From 0faa60938a66a09820ca21d55e7b5926bc58d59d Mon Sep 17 00:00:00 2001
From: Jay Foad <jay.foad@gmail.com>
Date: Tue, 1 Feb 2011 09:22:34 +0000
Subject: [PATCH] Make SwitchInst::removeCase() more efficient.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124659 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/llvm/Instructions.h |  3 ++-
 lib/VMCore/Instructions.cpp | 12 ++++--------
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/include/llvm/Instructions.h b/include/llvm/Instructions.h
index c79fda0deb4..17ff763c52b 100644
--- a/include/llvm/Instructions.h
+++ b/include/llvm/Instructions.h
@@ -2247,7 +2247,8 @@ public:
 
   /// removeCase - This method removes the specified successor from the switch
   /// instruction.  Note that this cannot be used to remove the default
-  /// destination (successor #0).
+  /// destination (successor #0). Also note that this operation may reorder the
+  /// remaining cases at index idx and above.
   ///
   void removeCase(unsigned idx);
 
diff --git a/lib/VMCore/Instructions.cpp b/lib/VMCore/Instructions.cpp
index db38a1568fc..6b561f34af2 100644
--- a/lib/VMCore/Instructions.cpp
+++ b/lib/VMCore/Instructions.cpp
@@ -3009,14 +3009,10 @@ void SwitchInst::removeCase(unsigned idx) {
   unsigned NumOps = getNumOperands();
   Use *OL = OperandList;
 
-  // Move everything after this operand down.
-  //
-  // FIXME: we could just swap with the end of the list, then erase.  However,
-  // client might not expect this to happen.  The code as it is thrashes the
-  // use/def lists, which is kinda lame.
-  for (unsigned i = (idx+1)*2; i != NumOps; i += 2) {
-    OL[i-2] = OL[i];
-    OL[i-2+1] = OL[i+1];
+  // Overwrite this case with the end of the list.
+  if ((idx + 1) * 2 != NumOps) {
+    OL[idx * 2] = OL[NumOps - 2];
+    OL[idx * 2 + 1] = OL[NumOps - 1];
   }
 
   // Nuke the last value.