BBVectorize: Cap the number of candidate pairs in each instruction group

For some basic blocks, it is possible to generate many candidate pairs for
relatively few pairable instructions. When many (tens of thousands) of these pairs
are generated for a single instruction group, the time taken to generate and
rank the different vectorization plans can become quite large. As a result, we now
cap the number of candidate pairs within each instruction group. This is done by
closing out the group once the threshold is reached (set now at 3000 pairs).

Although this will limit the overall compile-time impact, this may not be the best
way to achieve this result. It might be better, for example, to prune excessive
candidate pairs after the fact the prevent the generation of short, but highly-connected
groups. We can experiment with this in the future.

This change reduces the overall compile-time slowdown of the csa.ll test case in
PR15222 to ~5x. If 5x is still considered too large, a lower limit can be
used as the default.

This represents a functionality change, but only for very large inputs
(thus, there is no regression test).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175251 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 12 insertions, 1 deletions

diff --git a/include/llvm/Transforms/Vectorize.h b/include/llvm/Transforms/Vectorize.h
index 1ba4d22d5f..d205dbdede 100644
--- a/include/llvm/Transforms/Vectorize.h
+++ b/include/llvm/Transforms/Vectorize.h
@@ -84,6 +84,9 @@ struct VectorizeConfig {
   /// @brief The maximum number of pairable instructions per group.
   unsigned MaxInsts;
 
+  /// @brief The maximum number of candidate instruction pairs per group.
+  unsigned MaxPairs;
+
   /// @brief The maximum number of pairing iterations.
   unsigned MaxIter;
 
diff --git a/lib/Transforms/Vectorize/BBVectorize.cpp b/lib/Transforms/Vectorize/BBVectorize.cpp
index 37638ca3c8..4849a968b0 100644
--- a/lib/Transforms/Vectorize/BBVectorize.cpp
+++ b/lib/Transforms/Vectorize/BBVectorize.cpp
@@ -88,6 +88,10 @@ MaxInsts("bb-vectorize-max-instr-per-group", cl::init(500), cl::Hidden,
   cl::desc("The maximum number of pairable instructions per group"));
 
 static cl::opt<unsigned>
+MaxPairs("bb-vectorize-max-pairs-per-group", cl::init(3000), cl::Hidden,
+  cl::desc("The maximum number of candidate instruction pairs per group"));
+
+static cl::opt<unsigned>
 MaxCandPairsForCycleCheck("bb-vectorize-max-cycle-check-pairs", cl::init(200),
   cl::Hidden, cl::desc("The maximum number of candidate pairs with which to use"
                        " a full cycle check"));
@@ -1164,6 +1168,7 @@ namespace {
                        DenseSet<ValuePair> &FixedOrderPairs,
                        DenseMap<ValuePair, int> &CandidatePairCostSavings,
                        std::vector<Value *> &PairableInsts, bool NonPow2Len) {
+    size_t TotalPairs = 0;
     BasicBlock::iterator E = BB.end();
     if (Start == E) return false;
 
@@ -1210,6 +1215,7 @@ namespace {
         }
 
         CandidatePairs[I].push_back(J);
+        ++TotalPairs;
         if (TTI)
           CandidatePairCostSavings.insert(ValuePairWithCost(ValuePair(I, J),
                                                             CostSavings));
@@ -1233,7 +1239,8 @@ namespace {
         // If we have already found too many pairs, break here and this function
         // will be called again starting after the last instruction selected
         // during this invocation.
-        if (PairableInsts.size() >= Config.MaxInsts) {
+        if (PairableInsts.size() >= Config.MaxInsts ||
+            TotalPairs >= Config.MaxPairs) {
           ShouldContinue = true;
           break;
         }
@@ -3165,6 +3172,7 @@ VectorizeConfig::VectorizeConfig() {
   MaxCandPairsForCycleCheck = ::MaxCandPairsForCycleCheck;
   SplatBreaksChain = ::SplatBreaksChain;
   MaxInsts = ::MaxInsts;
+  MaxPairs = ::MaxPairs;
   MaxIter = ::MaxIter;
   Pow2LenOnly = ::Pow2LenOnly;
   NoMemOpBoost = ::NoMemOpBoost;