[BOLT] Always match stale entry blocks

Two (minor) improvements for stale matching:
- always match entry blocks to each other, even if there is a hash mismatch;
- ignore nops in (loose) hash computation.

I record a small improvement in inference quality on my benchmarks. Tests are not affected

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D159488

Author: spupyrev

bolt/lib/Core/HashUtilities.cpp

@@ -139,7 +139,8 @@ std::string hashBlockLoose(BinaryContext &BC, const BinaryBasicBlock &BB) {
   // instruction opcodes, which is then hashed with std::hash.
   std::set<std::string> Opcodes;
   for (const MCInst &Inst : BB) {
-    if (BC.MIB->isPseudo(Inst))
+    // Skip pseudo instructions and nops.
+    if (BC.MIB->isPseudo(Inst) || BC.MIB->isNoop(Inst))
       continue;
 
     // Ignore unconditional jumps, as they can be added / removed as a result

bolt/lib/Profile/StaleProfileMatching.cpp

@@ -389,6 +389,9 @@ void matchWeightsByHashes(BinaryContext &BC,
     assert(YamlBB.Hash != 0 && "empty hash of BinaryBasicBlockProfile");
     BlendedBlockHash YamlHash(YamlBB.Hash);
     const FlowBlock *MatchedBlock = Matcher.matchBlock(YamlHash);
+    // Always match the entry block.
+    if (MatchedBlock == nullptr && YamlBB.Index == 0)
+      MatchedBlock = Blocks[0];
     if (MatchedBlock != nullptr) {
       MatchedBlocks[YamlBB.Index] = MatchedBlock;
       BlendedBlockHash BinHash = BlendedHashes[MatchedBlock->Index - 1];