Update edge weights properly when merging blocks in if-conversion.

In if-conversion, there is a utility function MergeBlocks() that is used to merge blocks. However, when new edges are built in this function the edge weight is either not provided or not updated properly, leading to a modified CFG with incorrect edge weights. This patch corrects this issue.

Differential Revision: http://reviews.llvm.org/D12513

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248030 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/IfConversion.cpp b/lib/CodeGen/IfConversion.cpp
index b99e570a446149d55f484807b8ede8d6293ea5c8..ac0b99674a6de72d7421f9c039a47b26f1e27368 100644 (file)
--- a/lib/CodeGen/IfConversion.cpp
+++ b/lib/CodeGen/IfConversion.cpp
@@ -1686,19 +1686,83 @@ void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
   ToBBI.BB->splice(ToBBI.BB->end(),
                    FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
 
-  std::vector<MachineBasicBlock *> Succs(FromBBI.BB->succ_begin(),
-                                         FromBBI.BB->succ_end());
+  SmallVector<MachineBasicBlock *, 4> FromSuccs(FromBBI.BB->succ_begin(),
+                                                FromBBI.BB->succ_end());
   MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
   MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : nullptr;
 
-  for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
-    MachineBasicBlock *Succ = Succs[i];
+  // The edge weight from ToBBI.BB to FromBBI.BB, which is only needed when
+  // AddEdges is true and FromBBI.BB is a successor of ToBBI.BB.
+  uint32_t To2FromWeight = 0;
+  // WeightScale and SumWeight are for calculating successor probabilities of
+  // FromBBI.BB.
+  uint32_t WeightScale = 0;
+  uint32_t SumWeight = 0;
+  if (AddEdges && ToBBI.BB->isSuccessor(FromBBI.BB)) {
+    To2FromWeight = MBPI->getEdgeWeight(ToBBI.BB, FromBBI.BB);
+    // Set the edge weight from ToBBI.BB to FromBBI.BB to zero to avoid the edge
+    // weight being merged to other edges when this edge is removed later.
+    ToBBI.BB->setSuccWeight(
+        std::find(ToBBI.BB->succ_begin(), ToBBI.BB->succ_end(), FromBBI.BB), 0);
+    SumWeight = MBPI->getSumForBlock(FromBBI.BB, WeightScale);
+  }
+
+  for (unsigned i = 0, e = FromSuccs.size(); i != e; ++i) {
+    MachineBasicBlock *Succ = FromSuccs[i];
     // Fallthrough edge can't be transferred.
     if (Succ == FallThrough)
       continue;
+
+    uint32_t NewWeight = 0;
+    if (AddEdges) {
+      // Calculate the edge weight for the edge from ToBBI.BB to Succ, which is
+      // a portion of the edge weight from FromBBI.BB to Succ. The portion ratio
+      // is the edge probability from ToBBI.BB to FromBBI.BB (if FromBBI is a
+      // successor of ToBBI.BB. See comment below for excepion).
+      NewWeight = MBPI->getEdgeWeight(FromBBI.BB, Succ);
+
+      // To2FromWeight is 0 when FromBBI.BB is not a successor of ToBBI.BB. This
+      // only happens when if-converting a diamond CFG and FromBBI.BB is the
+      // tail BB.  In this case FromBBI.BB post-dominates ToBBI.BB and hence we
+      // could just use the weights on FromBBI.BB's out-edges when adding new
+      // successors.
+      if (To2FromWeight > 0) {
+        BranchProbability Prob(NewWeight / WeightScale, SumWeight);
+        NewWeight = Prob.scale(To2FromWeight);
+      }
+    }
+
     FromBBI.BB->removeSuccessor(Succ);
-    if (AddEdges && !ToBBI.BB->isSuccessor(Succ))
-      ToBBI.BB->addSuccessor(Succ);
+
+    if (AddEdges) {
+      // If the edge from ToBBI.BB to Succ already exists, update the weight of
+      // this edge by adding NewWeight to it. An example is shown below, in
+      // which A is ToBBI.BB and B is FromBBI.BB. In this case we don't have to
+      // set C as A's successor as it already is. We only need to update the
+      // edge weight on A->C. Note that B will not be immediately removed from
+      // A's successors. It is possible that B->D is not removed either if D is
+      // a fallthrough of B. Later the edge A->D (generated here) and B->D will
+      // be combined into one edge. To maintain correct edge weight of this
+      // combined edge, we need to set the edge weight of A->B to zero, which is
+      // already done above. The edge weight on A->D is calculated by scaling
+      // the original weight on A->B by the probability of B->D.
+      //
+      // Before ifcvt:      After ifcvt (assume B->D is kept):
+      //
+      //       A                A
+      //      /|               /|\
+      //     / B              / B|
+      //    | /|             |  ||
+      //    |/ |             |  |/
+      //    C  D             C  D
+      //
+      if (ToBBI.BB->isSuccessor(Succ))
+        ToBBI.BB->setSuccWeight(
+            std::find(ToBBI.BB->succ_begin(), ToBBI.BB->succ_end(), Succ),
+            MBPI->getEdgeWeight(ToBBI.BB, Succ) + NewWeight);
+      else
+        ToBBI.BB->addSuccessor(Succ, NewWeight);
+    }
   }
 
