Create a new interface addSuccessorWithoutWeight(MBB*) in MBB to add successors when optimization is disabled.

When optimization is disabled, edge weights that are stored in MBB won't be used so that we don't have to store them. Currently, this is done by adding successors with default weight 0, and if all successors have default weights, the weight list will be empty. But that the weight list is empty doesn't mean disabled optimization (as is stated several times in MachineBasicBlock.cpp): it may also mean all successors just have default weights.

We should discourage using default weights when adding successors, because it is very easy for users to forget update the correct edge weights instead of using default ones (one exception is that the MBB only has one successor). In order to detect such usages, it is better to differentiate using default weights from the case when optimizations is disabled.

In this patch, a new interface addSuccessorWithoutWeight(MBB*) is created for when optimization is disabled. In this case, MBB will try to maintain an empty weight list, but it cannot guarantee this as for many uses of addSuccessor() whether optimization is disabled or not is not checked. But it can guarantee that if optimization is enabled, then the weight list always has the same size of the successor list.

Differential revision: http://reviews.llvm.org/D13963

llvm-svn: 251429

llvm/include/llvm/CodeGen/MachineBasicBlock.h

@@ -425,6 +425,12 @@ public:
   /// Note that duplicate Machine CFG edges are not allowed.
   void addSuccessor(MachineBasicBlock *Succ, uint32_t Weight = 0);
 
+  /// Add Succ as a successor of this MachineBasicBlock.  The Predecessors list
+  /// of Succ is automatically updated. The weight is not provided because BPI
+  /// is not available (e.g. -O0 is used), in which case edge weights won't be
+  /// used. Using this interface can save some space.
+  void addSuccessorWithoutWeight(MachineBasicBlock *Succ);
+
   /// Set successor weight of a given iterator.
   void setSuccWeight(succ_iterator I, uint32_t Weight);
 

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -506,15 +506,19 @@ void MachineBasicBlock::updateTerminator() {
 }
 
 void MachineBasicBlock::addSuccessor(MachineBasicBlock *Succ, uint32_t Weight) {
-
-  // If we see non-zero value for the first time it means we actually use Weight
-  // list, so we fill all Weights with 0's.
-  if (Weight != 0 && Weights.empty())
-    Weights.resize(Successors.size());
-
-  if (Weight != 0 || !Weights.empty())
+  // Weight list is either empty (if successor list isn't empty, this means
+  // disabled optimization) or has the same size as successor list.
+  if (!(Weights.empty() && !Successors.empty()))
     Weights.push_back(Weight);
+  Successors.push_back(Succ);
+  Succ->addPredecessor(this);
+}
 
+void MachineBasicBlock::addSuccessorWithoutWeight(MachineBasicBlock *Succ) {
+  // We need to make sure weight list is either empty or has the same size of
+  // successor list. When this function is called, we can safely delete all
+  // weight in the list.
+  Weights.clear();
   Successors.push_back(Succ);
   Succ->addPredecessor(this);
 }

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1394,25 +1394,28 @@ void FastISel::fastEmitBranch(MachineBasicBlock *MSucc, DebugLoc DbgLoc) {
     TII.InsertBranch(*FuncInfo.MBB, MSucc, nullptr,
                      SmallVector<MachineOperand, 0>(), DbgLoc);
   }
-  uint32_t BranchWeight = 0;
-  if (FuncInfo.BPI)
-    BranchWeight = FuncInfo.BPI->getEdgeWeight(FuncInfo.MBB->getBasicBlock(),
-                                               MSucc->getBasicBlock());
-  FuncInfo.MBB->addSuccessor(MSucc, BranchWeight);
+  if (FuncInfo.BPI) {
+    uint32_t BranchWeight = FuncInfo.BPI->getEdgeWeight(
+        FuncInfo.MBB->getBasicBlock(), MSucc->getBasicBlock());
+    FuncInfo.MBB->addSuccessor(MSucc, BranchWeight);
+  } else
+    FuncInfo.MBB->addSuccessorWithoutWeight(MSucc);
 }
 
