From 650b6ea893c118bf0c6e40cd3002954cc34df2bb Mon Sep 17 00:00:00 2001
From: Nick Lewycky <nicholas@mxc.ca>
Date: Thu, 19 Jun 2014 03:35:49 +0000
Subject: [PATCH] Make instsimplify's analysis of icmp eq/ne use
 computeKnownBits to determine whether the icmp is always true or false. Patch
 by Suyog Sarda!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211251 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/InstructionSimplify.cpp        | 19 +++++++++++++++++++
 test/Transforms/InstCombine/align-2d-gep.ll |  2 +-
 test/Transforms/InstSimplify/compare.ll     | 19 +++++++++++++++++++
 3 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 3684fda854f..b06d9948ac1 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -2241,6 +2241,25 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
+  // If a bit is known to be zero for A and known to be one for B,
+  // then A and B cannot be equal.
+  if (ICmpInst::isEquality(Pred)) {
+    if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS)) {
+      uint32_t BitWidth = CI->getBitWidth();
+      APInt LHSKnownZero(BitWidth, 0);
+      APInt LHSKnownOne(BitWidth, 0);
+      computeKnownBits(LHS, LHSKnownZero, LHSKnownOne);
+      APInt RHSKnownZero(BitWidth, 0);
+      APInt RHSKnownOne(BitWidth, 0);
+      computeKnownBits(RHS, RHSKnownZero, RHSKnownOne);
+      if (((LHSKnownOne & RHSKnownZero) != 0) ||
+          ((LHSKnownZero & RHSKnownOne) != 0))
+        return (Pred == ICmpInst::ICMP_EQ)
+                   ? ConstantInt::getFalse(CI->getContext())
+                   : ConstantInt::getTrue(CI->getContext());
+    }
+  }
+
   // Special logic for binary operators.
   BinaryOperator *LBO = dyn_cast<BinaryOperator>(LHS);
   BinaryOperator *RBO = dyn_cast<BinaryOperator>(RHS);
diff --git a/test/Transforms/InstCombine/align-2d-gep.ll b/test/Transforms/InstCombine/align-2d-gep.ll
index 5bca46d5a21..f6a877684ce 100644
--- a/test/Transforms/InstCombine/align-2d-gep.ll
+++ b/test/Transforms/InstCombine/align-2d-gep.ll
@@ -31,7 +31,7 @@ bb1:
   store <2 x double><double 0.0, double 0.0>, <2 x double>* %r, align 8
 
   %indvar.next = add i64 %j, 2
-  %exitcond = icmp eq i64 %indvar.next, 557
+  %exitcond = icmp eq i64 %indvar.next, 556
   br i1 %exitcond, label %bb11, label %bb1
 
 bb11:
diff --git a/test/Transforms/InstSimplify/compare.ll b/test/Transforms/InstSimplify/compare.ll
index 105e244ed8c..89fd6362d78 100644
--- a/test/Transforms/InstSimplify/compare.ll
+++ b/test/Transforms/InstSimplify/compare.ll
@@ -883,3 +883,22 @@ define i1 @returns_nonnull() {
 ; CHECK: ret i1 false
 }
 
+; If a bit is known to be zero for A and known to be one for B,
+; then A and B cannot be equal.
+define i1 @icmp_eq_const(i32 %a) nounwind {
+  %b = mul nsw i32 %a, -2
+  %c = icmp eq i32 %b, 1
+  ret i1 %c
+
+; CHECK-LABEL: @icmp_eq_const
+; CHECK-NEXT: ret i1 false 
+}
+
+define i1 @icmp_ne_const(i32 %a) nounwind {
+  %b = mul nsw i32 %a, -2
+  %c = icmp ne i32 %b, 1
+  ret i1 %c
+
+; CHECK-LABEL: @icmp_ne_const
+; CHECK-NEXT: ret i1 true
+}