[BOLT] Run EliminateUnreachableBlocks in parallel (llvm#71299)

The wall time for this pass decreased on my laptop from ~80 sec to 5
sec processing the clang.

Author: yota9

bolt/include/bolt/Core/BinaryFunction.h

@@ -1445,7 +1445,8 @@ class BinaryFunction {
 
   /// Rebuilds BBs layout, ignoring dead BBs. Returns the number of removed
   /// BBs and the removed number of bytes of code.
-  std::pair<unsigned, uint64_t> eraseInvalidBBs();
+  std::pair<unsigned, uint64_t>
+  eraseInvalidBBs(const MCCodeEmitter *Emitter = nullptr);
 
   /// Get the relative order between two basic blocks in the original
   /// layout.  The result is > 0 if B occurs before A and < 0 if B

bolt/lib/Core/BinaryFunction.cpp

@@ -322,7 +322,8 @@ void BinaryFunction::markUnreachableBlocks() {
 
 // Any unnecessary fallthrough jumps revealed after calling eraseInvalidBBs
 // will be cleaned up by fixBranches().
-std::pair<unsigned, uint64_t> BinaryFunction::eraseInvalidBBs() {
+std::pair<unsigned, uint64_t>
+BinaryFunction::eraseInvalidBBs(const MCCodeEmitter *Emitter) {
   DenseSet<const BinaryBasicBlock *> InvalidBBs;
   unsigned Count = 0;
   uint64_t Bytes = 0;
@@ -331,7 +332,7 @@ std::pair<unsigned, uint64_t> BinaryFunction::eraseInvalidBBs() {
       assert(!isEntryPoint(*BB) && "all entry blocks must be valid");
       InvalidBBs.insert(BB);
       ++Count;
-      Bytes += BC.computeCodeSize(BB->begin(), BB->end());
+      Bytes += BC.computeCodeSize(BB->begin(), BB->end(), Emitter);
     }
   }
 

bolt/lib/Passes/BinaryPasses.cpp

@@ -317,38 +317,46 @@ void NormalizeCFG::runOnFunctions(BinaryContext &BC) {
 }
 
 void EliminateUnreachableBlocks::runOnFunction(BinaryFunction &Function) {
-  if (!Function.getLayout().block_empty()) {
-    unsigned Count;
-    uint64_t Bytes;
-    Function.markUnreachableBlocks();
-    LLVM_DEBUG({
-      for (BinaryBasicBlock &BB : Function) {
-        if (!BB.isValid()) {
-          dbgs() << "BOLT-INFO: UCE found unreachable block " << BB.getName()
-                 << " in function " << Function << "\n";
-          Function.dump();
-        }
+  BinaryContext &BC = Function.getBinaryContext();
+  unsigned Count;
+  uint64_t Bytes;
+  Function.markUnreachableBlocks();
+  LLVM_DEBUG({
+    for (BinaryBasicBlock &BB : Function) {
+      if (!BB.isValid()) {
+        dbgs() << "BOLT-INFO: UCE found unreachable block " << BB.getName()
+               << " in function " << Function << "\n";
+        Function.dump();
       }
-    });
-    std::tie(Count, Bytes) = Function.eraseInvalidBBs();
-    DeletedBlocks += Count;
-    DeletedBytes += Bytes;
-    if (Count) {
-      Modified.insert(&Function);
-      if (opts::Verbosity > 0)
-        outs() << "BOLT-INFO: removed " << Count
-               << " dead basic block(s) accounting for " << Bytes
-               << " bytes in function " << Function << '\n';
     }
+  });
+  BinaryContext::IndependentCodeEmitter Emitter =
+      BC.createIndependentMCCodeEmitter();
+  std::tie(Count, Bytes) = Function.eraseInvalidBBs(Emitter.MCE.get());
+  DeletedBlocks += Count;
+  DeletedBytes += Bytes;
+  if (Count) {
+    auto L = BC.scopeLock();
+    Modified.insert(&Function);
+    if (opts::Verbosity > 0)
+      outs() << "BOLT-INFO: removed " << Count
+             << " dead basic block(s) accounting for " << Bytes
+             << " bytes in function " << Function << '\n';
   }
 }
 
 void EliminateUnreachableBlocks::runOnFunctions(BinaryContext &BC) {
-  for (auto &It : BC.getBinaryFunctions()) {
-    BinaryFunction &Function = It.second;
-    if (shouldOptimize(Function))
-      runOnFunction(Function);
-  }
+  ParallelUtilities::WorkFuncTy WorkFun = [&](BinaryFunction &BF) {
+    runOnFunction(BF);
+  };
+
+  ParallelUtilities::PredicateTy SkipPredicate = [&](const BinaryFunction &BF) {
+    return !shouldOptimize(BF) || BF.getLayout().block_empty();
+  };
+
+  ParallelUtilities::runOnEachFunction(
+      BC, ParallelUtilities::SchedulingPolicy::SP_CONSTANT, WorkFun,
+      SkipPredicate, "elimininate-unreachable");
 
   if (DeletedBlocks)
     outs() << "BOLT-INFO: UCE removed " << DeletedBlocks << " blocks and "