r275115 - Prevent the creation of empty (forwarding) blocks resulting from nested ifs.

[Wolfgang Pieb via cfe-commits]

Author: wolfgangp
Date: Mon Jul 11 17:22:23 2016
New Revision: 275115

URL: http://llvm.org/viewvc/llvm-project?rev=275115&view=rev
Log:
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
 

Added:
    cfe/trunk/test/CodeGen/forwarding-blocks-if.c
Modified:
    cfe/trunk/lib/CodeGen/CGStmt.cpp

Modified: cfe/trunk/lib/CodeGen/CGStmt.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGStmt.cpp?rev=275115&r1=275114&r2=275115&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CGStmt.cpp (original)
+++ cfe/trunk/lib/CodeGen/CGStmt.cpp Mon Jul 11 17:22:23 2016
@@ -613,7 +613,14 @@ void CodeGenFunction::EmitIfStmt(const I
     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 I
     {
       // 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); 
     }
   }
 

Added: cfe/trunk/test/CodeGen/forwarding-blocks-if.c
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGen/forwarding-blocks-if.c?rev=275115&view=auto
==============================================================================
--- cfe/trunk/test/CodeGen/forwarding-blocks-if.c (added)
+++ cfe/trunk/test/CodeGen/forwarding-blocks-if.c Mon Jul 11 17:22:23 2016
@@ -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