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

[Matthias Braun 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()))
----------------
MatzeB wrote:

I think this is the only place that calls `analyzeBranch` with `AllowModify = true` though. I think that part is required for things like removing jump instructions to the immediately following block, so we better not skip it?

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