[llvm] [CodeLayout] Size-aware machine block placement (PR #109711)

[Kyungwoo Lee via llvm-commits]

================
@@ -3604,24 +3624,48 @@ void MachineBlockPlacement::applyExtTsp() {
     // not see a perf improvement with the exact block sizes.
     auto NonDbgInsts =
         instructionsWithoutDebug(MBB.instr_begin(), MBB.instr_end());
-    int NumInsts = std::distance(NonDbgInsts.begin(), NonDbgInsts.end());
+    size_t NumInsts = std::distance(NonDbgInsts.begin(), NonDbgInsts.end());
     BlockSizes[BlockIndex[&MBB]] = 4 * NumInsts;
     // Getting jump frequencies.
-    for (MachineBasicBlock *Succ : MBB.successors()) {
-      auto EP = MBPI->getEdgeProbability(&MBB, Succ);
-      BlockFrequency JumpFreq = BlockFreq * EP;
-      JumpCounts.push_back(
-          {BlockIndex[&MBB], BlockIndex[Succ], JumpFreq.getFrequency()});
+
+    if (!OptForSize) {
+      for (MachineBasicBlock *Succ : MBB.successors()) {
+        auto EP = MBPI->getEdgeProbability(&MBB, Succ);
+        BlockFrequency JumpFreq = BlockFreq * EP;
+        JumpCounts.push_back(
+            {BlockIndex[&MBB], BlockIndex[Succ], JumpFreq.getFrequency()});
+      }
+    } else {
+      Cond.clear();
+      MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
+      if (TII->analyzeBranch(MBB, TBB, FBB, Cond))
+        continue;
+
+      const MachineBasicBlock *FTB = MBB.getFallThrough();
+
+      Succs.clear();
+      if (TBB && TBB != FTB)
+        Succs.push_back(TBB);
+      if (FBB && FBB != FTB)
+        Succs.push_back(FBB);
+      if (FTB)
+        Succs.push_back(FTB);
+      // Absolute magnitude of non-zero counts does not matter for the
+      // optimization; prioritize slightly jumps with a single successor, since
+      // the corresponding jump instruction will be removed from the binary.
+      const uint64_t Freq = Succs.size() == 1 ? 110 : 100;
+      for (const MachineBasicBlock *Succ : Succs) {
+        JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succ], Freq});
+      }
     }
   }
 
   LLVM_DEBUG(dbgs() << "Applying ext-tsp layout for |V| = " << F->size()
                     << " with profile = " << F->getFunction().hasProfileData()
-                    << " (" << F->getName().str() << ")"
-                    << "\n");
-  LLVM_DEBUG(
-      dbgs() << format("  original  layout score: %0.2f\n",
-                       calcExtTspScore(BlockSizes, BlockCounts, JumpCounts)));
+                    << " (" << F->getName().str() << ")" << "\n");
----------------
kyulee-com wrote:

```suggestion
                    << " (" << F->getName() << ")" << "\n");
```

https://github.com/llvm/llvm-project/pull/109711