Prevent the creation of empty (forwarding) blocks resulting from nested ifs.

Summary:
Nested if statements can generate empty BBs whose terminator branches 
unconditionally to its successor. These branches are not eliminated
to help generate better line number information in some cases, but there
is no reason to keep the empty blocks that result from nested ifs.

Reviewers: mehdi_amini, dblaikie, echristo

Subscribers: mehdi_amini, cfe-commits

Differential review: http://reviews.llvm.org/D11360
 

llvm-svn: 275115

2 files changed, 49 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index 29889dd6b50..ee6e3ebb356 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/lib/CodeGen/CGStmt.cpp
@@ -613,7 +613,14 @@ void CodeGenFunction::EmitIfStmt(const IfStmt &S) {
     RunCleanupsScope ThenScope(*this);
     EmitStmt(S.getThen());
   }
-  EmitBranch(ContBlock);
+  {
+    auto CurBlock = Builder.GetInsertBlock();
+    EmitBranch(ContBlock);
+    // Eliminate any empty blocks that may have been created by nested
+    // control flow statements in the 'then' clause.
+    if (CurBlock)
+      SimplifyForwardingBlocks(CurBlock); 
+  }
 
   // Emit the 'else' code if present.
   if (const Stmt *Else = S.getElse()) {
@@ -629,7 +636,12 @@ void CodeGenFunction::EmitIfStmt(const IfStmt &S) {
     {
       // There is no need to emit line number for an unconditional branch.
       auto NL = ApplyDebugLocation::CreateEmpty(*this);
+      auto CurBlock = Builder.GetInsertBlock();
       EmitBranch(ContBlock);
+      // Eliminate any empty blocks that may have been created by nested
+      // control flow statements emitted in the 'else' clause.
+      if (CurBlock)
+        SimplifyForwardingBlocks(CurBlock); 
     }
   }
 
diff --git a/clang/test/CodeGen/forwarding-blocks-if.c b/clang/test/CodeGen/forwarding-blocks-if.c
new file mode 100644
index 00000000000..19c8d61d9cc
--- /dev/null
+++ b/clang/test/CodeGen/forwarding-blocks-if.c
@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 %s -emit-llvm -o - | FileCheck %s
+// Check that no empty blocks are generated for nested ifs.
+
+extern void func();
+
+int f0(int val) {
+  if (val == 0) {
+    func();
+  } else if (val == 1) {
+    func();
+  }
+  return 0;
+}
+
+// CHECK-LABEL: define i32 @f0
+// CHECK: call void {{.*}} @func
+// CHECK: call void {{.*}} @func
+// CHECK: br label %[[RETBLOCK1:[^ ]*]]
+// CHECK: [[RETBLOCK1]]:
+// CHECK-NOT: br label
+// CHECK: ret i32
+
+int f1(int val, int g) {
+  if (val == 0)
+    if (g == 1) {
+      func();
+    }
+  return 0;
+}
+
+// CHECK-LABEL: define i32 @f1
+// CHECK: call void {{.*}} @func
+// CHECK: br label %[[RETBLOCK2:[^ ]*]]
+// CHECK: [[RETBLOCK2]]:
+// CHECK-NOT: br label
+// CHECK: ret i32