[ifcnv] Add comment explaining why it's OK to duplicate convergent MIs in ifcnv.

Summary:
Also add a comment briefly explaining what ifcnv is.

No functional changes.

Reviewers: resistor

Subscribers: echristo, tra, llvm-commits

Differential Revision: http://reviews.llvm.org/D17430

llvm-svn: 261543

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index 41f9f0ba83d..a8e72f4be84 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/llvm/lib/CodeGen/IfConversion.cpp
@@ -7,7 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file implements the machine instruction level if-conversion pass.
+// This file implements the machine instruction level if-conversion pass, which
+// tries to convert conditional branches into predicated instructions.
 //
 //===----------------------------------------------------------------------===//
 
@@ -673,6 +674,9 @@ void IfConverter::ScanInstructions(BBInfo &BBI) {
     if (I->isDebugValue())
       continue;
 
+    // Don't need to check isConvergent(). It's OK to duplicate convergent
+    // instructions here, because we'll only push convergent operations up the
+    // CFG -- this can only *remove* control-flow dependencies.
     if (I->isNotDuplicable())
       BBI.CannotBeCopied = true;