From 97b087c77898c7f47252853df47b568f2b621d17 Mon Sep 17 00:00:00 2001
From: Eli Friedman <eli.friedman@gmail.com>
Date: Fri, 18 Dec 2009 08:22:35 +0000
Subject: [PATCH] Optimize icmp of null and select of two constants even if the
 select has multiple uses.  (The construct in question was found in gcc.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91675 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../Scalar/InstructionCombining.cpp           | 32 ++++++++++---------
 .../InstCombine/2009-12-17-CmpSelectNull.ll   | 16 ++++++++++
 2 files changed, 33 insertions(+), 15 deletions(-)
 create mode 100644 test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 2a82e0f2335..a6e0eef854d 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -6356,24 +6356,26 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         // comparison into the select arms, which will cause one to be
         // constant folded and the select turned into a bitwise or.
         Value *Op1 = 0, *Op2 = 0;
-        if (LHSI->hasOneUse()) {
-          if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1))) {
-            // Fold the known value into the constant operand.
-            Op1 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
-            // Insert a new ICmp of the other select operand.
-            Op2 = Builder->CreateICmp(I.getPredicate(), LHSI->getOperand(2),
-                                      RHSC, I.getName());
-          } else if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2))) {
-            // Fold the known value into the constant operand.
-            Op2 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
-            // Insert a new ICmp of the other select operand.
+        if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1)))
+          Op1 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
+        if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2)))
+          Op2 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
+
+        // We only want to perform this transformation if it will not lead to
+        // additional code. This is true if either both sides of the select
+        // fold to a constant (in which case the icmp is replaced with a select
+        // which will usually simplify) or this is the only user of the
+        // select (in which case we are trading a select+icmp for a simpler
+        // select+icmp).
+        if ((Op1 && Op2) || (LHSI->hasOneUse() && (Op1 || Op2))) {
+          if (!Op1)
             Op1 = Builder->CreateICmp(I.getPredicate(), LHSI->getOperand(1),
                                       RHSC, I.getName());
-          }
-        }
-
-        if (Op1)
+          if (!Op2)
+            Op2 = Builder->CreateICmp(I.getPredicate(), LHSI->getOperand(2),
+                                      RHSC, I.getName());
           return SelectInst::Create(LHSI->getOperand(0), Op1, Op2);
+        }
         break;
       }
       case Instruction::Call:
diff --git a/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll b/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll
new file mode 100644
index 00000000000..fb7497b395d
--- /dev/null
+++ b/test/Transforms/InstCombine/2009-12-17-CmpSelectNull.ll
@@ -0,0 +1,16 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+@.str254 = internal constant [2 x i8] c".\00"
+@.str557 = internal constant [3 x i8] c"::\00"
+
+define i8* @demangle_qualified(i32 %isfuncname) nounwind {
+entry:
+  %tobool272 = icmp ne i32 %isfuncname, 0
+  %cond276 = select i1 %tobool272, i8* getelementptr inbounds ([2 x i8]* @.str254, i32 0, i32 0), i8* getelementptr inbounds ([3 x i8]* @.str557, i32 0, i32 0) ; <i8*> [#uses=4]
+  %cmp.i504 = icmp eq i8* %cond276, null
+  %rval = getelementptr i8* %cond276, i1 %cmp.i504
+  ret i8* %rval
+}
+
+; CHECK: %cond276 = select i1
+; CHECK: ret i8* %cond276