commit 07eeb8001e0cc9705366ecc63f79ed5f2a5f51fc
author Cong Hou <congh@google.com> Tue Oct 27 17:59:36 2015 +0000
committer Cong Hou <congh@google.com> Tue Oct 27 17:59:36 2015 +0000
tree 1334a821bb7eba1b1a7d3515971326366cce4878
parent ab3215fa115d336c47bbf2e6387f7b72eb9a9a01

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/lib/CodeGen/SelectionDAG/FastISel.cpp

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 99f4ef7..adc51c5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1394,25 +1394,28 @@
     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);
 }