[llvm] [CodeLayout] Do not flip branch condition when using optsize (PR #114607)

[Ellis Hoag via llvm-commits]

================
@@ -2916,24 +2916,30 @@ void MachineBlockPlacement::optimizeBranches() {
   // a fallthrough when it occurs after predicated terminators.
   for (MachineBasicBlock *ChainBB : FunctionChain) {
     Cond.clear();
-    MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
-    if (!TII->analyzeBranch(*ChainBB, TBB, FBB, Cond, /*AllowModify*/ true)) {
-      // If PrevBB has a two-way branch, try to re-order the branches
-      // such that we branch to the successor with higher probability first.
-      if (TBB && !Cond.empty() && FBB &&
-          MBPI->getEdgeProbability(ChainBB, FBB) >
-              MBPI->getEdgeProbability(ChainBB, TBB) &&
-          !TII->reverseBranchCondition(Cond)) {
-        LLVM_DEBUG(dbgs() << "Reverse order of the two branches: "
-                          << getBlockName(ChainBB) << "\n");
-        LLVM_DEBUG(dbgs() << "    Edge probability: "
-                          << MBPI->getEdgeProbability(ChainBB, FBB) << " vs "
-                          << MBPI->getEdgeProbability(ChainBB, TBB) << "\n");
-        DebugLoc dl; // FIXME: this is nowhere
-        TII->removeBranch(*ChainBB);
-        TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
-      }
-    }
+    MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
+    if (TII->analyzeBranch(*ChainBB, TBB, FBB, Cond, /*AllowModify*/ true))
+      continue;
+    if (!TBB || !FBB || Cond.empty())
+      continue;
+    // If we are optimizing for size we do not consider the runtime performance.
+    // Instead, we retain the original branch condition so we have more uniform
+    // instructions which will benefit ICF.
+    if (llvm::shouldOptimizeForSize(ChainBB, PSI, MBFI.get()))
----------------
ellishg wrote:

I changed the iterator to iterate over blocks instead of chains, and I saw negligible size changes. I'm not sure if there is some reason we are using chains, or if we can switch all of them to iterate over blocks. I'll have to do more research on this.

https://github.com/llvm/llvm-project/pull/114607