[llvm] 6f9640d - [RegAlloc] Add a complexity limit in growRegion() to cap compilation time.

[Vasileios Porpodas via llvm-commits]

Author: Vasileios Porpodas
Date: 2022-03-03T11:31:07-08:00
New Revision: 6f9640d6a3d57eaa2d0467d94a7b87963fbe53d7

URL: https://github.com/llvm/llvm-project/commit/6f9640d6a3d57eaa2d0467d94a7b87963fbe53d7
DIFF: https://github.com/llvm/llvm-project/commit/6f9640d6a3d57eaa2d0467d94a7b87963fbe53d7.diff

LOG: [RegAlloc] Add a complexity limit in growRegion() to cap compilation time.

growRegion() does not scale in code with BBs with a very large number of edges.
In such code growRegion() becomes a compile-time bottleneck, consuming 60% of
the total compilation time.
This patch adds a limit to the complexity of growRegion() by incrementing a counter
in each iteration. We bail out once the limit is reached.

Differential Revision: https://reviews.llvm.org/D120752

Added: 
    

Modified: 
    llvm/lib/CodeGen/RegAllocGreedy.cpp

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index 817aeb6d4d9ba..85e11d1f6abc4 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -133,6 +133,12 @@ static cl::opt<bool> ConsiderLocalIntervalCost(
              "candidate when choosing the best split candidate."),
     cl::init(false));
 
+static cl::opt<long> GrowRegionComplexityBudget(
+    "grow-region-complexity-budget",
+    cl::desc("growRegion() does not scale with the number of BB edges, so "
+             "limit its budget and bail out once we reach the limit."),
+    cl::init(10000), cl::Hidden);
+
 static RegisterRegAlloc greedyRegAlloc("greedy", "greedy register allocator",
                                        createGreedyRegisterAllocator);
 
@@ -784,6 +790,7 @@ bool RAGreedy::growRegion(GlobalSplitCandidate &Cand) {
   unsigned Visited = 0;
 #endif
 
+  long Budget = GrowRegionComplexityBudget;
   while (true) {
     ArrayRef<unsigned> NewBundles = SpillPlacer->getRecentPositive();
     // Find new through blocks in the periphery of PrefRegBundles.
@@ -791,6 +798,9 @@ bool RAGreedy::growRegion(GlobalSplitCandidate &Cand) {
       // Look at all blocks connected to Bundle in the full graph.
       ArrayRef<unsigned> Blocks = Bundles->getBlocks(Bundle);
       for (unsigned Block : Blocks) {
+        // Limit compilation time by bailing out after we use all our budget.
+        if (Budget-- == 0)
+          return false;
         if (!Todo.test(Block))
           continue;
         Todo.reset(Block);