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

[Johannes Doerfert via llvm-commits]

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);
+  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;
   }