[llvm] r202106 - [SROA] Add a debugging tool which shuffles the slices sequence prior to

[Adrian Prantl]

On Feb 24, 2014, at 19:59, Chandler Carruth <chandlerc at gmail.com> wrote:

> Author: chandlerc
> Date: Mon Feb 24 21:59:29 2014
> New Revision: 202106
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=202106&view=rev
> Log:
> [SROA] Add a debugging tool which shuffles the slices sequence prior to
> sorting it. This helps uncover latent reliance on the original ordering
> which aren't guaranteed to be preserved by std::sort (but often are),
> and which are based on the use-def chain orderings which also aren't
> (technically) guaranteed.
> 
> Only available in C++11 debug builds, and behind a flag to prevent noise
> at the moment, but this is generally useful so figured I'd put it in the
> tree rather than keeping it out-of-tree.
> 
> Modified:
>    llvm/trunk/lib/Transforms/Scalar/SROA.cpp
> 
> Modified: llvm/trunk/lib/Transforms/Scalar/SROA.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SROA.cpp?rev=202106&r1=202105&r2=202106&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/SROA.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/SROA.cpp Mon Feb 24 21:59:29 2014
> @@ -51,10 +51,17 @@
> #include "llvm/Support/Debug.h"
> #include "llvm/Support/ErrorHandling.h"
> #include "llvm/Support/MathExtras.h"
> +#include "llvm/Support/TimeValue.h"
> #include "llvm/Support/raw_ostream.h"
> #include "llvm/Transforms/Utils/Local.h"
> #include "llvm/Transforms/Utils/PromoteMemToReg.h"
> #include "llvm/Transforms/Utils/SSAUpdater.h"
> +
> +#if __cplusplus >= 201103L && !defined(NDEBUG)
> +// We only use this for a debug check in C++11
> +#include <random>
> +#endif
> +
> using namespace llvm;
> 
> STATISTIC(NumAllocasAnalyzed, "Number of allocas analyzed for replacement");
> @@ -73,6 +80,11 @@ STATISTIC(NumVectorized, "Number of vect
> static cl::opt<bool>
> ForceSSAUpdater("force-ssa-updater", cl::init(false), cl::Hidden);
> 
> +/// Hidden option to enable randomly shuffling the slices to help uncover
> +/// instability in their order.
> +static cl::opt<bool> SROARandomShuffleSlices("sroa-random-shuffle-slices",
> +                                             cl::init(false), cl::Hidden);
> +
> namespace {
> /// \brief A custom IRBuilder inserter which prefixes all names if they are
> /// preserved.
> @@ -690,6 +702,13 @@ AllocaSlices::AllocaSlices(const DataLay
>                               std::mem_fun_ref(&Slice::isDead)),
>                Slices.end());
> 
> +#if __cplusplus >= 201103L && !defined(NDEBUG)
> +  if (SROARandomShuffleSlices) {
> +    std::mt19937 MT(static_cast<unsigned>(sys::TimeValue::now().msec()));
> +    std::shuffle(Slices.begin(), Slices.end(), MT);
> +  }
> +#endif
> +
>   // Sort the uses. This arranges for the offsets to be in ascending order,
>   // and the sizes to be in descending order.
>   std::sort(Slices.begin(), Slices.end());

Maybe I’m missing something obvious here, but isn’t the shuffle a no-op if it is followed immediately by a sort?

-- adrian