[llvm] r261543 - [ifcnv] Add comment explaining why it's OK to duplicate convergent MIs in ifcnv.

[Justin Lebar via llvm-commits]

Author: jlebar
Date: Mon Feb 22 11:51:30 2016
New Revision: 261543

URL: http://llvm.org/viewvc/llvm-project?rev=261543&view=rev
Log:
[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

Modified:
    llvm/trunk/lib/CodeGen/IfConversion.cpp

Modified: llvm/trunk/lib/CodeGen/IfConversion.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/IfConversion.cpp?rev=261543&r1=261542&r2=261543&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/IfConversion.cpp (original)
+++ llvm/trunk/lib/CodeGen/IfConversion.cpp Mon Feb 22 11:51:30 2016
@@ -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(BBInf
     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;