[PATCH] Update TailCallElim to avoid making redundant changes

[Duncan P. N. Exon Smith]

Code is obviously correct, but can you add a testcase?

I think you can observe this misbehaviour in opt by setting
-debug-pass=Executions, which should print a "Made Modifications"
line to dbgs() when a pass reports change things.  You’ll also need
a "REQUIRES: asserts" line.

On 2014 Mar 21, at 17:15, Kevin Modzelewski <kmod at dropbox.com> wrote:

> kmod added you to the CC list for the revision "Update TailCallElim to avoid making redundant changes".
> 
> Hi #llvm,
> 
> While experimenting with an iterated optimization pipeline, I noticed that
> the standard optimization pipeline would always change (most) functions.
> I tracked it down to TailCallElim always calling setTailCall(), even if the call
> is already a tail call; with this change the pipeline does convege after a few
> iterations.
> 
> http://llvm-reviews.chandlerc.com/D3144
> 
> Files:
>  lib/Transforms/Scalar/TailRecursionElimination.cpp
> 
> Index: lib/Transforms/Scalar/TailRecursionElimination.cpp
> ===================================================================
> --- lib/Transforms/Scalar/TailRecursionElimination.cpp
> +++ lib/Transforms/Scalar/TailRecursionElimination.cpp
> @@ -252,8 +252,10 @@
>       for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
>         if (CallInst *CI = dyn_cast<CallInst>(I)) {
>           if (!ACT.UsesAlloca.count(CI)) {
> -            CI->setTailCall();
> -            MadeChange = true;
> +            if (!CI->isTailCall()) {
> +              CI->setTailCall();
> +              MadeChange = true;
> +            }
>           }
>         }
>       }
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