PGO: preserve branch-weight metadata when simplifying a switch with a single

case to a conditional branch and when removing dead cases.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163942 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/Local.cpp

@@ -200,8 +200,20 @@ bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
             "cond");
 
         // Insert the new branch.
-        Builder.CreateCondBr(Cond, FirstCase.getCaseSuccessor(),
+        BranchInst *NewBr = Builder.CreateCondBr(Cond,
+                                FirstCase.getCaseSuccessor(),
                                 SI->getDefaultDest());
+        MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
+        if (MD && MD->getNumOperands() == 3) {
+          ConstantInt *SICase = dyn_cast<ConstantInt>(MD->getOperand(2));
+          ConstantInt *SIDef = dyn_cast<ConstantInt>(MD->getOperand(1));
+          assert(SICase && SIDef);
+          // The TrueWeight should be the weight for the single case of SI.
+          NewBr->setMetadata(LLVMContext::MD_prof,
+                 MDBuilder(BB->getContext()).
+                 createBranchWeights(SICase->getValue().getZExtValue(),
+                                     SIDef->getValue().getZExtValue()));
+        }
 
         // Delete the old switch.
         SI->eraseFromParent();

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -667,13 +667,32 @@ SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
     DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
                  << "Through successor TI: " << *TI);
 
+    // Collect branch weights into a vector.
+    SmallVector<uint32_t, 8> Weights;
+    MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
+    bool HasWeight = MD && (MD->getNumOperands() == 2 + SI->getNumCases());
+    if (HasWeight)
+      for (unsigned MD_i = 1, MD_e = MD->getNumOperands(); MD_i < MD_e;
+           ++MD_i) {
+        ConstantInt* CI = dyn_cast<ConstantInt>(MD->getOperand(MD_i));
+        assert(CI);
+        Weights.push_back(CI->getValue().getZExtValue());
+      }
     for (SwitchInst::CaseIt i = SI->case_end(), e = SI->case_begin(); i != e;) {
       --i;
       if (DeadCases.count(i.getCaseValue())) {
+        if (HasWeight) {
+          std::swap(Weights[i.getCaseIndex()+1], Weights.back());
+          Weights.pop_back();
+        }
         i.getCaseSuccessor()->removePredecessor(TI->getParent());
         SI->removeCase(i);
       }
     }
+    if (HasWeight)
+      SI->setMetadata(LLVMContext::MD_prof,
+                      MDBuilder(SI->getParent()->getContext()).
+                      createBranchWeights(Weights));
 
     DEBUG(dbgs() << "Leaving: " << *TI << "\n");
     return true;

test/Transforms/SimplifyCFG/preserve-branchweights.ll

@@ -109,12 +109,60 @@ sw.epilog:
   ret void
 }
 
+;; test6 - Some cases of the second switch are pruned during optimization.
+;; Then the second switch will be converted to a branch, finally, the first
+;; switch and the branch will be merged into a single switch.
+define void @test6(i32 %M, i32 %N) nounwind uwtable {
+entry:
+  switch i32 %N, label %sw2 [
+    i32 1, label %sw2
+    i32 2, label %sw.bb
+    i32 3, label %sw.bb1
+  ], !prof !4
+; CHECK: test6
+; CHECK: switch i32 %N, label %sw.epilog
+; CHECK: i32 3, label %sw.bb1
+; CHECK: i32 2, label %sw.bb
+; CHECK: i32 4, label %sw.bb5
+; CHECK: ], !prof !3
+
+sw.bb:
+  call void @helper(i32 0)
+  br label %sw.epilog
+
+sw.bb1:
+  call void @helper(i32 1)
+  br label %sw.epilog
+
+sw2:
+;; Here "case 2" is invalidated since the default case of the first switch
+;; does not include "case 2".
+  switch i32 %N, label %sw.epilog [
+    i32 2, label %sw.bb4
+    i32 4, label %sw.bb5
+  ], !prof !5
+
+sw.bb4:
+  call void @helper(i32 2)
+  br label %sw.epilog
+
+sw.bb5:
+  call void @helper(i32 3)
+  br label %sw.epilog
+
+sw.epilog:
+  ret void
+}
+
 !0 = metadata !{metadata !"branch_weights", i32 3, i32 5}
 !1 = metadata !{metadata !"branch_weights", i32 1, i32 1}
 !2 = metadata !{metadata !"branch_weights", i32 1, i32 2}
 !3 = metadata !{metadata !"branch_weights", i32 4, i32 3, i32 2, i32 1}
+!4 = metadata !{metadata !"branch_weights", i32 4, i32 3, i32 2, i32 1}
+!5 = metadata !{metadata !"branch_weights", i32 7, i32 6, i32 5}
 
 ; CHECK: !0 = metadata !{metadata !"branch_weights", i32 5, i32 11}
 ; CHECK: !1 = metadata !{metadata !"branch_weights", i32 1, i32 5}
 ; CHECK: !2 = metadata !{metadata !"branch_weights", i32 7, i32 1, i32 2}
-; CHECK-NOT: !3
+; CHECK: !3 = metadata !{metadata !"branch_weights", i32 49, i32 12, i32 24, i32 35}
+; CHECK-NOT: !4