From e9d448e9979aa7bcfe263eea675d7171ea347f17 Mon Sep 17 00:00:00 2001
From: Nick Lewycky <nicholas@mxc.ca>
Date: Tue, 1 Mar 2011 08:15:50 +0000
Subject: [PATCH] Optimize "icmp pred (urem X, Y), Y" --> true/false depending
 on pred. There's more work to do here, "icmp ult (urem X, 10), 11" doesn't
 optimize away yet. Fixes example 3 from PR9343!

llvm-svn: 126741
---
 lib/Analysis/InstructionSimplify.cpp    | 18 ++++++++++++++++++
 test/Transforms/InstSimplify/compare.ll | 16 ++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 982dacb50bf..c23abb78089 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -1434,6 +1434,9 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
         return ConstantInt::getTrue(CI->getContext());
       break;
     }
+
+    // TODO: integer div and rem with constant RHS all produce values in a
+    // constant range. We should check whether the comparison is a tautology.
   }
 
   // Compare of cast, for example (zext X) != 0 -> X != 0
@@ -1644,6 +1647,21 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
+  if (LBO && match(LBO, m_URem(m_Value(), m_Specific(RHS)))) {
+    switch (Pred) {
+    default:
+      break;
+    case ICmpInst::ICMP_EQ:
+    case ICmpInst::ICMP_UGT:
+    case ICmpInst::ICMP_UGE:
+      return ConstantInt::getFalse(RHS->getContext());
+    case ICmpInst::ICMP_NE:
+    case ICmpInst::ICMP_ULT:
+    case ICmpInst::ICMP_ULE:
+      return ConstantInt::getTrue(RHS->getContext());
+    }
+  }
+
   // If the comparison is with the result of a select instruction, check whether
   // comparing with either branch of the select always yields the same value.
   if (isa<SelectInst>(LHS) || isa<SelectInst>(RHS))
diff --git a/test/Transforms/InstSimplify/compare.ll b/test/Transforms/InstSimplify/compare.ll
index 250e44ce340..f392d3a5e7c 100644
--- a/test/Transforms/InstSimplify/compare.ll
+++ b/test/Transforms/InstSimplify/compare.ll
@@ -187,3 +187,19 @@ define i1 @select4(i1 %cond) {
   ret i1 %c
 ; CHECK: ret i1 %cond
 }
+
+define i1 @urem1(i32 %X, i32 %Y) {
+; CHECK: @urem1
+  %A = urem i32 %X, %Y
+  %B = icmp ult i32 %A, %Y
+  ret i1 %B
+; CHECK: ret i1 true
+}
+
+define i1 @urem2(i32 %X, i32 %Y) {
+; CHECK: @urem2
+  %A = urem i32 %X, %Y
+  %B = icmp eq i32 %A, %Y
+  ret i1 %B
+; CHECK ret i1 false
+}