[polly] r269045 - Prevent complex access ranges with low number of pieces.

[Michael Kruse via llvm-commits]

2016-05-10 13:46 GMT+02:00 Johannes Doerfert via llvm-commits
<llvm-commits at lists.llvm.org>:
> Author: jdoerfert
> Date: Tue May 10 06:46:57 2016
> New Revision: 269045
>
> URL: http://llvm.org/viewvc/llvm-project?rev=269045&view=rev
> Log:
> Prevent complex access ranges with low number of pieces.
>
>   Previously we checked the number of pieces to decide whether or not a
>   invariant load was to complex to be generated. However, there are
>   cases when e.g., divisions cause the complexity to spike regardless of
>   the number of pieces. To this end we now check the number of totally
>   involved dimensions which will increase with the number of pieces but
>   also the number of divisions.
>
>
> Modified:
>     polly/trunk/lib/CodeGen/IslNodeBuilder.cpp
>
> Modified: polly/trunk/lib/CodeGen/IslNodeBuilder.cpp
> URL: http://llvm.org/viewvc/llvm-project/polly/trunk/lib/CodeGen/IslNodeBuilder.cpp?rev=269045&r1=269044&r2=269045&view=diff
> ==============================================================================
> --- polly/trunk/lib/CodeGen/IslNodeBuilder.cpp (original)
> +++ polly/trunk/lib/CodeGen/IslNodeBuilder.cpp Tue May 10 06:46:57 2016
> @@ -48,11 +48,11 @@
>  using namespace polly;
>  using namespace llvm;
>
> -// The maximal number of basic sets we allow during invariant load construction.
> +// The maximal number of dimensions we allow during invariant load construction.
>  // More complex access ranges will result in very high compile time and are also
>  // unlikely to result in good code. This value is very high and should only
>  // trigger for corner cases (e.g., the "dct_luma" function in h264, SPEC2006).
> -static int const MaxDisjunctionsInAccessRange = 80;
> +static int const MaxDimensionsInAccessRange = 9;
>
>  __isl_give isl_ast_expr *
>  IslNodeBuilder::getUpperBound(__isl_keep isl_ast_node *For,
> @@ -922,10 +922,22 @@ bool IslNodeBuilder::materializeParamete
>    return true;
>  }
>
> +/// @brief Add the number of dimensions in @p BS to @p U.
> +static isl_stat countTotalDims(isl_basic_set *BS, void *U) {
> +  unsigned *NumTotalDim = static_cast<unsigned *>(U);
> +  *NumTotalDim += isl_basic_set_total_dim(BS);

The total number of dimensions doesn't look like a useful metric for
complexity. The number of parameters and domain contribute to it even
if there are no constraints on it. With the current threshold, the
test will fail eg. with 2 basic sets in a space of 5 parameters or
even unconditionally with 10 parameters.

I suggest to use total number of constraints and/or
isl_basic_set_dim(BS, isl_dim_div).

Michael

> +  isl_basic_set_free(BS);
> +  return isl_stat_ok;
> +}
> +
>  Value *IslNodeBuilder::preloadUnconditionally(isl_set *AccessRange,
>                                                isl_ast_build *Build,
>                                                Instruction *AccInst) {
> -  if (isl_set_n_basic_set(AccessRange) > MaxDisjunctionsInAccessRange) {
> +
> +  // TODO: This check could be performed in the ScopInfo already.
> +  unsigned NumTotalDim = 0;
> +  isl_set_foreach_basic_set(AccessRange, countTotalDims, &NumTotalDim);
> +  if (NumTotalDim > MaxDimensionsInAccessRange) {
>      isl_set_free(AccessRange);
>      return nullptr;
>    }
>
>
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