[PATCH] Teach TailRecursionElimination (TRE) to handle certain cases of nocapture escaping allocas.

[Michael Gottesman]

Thanks Nick!

Michael

On Jul 5, 2013, at 6:58 PM, Nick Lewycky <nicholas at mxc.ca> wrote:

> Michael Gottesman wrote:
>> Hello llvm-commits!
>> 
>> This patch teaches TRE how to understand/handle no capture allocas in order
>> to allow for more call sites to have the tail marker placed on them by the TRE pass.
> 
> Also relevant, this is the second time this patch has been picked up and continued. Here's a link to the first time:
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20130204/165122.html
>  continued http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20130211/165176.html
> 
> Nick
> 
>> Without the changes introduced into this patch, if TRE saw any allocas at all,
>> TRE would not perform TRE *or* mark callsites with the tail marker.
>> 
>> Because TRE runs after mem2reg, this inadequacy is not a death sentence. But
>> given a callsite A without escaping alloca argument, A may not be able to have
>> the tail marker placed on it due to a separate callsite B having a write-back
>> parameter passed in via an argument with the nocapture attribute.
>> 
>> Assume that B is the only other callsite besides A and B only has nocapture
>> escaping alloca arguments (*NOTE* B may have other arguments that are not passed
>> allocas). In this case not marking A with the tail marker is unnecessarily
>> conservative since:
>> 
>>   1. By assumption A has no escaping alloca arguments itself so it can not
>>      access the caller's stack via its arguments.
>> 
>>   2. Since all of B's escaping alloca arguments are passed as parameters with
>>      the nocapture attribute, we know that B does not stash said escaping
>>      allocas in a manner that outlives B itself and thus could be accessed
>>      indirectly by A.
>> 
>> With the changes introduced by this patch:
>> 
>>   1. If we see any escaping allocas passed as a capturing argument, we do
>>      nothing and bail early.
>> 
>>   2. If we do not see any escaping allocas passed as captured arguments but we
>>      do see escaping allocas passed as nocapture arguments:
>> 
>>        i. We do not perform TRE to avoid PR962 since the code generator produces
>>           significantly worse code for the dynamic allocas that would be created
>>           by the TRE algorithm.
>> 
>>        ii. If we do not return twice, mark call sites without escaping allocas
>>            with the tail marker. *NOTE* This excludes functions with escaping
>>            nocapture allocas.
>> 
>>   3. If we do not see any escaping allocas at all (whether captured or not):
>> 
>>        i. If we do not have usage of setjmp, mark all callsites with the tail
>>           marker.
>> 
>>        ii. If there are no dynamic/variable sized allocas in the function,
>>            attempt to perform TRE on all callsites in the function.
>> 
>> Please Review,
>> Michael
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
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