[PATCH] fix to compile-time blowup in LoopStrengthReduce

[Andrew Trick]

LGTM

And committed: r220622.

Thanks Daniel.

-Andy

> On Oct 24, 2014, at 3:04 PM, Daniel Reynaud <dreynaud at apple.com> wrote:
> 
> This is a simple fix that brings the compilation time from 5min to 5s on a specific example I've encountered.
> 
> It's tricky to come up with a test case, and it would just take a long time to build anyway (before the fix).
> 
> Thanks,
> -- daniel
> 
> 
> 
> diff --git a/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/lib/Transforms/Scalar/LoopStrengthReduce.cpp
> index fbc8e0e..e1d18e8 100644
> --- a/lib/Transforms/Scalar/LoopStrengthReduce.cpp
> +++ b/lib/Transforms/Scalar/LoopStrengthReduce.cpp
> @@ -3117,10 +3117,15 @@ void
> LSRInstance::CollectLoopInvariantFixupsAndFormulae() {
>   SmallVector<const SCEV *, 8> Worklist(RegUses.begin(), RegUses.end());
>   SmallPtrSet<const SCEV *, 8> Inserted;
> +  SmallPtrSet<const SCEV *, 32> Done;
> 
>   while (!Worklist.empty()) {
>     const SCEV *S = Worklist.pop_back_val();
> 
> +    // Don't process the same SCEV twice
> +    if (!Done.insert(S))
> +      continue;
> +
>     if (const SCEVNAryExpr *N = dyn_cast<SCEVNAryExpr>(S))
>       Worklist.append(N->op_begin(), N->op_end());
>     else if (const SCEVCastExpr *C = dyn_cast<SCEVCastExpr>(S))
> 
> 
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