +    virtual bool shouldExplore(Use *U);
>
>      /// captured - Information about the pointer was captured by the user of
>      /// use U. Return true to stop the traversal or false to continue looking
>      /// for more capturing instructions.
>      virtual bool captured(Use *U) = 0;
> +
> +    /// temporarilyCaptured - Information about the pointer was captured by the
> +    /// user of use U, but only for a known duration, such as the length of a
> +    /// single function call. Return true to stop the traversal.
> +    virtual bool temporarilyCaptured(Use *U);

this seems way too vaguely defined to me.  Is it just for the duration of a call
or not?  And how can you determine that anyway: what if the callee stores it to
a global?

Ciao, Duncan.

>    };
>
>    /// PointerMayBeCaptured - Visit the value and the values derived from it and