From cb578f84e091e9198b25c687d0f5f1dc558f27c5 Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault@amd.com>
Date: Tue, 1 Nov 2016 18:34:00 +0000
Subject: BranchRelaxation: Expand unconditional branches first

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.

llvm-svn: 285722
---
 llvm/lib/CodeGen/BranchRelaxation.cpp | 37 +++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 15 deletions(-)

(limited to 'llvm/lib/CodeGen/BranchRelaxation.cpp')

diff --git a/llvm/lib/CodeGen/BranchRelaxation.cpp b/llvm/lib/CodeGen/BranchRelaxation.cpp
index a36906540a2..ddf8797d479 100644
--- a/llvm/lib/CodeGen/BranchRelaxation.cpp
+++ b/llvm/lib/CodeGen/BranchRelaxation.cpp
@@ -404,6 +404,28 @@ bool BranchRelaxation::relaxBranchInstructions() {
   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) {
@@ -437,21 +459,6 @@ bool BranchRelaxation::relaxBranchInstructions() {
           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;
-      }
     }
   }
 
-- 
cgit v1.2.3