Except in asm-verbose mode, avoid printing labels for blocks that are

only reachable via fall-through edges. This dramatically reduces the
number of labels printed, and thus also the number of labels the
assembler must parse and remember.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68073 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 15 insertions, 1 deletions

diff --git a/include/llvm/CodeGen/MachineBasicBlock.h b/include/llvm/CodeGen/MachineBasicBlock.h
index 1ba7e798f8..f962273d2b 100644
--- a/include/llvm/CodeGen/MachineBasicBlock.h
+++ b/include/llvm/CodeGen/MachineBasicBlock.h
@@ -253,6 +253,15 @@ public:
   /// it returns end()
   iterator getFirstTerminator();
 
+  /// isOnlyReachableViaFallthough - Return true if this basic block has
+  /// exactly one predecessor and the control transfer mechanism between
+  /// the predecessor and this block is a fall-through.
+  bool isOnlyReachableByFallthrough() const {
+    return !pred_empty() &&
+           next(pred_begin()) == pred_end() &&
+           (*pred_begin())->getFirstTerminator() == (*pred_begin())->end();
+  }
+
   void pop_front() { Insts.pop_front(); }
   void pop_back() { Insts.pop_back(); }
   void push_back(MachineInstr *MI) { Insts.push_back(MI); }
diff --git a/lib/Target/X86/AsmPrinter/X86ATTAsmPrinter.cpp b/lib/Target/X86/AsmPrinter/X86ATTAsmPrinter.cpp
index 3bdcf88365..d8a33957d5 100644
--- a/lib/Target/X86/AsmPrinter/X86ATTAsmPrinter.cpp
+++ b/lib/Target/X86/AsmPrinter/X86ATTAsmPrinter.cpp
@@ -238,7 +238,12 @@ bool X86ATTAsmPrinter::runOnMachineFunction(MachineFunction &MF) {
   for (MachineFunction::const_iterator I = MF.begin(), E = MF.end();
        I != E; ++I) {
     // Print a label for the basic block.
-    if (!I->pred_empty()) {
+    if (!VerboseAsm && (I->pred_empty() || I->isOnlyReachableByFallthrough())) {
+      // This is an entry block or a block that's only reachable via a
+      // fallthrough edge. In non-VerboseAsm mode, don't print the label.
+      assert((I->pred_empty() || (*I->pred_begin())->isLayoutSuccessor(I)) &&
+             "Fall-through predecessor not adjacent to its successor!");
+    } else {
       printBasicBlockLabel(I, true, true, VerboseAsm);
       O << '\n';
     }