 void FastISel::finishCondBranch(const BasicBlock *BranchBB,
                                 MachineBasicBlock *TrueMBB,
                                 MachineBasicBlock *FalseMBB) {
-  uint32_t BranchWeight = 0;
-  if (FuncInfo.BPI)
-    BranchWeight = FuncInfo.BPI->getEdgeWeight(BranchBB,
-                                               TrueMBB->getBasicBlock());
   // Add TrueMBB as successor unless it is equal to the FalseMBB: This can
   // happen in degenerate IR and MachineIR forbids to have a block twice in the
   // successor/predecessor lists.
-  if (TrueMBB != FalseMBB)
-    FuncInfo.MBB->addSuccessor(TrueMBB, BranchWeight);
+  if (TrueMBB != FalseMBB) {
+    if (FuncInfo.BPI) {
+      uint32_t BranchWeight =
+          FuncInfo.BPI->getEdgeWeight(BranchBB, TrueMBB->getBasicBlock());
+      FuncInfo.MBB->addSuccessor(TrueMBB, BranchWeight);
+    } else
+      FuncInfo.MBB->addSuccessorWithoutWeight(TrueMBB);
+  }
 
   fastEmitBranch(FalseMBB, DbgLoc);
 }

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -1499,9 +1499,13 @@ uint32_t SelectionDAGBuilder::getEdgeWeight(const MachineBasicBlock *Src,
 void SelectionDAGBuilder::
 addSuccessorWithWeight(MachineBasicBlock *Src, MachineBasicBlock *Dst,
                        uint32_t Weight /* = 0 */) {
-  if (!Weight)
-    Weight = getEdgeWeight(Src, Dst);
-  Src->addSuccessor(Dst, Weight);
+  if (!FuncInfo.BPI)
+    Src->addSuccessorWithoutWeight(Dst);
+  else {
+    if (!Weight)
+      Weight = getEdgeWeight(Src, Dst);
+    Src->addSuccessor(Dst, Weight);
+  }
 }
 
 

llvm/lib/Target/AArch64/AArch64FastISel.cpp

@@ -2376,11 +2376,12 @@ bool AArch64FastISel::selectBranch(const Instruction *I) {
         .addMBB(Target);
 
     // Obtain the branch weight and add the target to the successor list.
-    uint32_t BranchWeight = 0;
-    if (FuncInfo.BPI)
-      BranchWeight = FuncInfo.BPI->getEdgeWeight(BI->getParent(),
-                                                 Target->getBasicBlock());
-    FuncInfo.MBB->addSuccessor(Target, BranchWeight);
+    if (FuncInfo.BPI) {
+      uint32_t BranchWeight =
+          FuncInfo.BPI->getEdgeWeight(BI->getParent(), Target->getBasicBlock());
+      FuncInfo.MBB->addSuccessor(Target, BranchWeight);
+    } else
+      FuncInfo.MBB->addSuccessorWithoutWeight(Target);
     return true;
   } else if (foldXALUIntrinsic(CC, I, BI->getCondition())) {
     // Fake request the condition, otherwise the intrinsic might be completely

llvm/test/CodeGen/MIR/X86/newline-handling.mir

@@ -35,7 +35,7 @@ liveins:
 # CHECK-LABEL: name: foo
 # CHECK: body: |
 # CHECK-NEXT: bb.0.entry:
-# CHECK-NEXT: successors: %bb.1.less, %bb.2.exit
+# CHECK-NEXT: successors: %bb.1.less(0), %bb.2.exit(0)
 # CHECK-NEXT: liveins: %edi
 # CHECK:      CMP32ri8 %edi, 10, implicit-def %eflags
 # CHECK-NEXT: JG_1 %bb.2.exit, implicit killed %eflags
@@ -79,7 +79,7 @@ liveins:
 # CHECK-LABEL: name: bar
 # CHECK: body: |
 # CHECK-NEXT: bb.0.entry:
-# CHECK-NEXT: successors: %bb.1.less, %bb.2.exit
+# CHECK-NEXT: successors: %bb.1.less(0), %bb.2.exit(0)
 # CHECK-NEXT: liveins: %edi
 # CHECK:      CMP32ri8 %edi, 10, implicit-def %eflags
 # CHECK-NEXT: JG_1 %bb.2.exit, implicit killed %eflags

llvm/test/CodeGen/MIR/X86/successor-basic-blocks.mir

@@ -32,7 +32,7 @@
 name:            foo
 body: |
   ; CHECK-LABEL: bb.0.entry:
-  ; CHECK:         successors: %bb.1.less, %bb.2.exit
+  ; CHECK:         successors: %bb.1.less(0), %bb.2.exit(0)
   ; CHECK-LABEL: bb.1.less:
   bb.0.entry:
     successors: %bb.1.less, %bb.2.exit
@@ -58,7 +58,7 @@ body: |
   ; Verify that we can have multiple lists of successors that will be merged
   ; into one.
   ; CHECK-LABEL: bb.0.entry:
-  ; CHECK:         successors: %bb.1, %bb.2
+  ; CHECK:         successors: %bb.1(0), %bb.2(0)
   bb.0.entry:
     liveins: %edi
     successors: %bb.1