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

[Nick Lewycky]

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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>
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