BranchRelaxation: Expand unconditional branches first

[arsenm]

Summary

It's likely if a conditional branch needs to be expanded, the following
unconditional branch will also need expansion. By expanding the
unconditional branch first, the conditional branch can be simply
inverted to jump over the inserted indirect branch block. If the
conditional branch is expanded first, it results in an additional
branch.

This avoids test regressions in future commits.

[arsenm]

ping

[• tstellarAMD]

LGTM.

[arsenm]

r285722

lib/CodeGen/BranchRelaxation.cpp

 bool BranchRelaxation::relaxBranchInstructions() {
   bool Changed = false;
 
   // Relaxing branches involves creating new basic blocks, so re-eval
   // end() for termination.
   for (MachineFunction::iterator I = MF->begin(); I != MF->end(); ++I) {
     MachineBasicBlock &MBB = *I;
 
+    auto Last = MBB.rbegin();
+    if (Last == MBB.rend()) // Empty block.
+      continue;
+
+    // Expand the unconditional branch first if necessary. If there is a
+    // conditional branch, this will end up changing the branch destination of
+    // it to be over the newly inserted indirect branch block, which may avoid
+    // the need to try expanding the conditional branch first, saving an extra
+    // jump.
+    if (Last->isUnconditionalBranch()) {
+      // Unconditional branch destination might be unanalyzable, assume these
+      // are OK.
+      if (MachineBasicBlock *DestBB = TII->getBranchDestBlock(*Last)) {
+        if (!isBlockInRange(*Last, *DestBB)) {
+          fixupUnconditionalBranch(*Last);
+          ++NumUnconditionalRelaxed;
+          Changed = true;
+        }
+      }
+    }
+
+    // Loop over the conditional branches.
     MachineBasicBlock::iterator Next;
     for (MachineBasicBlock::iterator J = MBB.getFirstTerminator();
          J != MBB.end(); J = Next) {
       Next = std::next(J);
       MachineInstr &MI = *J;
 
       if (MI.isConditionalBranch()) {
         MachineBasicBlock *DestBB = TII->getBranchDestBlock(MI);
for (MachineBasicBlock::iterator J = MBB.getFirstTerminator();
           }
 
           Changed = true;
 
           // This may have modified all of the terminators, so start over.
           Next = MBB.getFirstTerminator();
         }
       }
-
-      if (MI.isUnconditionalBranch()) {
-        // Unconditional branch destination might be unanalyzable, assume these
-        // are OK.
-        if (MachineBasicBlock *DestBB = TII->getBranchDestBlock(MI)) {
-          if (!isBlockInRange(MI, *DestBB)) {
-            fixupUnconditionalBranch(MI);
-            ++NumUnconditionalRelaxed;
-            Changed = true;
-          }
-        }
-
-        // Unconditional branch is the last terminator.
-        break;
-      }
     }
   }
 
   return Changed;
 }
 
 bool BranchRelaxation::runOnMachineFunction(MachineFunction &mf) {
   MF = &mf;