   // Now FromBBI always falls through to the next block!

diff --git a/test/CodeGen/ARM/ifcvt-iter-indbr.ll b/test/CodeGen/ARM/ifcvt-iter-indbr.ll
index 75e9d77d79207a8479a6c0c1f2338de45eeece17..69abbf395dca3e0e76b61e39980dc3fdda477f17 100644 (file)
--- a/test/CodeGen/ARM/ifcvt-iter-indbr.ll
+++ b/test/CodeGen/ARM/ifcvt-iter-indbr.ll
@@ -1,4 +1,5 @@
 ; RUN: llc < %s -mtriple thumbv7s-apple-darwin  -asm-verbose=false | FileCheck %s
+; RUN: llc < %s -mtriple thumbv7s-apple-darwin  -asm-verbose=false -print-machineinstrs=if-converter 2>&1 | FileCheck --check-prefix=CHECK-WEIGHT %s
 
 declare i32 @foo(i32)
 declare i8* @bar(i32, i8*, i8*)
@@ -27,6 +28,11 @@ declare i8* @bar(i32, i8*, i8*)
 ; CHECK-NEXT:  movw r0, #4567
 ; CHECK-NEXT: [[FOOCALL]]:
 ; CHECK-NEXT:  blx _foo
+;
+; CHECK-WEIGHT: BB#0:
+; CHECK-WEIGHT: Successors according to CFG: BB#1(16) BB#2(8) BB#4(8)
+; CHECK-WEIGHT: BB#1:
+; CHECK-WEIGHT: Successors according to CFG: BB#2(24) BB#4(8)
 
 define i32 @test(i32 %a, i32 %a2, i32* %p, i32* %p2) {
 entry:

diff --git a/test/CodeGen/Hexagon/ifcvt-edge-weight.ll b/test/CodeGen/Hexagon/ifcvt-edge-weight.ll
new file mode 100644 (file)
index 0000000..89bb1a6
--- /dev/null
+++ b/test/CodeGen/Hexagon/ifcvt-edge-weight.ll
@@ -0,0 +1,64 @@
+; RUN: llc -march=hexagon -mcpu=hexagonv5  -print-machineinstrs=if-converter %s -o /dev/null 2>&1 | FileCheck %s
+; Check that the edge weights are updated correctly after if-conversion.
+
+; CHECK: BB#3:
+; CHECK: Successors according to CFG: BB#2(10) BB#1(90)
+@a = external global i32
+@d = external global i32
+
+; In the following CFG, A,B,C,D will be if-converted into a single block.
+; Check if the edge weights on edges to E and F are maintained correctly.
+;
+;    A
+;   / \
+;  B   C
+;   \ /
+;    D
+;   / \
+;  E   F
+;
+define void @test1(i8 zeroext %la, i8 zeroext %lb) {
+entry:
+  %cmp0 = call i1 @pred()
+  br i1 %cmp0, label %if.else2, label %if.then0, !prof !1
+
+if.else2:
+  call void @bar(i32 2)
+  br label %if.end2
+
+if.end2:
+  call void @foo(i32 2)
+  br label %return
+
+if.end:
+  %storemerge = phi i32 [ %and, %if.else ], [ %shl, %if.then ]
+  store i32 %storemerge, i32* @a, align 4
+  %0 = load i32, i32* @d, align 4
+  %cmp2 = call i1 @pred()
+  br i1 %cmp2, label %if.end2, label %if.else2, !prof !2
+
+if.then0:
+  %cmp = icmp eq i8 %la, %lb
+  br i1 %cmp, label %if.then, label %if.else, !prof !1
+
+if.then:
+  %conv1 = zext i8 %la to i32
+  %shl = shl nuw nsw i32 %conv1, 16
+  br label %if.end
+
+if.else:
+  %and8 = and i8 %lb, %la
+  %and = zext i8 %and8 to i32
+  br label %if.end
+
+return:
+  call void @foo(i32 2)
+  ret void
+}
+
+declare void @foo(i32)
+declare void @bar(i32)
+declare i1 @pred()
+
+!1 = !{!"branch_weights", i32 80, i32 20}
+!2 = !{!"branch_weights", i32 10, i32 90}