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

[Nick Lewycky]

Duncan Sands wrote:
> 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?

I agree. How about:

   /// used - This is a non-capturing use.
   virtual bool used(Use *U);

If unused, the additional function calls would melt away in any 
optimized build.

Nick

> Ciao, Duncan.
>
>> };
>>
>> /// PointerMayBeCaptured - Visit the value and the values derived from
>> it and
>
>
>
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