From 044b644f54ead0ef4d61f1935aa604c017c32fa4 Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Tue, 25 Nov 2014 02:55:48 +0000
Subject: [PATCH] InstSimplify: Handle some simple tautological comparisons

This handles cases where we are comparing a masked value against itself.
The analysis could be further improved by making it recursive but such
expense is not currently justified.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222716 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/InstructionSimplify.cpp    | 34 +++++++++++++
 test/Transforms/InstSimplify/compare.ll | 64 +++++++++++++++++++++++++
 2 files changed, 98 insertions(+)

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 7cbe6eff4ba..014bc0f09ba 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -2477,6 +2477,40 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
+  // icmp pred (or X, Y), X
+  if (LBO && match(LBO, m_CombineOr(m_Or(m_Value(), m_Specific(RHS)),
+                                    m_Or(m_Specific(RHS), m_Value())))) {
+    if (Pred == ICmpInst::ICMP_ULT)
+      return getFalse(ITy);
+    if (Pred == ICmpInst::ICMP_UGE)
+      return getTrue(ITy);
+  }
+  // icmp pred X, (or X, Y)
+  if (RBO && match(RBO, m_CombineOr(m_Or(m_Value(), m_Specific(LHS)),
+                                    m_Or(m_Specific(LHS), m_Value())))) {
+    if (Pred == ICmpInst::ICMP_ULE)
+      return getTrue(ITy);
+    if (Pred == ICmpInst::ICMP_UGT)
+      return getFalse(ITy);
+  }
+
+  // icmp pred (and X, Y), X
+  if (LBO && match(LBO, m_CombineOr(m_And(m_Value(), m_Specific(RHS)),
+                                    m_And(m_Specific(RHS), m_Value())))) {
+    if (Pred == ICmpInst::ICMP_UGT)
+      return getFalse(ITy);
+    if (Pred == ICmpInst::ICMP_ULE)
+      return getTrue(ITy);
+  }
+  // icmp pred X, (and X, Y)
+  if (RBO && match(RBO, m_CombineOr(m_And(m_Value(), m_Specific(LHS)),
+                                    m_And(m_Specific(LHS), m_Value())))) {
+    if (Pred == ICmpInst::ICMP_UGE)
+      return getTrue(ITy);
+    if (Pred == ICmpInst::ICMP_ULT)
+      return getFalse(ITy);
+  }
+
   // 0 - (zext X) pred C
   if (!CmpInst::isUnsigned(Pred) && match(LHS, m_Neg(m_ZExt(m_Value())))) {
     if (ConstantInt *RHSC = dyn_cast<ConstantInt>(RHS)) {
diff --git a/test/Transforms/InstSimplify/compare.ll b/test/Transforms/InstSimplify/compare.ll
index 38fd747172f..10c7ca6754e 100644
--- a/test/Transforms/InstSimplify/compare.ll
+++ b/test/Transforms/InstSimplify/compare.ll
@@ -1100,3 +1100,67 @@ define i1 @icmp_shl_1_V_ne_31(i32 %V) {
 ; CHECK-LABEL: @icmp_shl_1_V_ne_31(
 ; CHECK-NEXT: ret i1 true
 }
+
+define i1 @tautological1(i32 %A, i32 %B) {
+  %C = and i32 %A, %B
+  %D = icmp ugt i32 %C, %A
+  ret i1 %D
+; CHECK-LABEL: @tautological1(
+; CHECK: ret i1 false
+}
+
+define i1 @tautological2(i32 %A, i32 %B) {
+  %C = and i32 %A, %B
+  %D = icmp ule i32 %C, %A
+  ret i1 %D
+; CHECK-LABEL: @tautological2(
+; CHECK: ret i1 true
+}
+
+define i1 @tautological3(i32 %A, i32 %B) {
+  %C = or i32 %A, %B
+  %D = icmp ule i32 %A, %C
+  ret i1 %D
+; CHECK-LABEL: @tautological3(
+; CHECK: ret i1 true
+}
+
+define i1 @tautological4(i32 %A, i32 %B) {
+  %C = or i32 %A, %B
+  %D = icmp ugt i32 %A, %C
+  ret i1 %D
+; CHECK-LABEL: @tautological4(
+; CHECK: ret i1 false
+}
+
+define i1 @tautological5(i32 %A, i32 %B) {
+  %C = or i32 %A, %B
+  %D = icmp ult i32 %C, %A
+  ret i1 %D
+; CHECK-LABEL: @tautological5(
+; CHECK: ret i1 false
+}
+
+define i1 @tautological6(i32 %A, i32 %B) {
+  %C = or i32 %A, %B
+  %D = icmp uge i32 %C, %A
+  ret i1 %D
+; CHECK-LABEL: @tautological6(
+; CHECK: ret i1 true
+}
+
+define i1 @tautological7(i32 %A, i32 %B) {
+  %C = and i32 %A, %B
+  %D = icmp uge i32 %A, %C
+  ret i1 %D
+; CHECK-LABEL: @tautological7(
+; CHECK: ret i1 true
+}
+
+define i1 @tautological8(i32 %A, i32 %B) {
+  %C = and i32 %A, %B
+  %D = icmp ult i32 %A, %C
+  ret i1 %D
+; CHECK-LABEL: @tautological8(
+; CHECK: ret i1 false
+}
-- 
2.34.1