Index: llvm/lib/CodeGen/CodeGenPrepare.cpp
===================================================================
--- llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -215,6 +215,10 @@
     "addr-sink-combine-scaled-reg", cl::Hidden, cl::init(true),
     cl::desc("Allow combining of ScaledReg field in Address sinking."));
 
+static cl::opt<unsigned> BasicBlockMaxSize(
+    "bb-max-size", cl::Hidden, cl::init(1000),
+    cl::desc("Split basic blocks bigger than this size when compiling for Os"));
+
 namespace {
 
 using SetOfInstrs = SmallPtrSet<Instruction *, 16>;
@@ -472,6 +476,31 @@
     EverMadeChange |= MadeChange;
   }
 
+  // Split big basic blocks. We're doing it to save compile time, which is not
+  // a concern on O3.
+  if (!OptSize && BasicBlockMaxSize != 0) {
+    SmallVector<BasicBlock*, 4> HugeBlocksToSplit;
+    for (BasicBlock &BB : F)
+      if (BB.size() > BasicBlockMaxSize)
+        HugeBlocksToSplit.push_back(&BB);
+    MadeChange = false;
+
+    for (BasicBlock *BB : HugeBlocksToSplit) {
+      while (BB->size() > BasicBlockMaxSize) {
+        unsigned n = 0;
+        for (Instruction &I : *BB) {
+          if (n == BasicBlockMaxSize) {
+            BB = SplitBlock(BB, &I);
+            MadeChange = true;
+            break;
+          }
+          n++;
+        }
+      }
+    }
+    EverMadeChange |= MadeChange;
+  }
+
   if (!DisableGCOpts) {
     SmallVector<Instruction *, 2> Statepoints;
     for (BasicBlock &BB : F)