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

[via llvm-commits]

spupyrev wrote:

You could try something along these lines. I haven't fine-tuned or polished the code, so be careful.

```diff
       // 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});
+      if (Succs.size() == 2) {
+        // NEW
+        BranchProbability BP0 = MBPI->getEdgeProbability(&MBB, Succs[0]);
+        BlockFrequency BF0 = BlockFrequency(100) * BP0;
+        size_t Pct0 = BF0.getFrequency();
+        assert(Pct0 <= 100);
+        BranchProbability BP1 = MBPI->getEdgeProbability(&MBB, Succs[1]);
+        BlockFrequency BF1 = BlockFrequency(100) * BP1;
+        size_t Pct1 = BF1.getFrequency();
+        assert(Pct1 <= 100);
+
+        if (Pct0 <= 5) {
+          JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succs[0]], 10});
+          JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succs[1]], Freq});
+        } else if (Pct1 <= 5) {
+          JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succs[1]], 10});
+          JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succs[0]], Freq});
+        } else {
+          // In case of ties, the last element of JumpCounts is prioritized.
+          // Thus, make sure that Succs[1] is the original fall-through.
+          if (BlockIndex[&MBB] + 1 == BlockIndex[Succs[0]])
+            std::swap(Succs[0], Succs[1]);
+          JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succs[0]], Freq});
+          JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succs[1]], Freq});
+        }
+      } else {
+        // OLD
+        for (const MachineBasicBlock *Succ : Succs)
+          JumpCounts.push_back({BlockIndex[&MBB], BlockIndex[Succ], Freq});
+      }
     } else {
       for (MachineBasicBlock *Succ : MBB.successors()) {
         auto EP = MBPI->getEdgeProbability(&MBB, Succ);
```

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