[PATCH] D120752: [RegAlloc] Add a complexity limit in growRegion() to cap compilation time.

[Vasileios Porpodas via Phabricator via llvm-commits]

vporpo created this revision.
vporpo added reviewers: qcolombet, wenlei.
Herald added subscribers: hiraditya, MatzeB.
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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 time spent in growRegion(). We bail out once
the limit is reached.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D120752

Files:
  llvm/lib/CodeGen/RegAllocGreedy.cpp


Index: llvm/lib/CodeGen/RegAllocGreedy.cpp
===================================================================
--- llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -133,6 +133,12 @@
              "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, so limit its budget. "
+             "Bail out if we explore this many edges."),
+    cl::init(10000), cl::Hidden);
+
 static RegisterRegAlloc greedyRegAlloc("greedy", "greedy register allocator",
                                        createGreedyRegisterAllocator);
 
@@ -784,6 +790,7 @@
   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 @@
       // 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);


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