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

[Nick Lewycky]

Ping! Surely somebody on llvm-commits remembers how tail recursion
elimination works, right?

Nick

On 8 October 2012 16:35, Nick Lewycky <nlewycky at google.com> wrote:

> Here's a new way to handle it. shouldExplore() is handed each value we're
> going to inspect for the potential that the pointer is leaked. Scan its
> use-list and see if any users are calls or invokes and add those to the
> list.
>
> New patch attached. Please review!
>
> Nick
>
>
> On 4 October 2012 01:42, Nick Lewycky <nicholas at mxc.ca> wrote:
>
>> 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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>>>
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>
>
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