Tweak the loop rotation logic to check whether the loop is naturally

laid out in a form with a fallthrough into the header and a fallthrough
out of the bottom. In that case, leave the loop alone because any
rotation will introduce unnecessary branches. If either side looks like
it will require an explicit branch, then the rotation won't add any, do
it to ensure the branch occurs outside of the loop (if possible) and
maximize the benefit of the fallthrough in the bottom.

llvm-svn: 154806

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -215,6 +215,8 @@ class MachineBlockPlacement : public MachineFunctionPass {
                                       MachineLoop &L,
                                       const BlockFilterSet &LoopBlockSet);
   void buildLoopChains(MachineFunction &F, MachineLoop &L);
+  void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
+                  const BlockFilterSet &LoopBlockSet);
   void buildCFGChains(MachineFunction &F);
 
 public:
@@ -659,6 +661,54 @@ MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
   return ExitingBB;
 }
 
+/// \brief Attempt to rotate an exiting block to the bottom of the loop.
+///
+/// Once we have built a chain, try to rotate it to line up the hot exit block
+/// with fallthrough out of the loop if doing so doesn't introduce unnecessary
+/// branches. For example, if the loop has fallthrough into its header and out
+/// of its bottom already, don't rotate it.
+void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
+                                       MachineBasicBlock *ExitingBB,
+                                       const BlockFilterSet &LoopBlockSet) {
+  if (!ExitingBB)
+    return;
+
+  MachineBasicBlock *Top = *LoopChain.begin();
+  bool ViableTopFallthrough = false;
+  for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
+                                        PE = Top->pred_end();
+       PI != PE; ++PI) {
+    BlockChain *PredChain = BlockToChain[*PI];
+    if (!LoopBlockSet.count(*PI) &&
+        (!PredChain || *PI == *llvm::prior(PredChain->end()))) {
+      ViableTopFallthrough = true;
+      break;
+    }
+  }
+
+  // If the header has viable fallthrough, check whether the current loop
+  // bottom is a viable exiting block. If so, bail out as rotating will
+  // introduce an unnecessary branch.
+  if (ViableTopFallthrough) {
+    MachineBasicBlock *Bottom = *llvm::prior(LoopChain.end());
+    for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
+                                          SE = Bottom->succ_end();
+         SI != SE; ++SI) {
+      BlockChain *SuccChain = BlockToChain[*SI];
+      if (!LoopBlockSet.count(*SI) &&
+          (!SuccChain || *SI == *SuccChain->begin()))
+        return;
+    }
+  }
+
+  BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
+                                          ExitingBB);
+  if (ExitIt == LoopChain.end())
+    return;
+
+  std::rotate(LoopChain.begin(), llvm::next(ExitIt), LoopChain.end());
+}
+
 /// \brief Forms basic block chains from the natural loop structures.
 ///
 /// These chains are designed to preserve the existing *structure* of the code
@@ -709,17 +759,7 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
   }
 
   buildChain(L.getHeader(), LoopChain, BlockWorkList, &LoopBlockSet);
-
-  // Once we have built a chain, try to rotate it to line up the hot exit block
-  // with fallthrough out of the loop (if we have a valid exit block for that).
-  if (ExitingBB) {
-    BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
-                                            ExitingBB);
-
-    if (ExitIt != LoopChain.end()) {
-      std::rotate(LoopChain.begin(), llvm::next(ExitIt), LoopChain.end());
-    }
-  }
+  rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
 
   DEBUG({
     // Crash at the end so we get all of the debugging output first.

llvm/test/CodeGen/X86/block-placement.ll

@@ -122,14 +122,14 @@ define i32 @test_loop_early_exits(i32 %i, i32* %a) {
 ; Check that we sink early exit blocks out of loop bodies.
 ; CHECK: test_loop_early_exits:
 ; CHECK: %entry
+; CHECK: %body1
 ; CHECK: %body2
 ; CHECK: %body3
 ; CHECK: %body4
-; CHECK: %body1
+; CHECK: %exit
 ; CHECK: %bail1
 ; CHECK: %bail2
 ; CHECK: %bail3
-; CHECK: %exit
 
 entry:
   br label %body1
@@ -199,6 +199,36 @@ exit:
   ret i32 %base
 }
 
+define i32 @test_no_loop_rotate(i32 %i, i32* %a) {
+; Check that we don't try to rotate a loop which is already laid out with
+; fallthrough opportunities into the top and out of the bottom.
+; CHECK: test_no_loop_rotate:
+; CHECK: %entry
+; CHECK: %body0
+; CHECK: %body1
+; CHECK: %exit
+
+entry:
+  br label %body0
+
+body0:
+  %iv = phi i32 [ 0, %entry ], [ %next, %body1 ]
+  %base = phi i32 [ 0, %entry ], [ %sum, %body1 ]
+  %arrayidx = getelementptr inbounds i32* %a, i32 %iv
+  %0 = load i32* %arrayidx
+  %sum = add nsw i32 %0, %base
+  %bailcond1 = icmp eq i32 %sum, 42
+  br i1 %bailcond1, label %exit, label %body1
+
+body1:
+  %next = add i32 %iv, 1
+  %exitcond = icmp eq i32 %next, %i
+  br i1 %exitcond, label %exit, label %body0
+
+exit:
+  ret i32 %base
+}
+
 define void @test_loop_rotate_reversed_blocks() {
 ; This test case (greatly reduced from an Olden bencmark) ensures that the loop
 ; rotate implementation doesn't assume that loops are laid out in a particular