[llvm] r233881 - Fix a bug indicated by -fsanitize=shift-exponent.

[Justin Bogner]

Alexey Samsonov <vonosmas at gmail.com> writes:
> Author: samsonov
> Date: Wed Apr  1 20:30:10 2015
> New Revision: 233881
>
> URL: http://llvm.org/viewvc/llvm-project?rev=233881&view=rev
> Log:
> Fix a bug indicated by -fsanitize=shift-exponent.

I noticed a significant performance regression Benchmarks/Shootout/sieve
after this change. I suspect this isn't quite the right fix for the
undefined behaviour.

You can see the performance jump in lnt here:

    http://llvm-lnt.herokuapp.com/db_default/v4/nts/graph?plot.0=3.794.3&highlight_run=9976

There's a big regression between r233879 and r233882. This is the only
interesting change in that range.

> Modified:
>     llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp
>
> Modified: llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp?rev=233881&r1=233880&r2=233881&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp (original)
> +++ llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp Wed Apr  1 20:30:10 2015
> @@ -320,7 +320,7 @@ bool llvm::UnrollRuntimeLoopProlog(Loop
>    // This constraint lets us deal with an overflowing trip count easily; see the
>    // comment on ModVal below.  This check is equivalent to `Log2(Count) <
>    // BEWidth`.
> -  if (static_cast<uint64_t>(Count) > (1ULL << BEWidth))
> +  if (BEWidth < 64 && static_cast<uint64_t>(Count) > (1ULL << BEWidth))

To match what the comment says we're doing, I guess we want:

  if (BEWidth >= 64 || static_cast<uint64_t>(Count) > (1ULL << BEWidth))

since Log2(Count) is guaranteed to be less than 64.

>      return false;
>  
>    // If this loop is nested, then the loop unroller changes the code in
>
>
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