[MBB] add a missing corner case in UpdateTerminator()

After the block placement, if a block ends with a conditional branch, but the
next block is not its successor. The conditional branch should be changed to
unconditional branch.  This patch fixes PR28307, PR28297, PR28402.

Differential Revision: http://reviews.llvm.org/D21811

llvm-svn: 274470

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -470,17 +470,27 @@ void MachineBasicBlock::updateTerminator() {
     FallthroughBB = *SI;
   }
 
-  if (!FallthroughBB && canFallThrough()) {
-    // We fallthrough to the same basic block as the conditional jump targets.
-    // Remove the conditional jump, leaving unconditional fallthrough.
-    // FIXME: This does not seem like a reasonable pattern to support, but it
-    // has been seen in the wild coming out of degenerate ARM test cases.
-    TII->RemoveBranch(*this);
+  if (!FallthroughBB) {
+    if (canFallThrough()) {
+      // We fallthrough to the same basic block as the conditional jump targets.
+      // Remove the conditional jump, leaving unconditional fallthrough.
+      // FIXME: This does not seem like a reasonable pattern to support, but it
+      // has been seen in the wild coming out of degenerate ARM test cases.
+      TII->RemoveBranch(*this);
+  
+      // Finally update the unconditional successor to be reached via a branch if
+      // it would not be reached by fallthrough.
+      if (!isLayoutSuccessor(TBB))
+        TII->InsertBranch(*this, TBB, nullptr, Cond, DL);
+      return;
+    }
 
-    // Finally update the unconditional successor to be reached via a branch if
-    // it would not be reached by fallthrough.
-    if (!isLayoutSuccessor(TBB))
-      TII->InsertBranch(*this, TBB, nullptr, Cond, DL);
+    // We enter here iff exactly one successor is TBB which cannot fallthrough
+    // and the rest successors if any are EHPads.  In this case, we need to
+    // change the conditional branch into unconditional branch.
+    TII->RemoveBranch(*this);
+    Cond.clear();
+    TII->InsertBranch(*this, TBB, nullptr, Cond, DL);
     return;
   }
 

llvm/test/CodeGen/X86/update-terminator.mir

@@ -0,0 +1,57 @@
+# RUN: llc -march=x86-64 -verify-machineinstrs -run-pass block-placement -o /dev/null %s 2>&1 | FileCheck %s
+# Check the conditional jump in bb.1 is changed to unconditional after block placement swaps bb.2 and bb.3.
+
+--- |
+  @a = external global i16
+  @b = external global i32
+
+  ; Function Attrs: nounwind
+  define void @f2() {
+    br i1 undef, label %bb1, label %bb3
+
+  bb1:
+    br i1 undef, label %bb2, label %bb2
+
+  bb2:
+    br label %bb4
+
+  bb3:
+    br label %bb2
+
+  bb4:
+    ret void
+  }
+
+
+...
+---
+# CHECK-LABEL: name: f2
+# CHECK: bb.1:
+# CHECK: JMP_1 %bb.2
+# CHECK: bb.3:
+# CHECK: bb.2:
+name:            f2
+body:             |
+  bb.0 (%ir-block.0):
+    successors: %bb.1(50), %bb.3(50)
+
+    JNE_1 %bb.1, implicit %eflags
+    JMP_1 %bb.3
+  bb.1:
+    successors: %bb.2(100)
+
+    JNE_1 %bb.2, implicit %eflags
+
+  bb.2:
+    successors: %bb.4(100)
+
+    JMP_1 %bb.4
+
+  bb.3:
+    successors: %bb.2(100)
+    JMP_1 %bb.2
+
+  bb.4:
+    RETQ
+
+...