From 925681053dd50567db83f254bf47107689373ca1 Mon Sep 17 00:00:00 2001
From: Sanjoy Das <sanjoy@playingwithpointers.com>
Date: Tue, 10 Nov 2015 23:56:20 +0000
Subject: [PATCH] [ValueTracking] Teach isImpliedCondition a new bitwise trick

Summary:
This change teaches isImpliedCondition to prove things like

  (A | 15) < L  ==>  (A | 14) < L

if the low 4 bits of A are known to be zero.

Depends on D14391

Reviewers: majnemer, reames, hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14392

llvm-svn: 252673
---
 llvm/lib/Analysis/ValueTracking.cpp          | 30 ++++++++
 llvm/test/Transforms/InstSimplify/implies.ll | 78 ++++++++++++++++++++
 2 files changed, 108 insertions(+)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e4cbdb9249f6..f851892d3d3e 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4136,6 +4136,36 @@ static bool isTruePredicate(CmpInst::Predicate Pred, Value *LHS, Value *RHS,
         return C->isMinValue();
       return true;
     }
+
+    // Match A to (X +_{nuw} CA) and B to (X +_{nuw} CB)
+    auto MatchNUWAddsToSameValue = [&](Value *A, Value *B, Value *&X,
+                                       const APInt *&CA, const APInt *&CB) {
+      if (match(A, m_NUWAdd(m_Value(X), m_APInt(CA))) &&
+          match(B, m_NUWAdd(m_Specific(X), m_APInt(CB))))
+        return true;
+
+      // If X & C == 0 then (X | C) == X +_{nuw} C
+      if (match(A, m_Or(m_Value(X), m_APInt(CA))) &&
+          match(B, m_Or(m_Specific(X), m_APInt(CB)))) {
+        unsigned BitWidth = CA->getBitWidth();
+        APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
+        computeKnownBits(X, KnownZero, KnownOne, DL, Depth + 1, AC, CxtI, DT);
+
+        if ((KnownZero & *CA) == *CA && (KnownZero & *CB) == *CB)
+          return true;
+      }
+
+      return false;
+    };
+
+    Value *X;
+    const APInt *CLHS, *CRHS;
+    if (MatchNUWAddsToSameValue(LHS, RHS, X, CLHS, CRHS)) {
+      if (Pred == CmpInst::ICMP_ULE)
+        return CLHS->ule(*CRHS);
+      return CLHS->ult(*CRHS);
+    }
+
     return false;
   }
   }
diff --git a/llvm/test/Transforms/InstSimplify/implies.ll b/llvm/test/Transforms/InstSimplify/implies.ll
index eb8db4224faf..2e3c9591b079 100644
--- a/llvm/test/Transforms/InstSimplify/implies.ll
+++ b/llvm/test/Transforms/InstSimplify/implies.ll
@@ -127,6 +127,84 @@ define i1 @test9(i32 %length.i, i32 %i) {
   ret i1 %res
 }
 
+define i1 @test10(i32 %length.i, i32 %x.full) {
+; CHECK-LABEL: @test10(
+; CHECK:  ret i1 true
+
+  %x = and i32 %x.full, 4294901760  ;; 4294901760 == 0xffff0000
+  %large = or i32 %x, 100
+  %small = or i32 %x, 90
+  %known = icmp ult i32 %large, %length.i
+  %to.prove = icmp ult i32 %small, %length.i
+  %res = icmp ule i1 %known, %to.prove
+  ret i1 %res
+}
+
+define i1 @test11(i32 %length.i, i32 %x) {
+; CHECK-LABEL: @test11(
+; CHECK: %res = icmp ule i1 %known, %to.prove
+; CHECK: ret i1 %res
+
+  %large = or i32 %x, 100
+  %small = or i32 %x, 90
+  %known = icmp ult i32 %large, %length.i
+  %to.prove = icmp ult i32 %small, %length.i
+  %res = icmp ule i1 %known, %to.prove
+  ret i1 %res
+}
+
+define i1 @test12(i32 %length.i, i32 %x.full) {
+; CHECK-LABEL: @test12(
+; CHECK: %res = icmp ule i1 %known, %to.prove
+; CHECK: ret i1 %res
+
+  %x = and i32 %x.full, 4294901760  ;; 4294901760 == 0xffff0000
+  %large = or i32 %x, 65536 ;; 65536 == 0x00010000
+  %small = or i32 %x, 90
+  %known = icmp ult i32 %large, %length.i
+  %to.prove = icmp ult i32 %small, %length.i
+  %res = icmp ule i1 %known, %to.prove
+  ret i1 %res
+}
+
+define i1 @test13(i32 %length.i, i32 %x) {
+; CHECK-LABEL: @test13(
+; CHECK:  ret i1 true
+
+  %large = add nuw i32 %x, 100
+  %small = add nuw i32 %x, 90
+  %known = icmp ult i32 %large, %length.i
+  %to.prove = icmp ult i32 %small, %length.i
+  %res = icmp ule i1 %known, %to.prove
+  ret i1 %res
+}
+
+define i1 @test14(i32 %length.i, i32 %x.full) {
+; CHECK-LABEL: @test14(
+; CHECK:  ret i1 true
+
+  %x = and i32 %x.full, 4294905615  ;; 4294905615 == 0xffff0f0f
+  %large = or i32 %x, 8224 ;; == 0x2020
+  %small = or i32 %x, 4112 ;; == 0x1010
+  %known = icmp ult i32 %large, %length.i
+  %to.prove = icmp ult i32 %small, %length.i
+  %res = icmp ule i1 %known, %to.prove
+  ret i1 %res
+}
+
+define i1 @test15(i32 %length.i, i32 %x) {
+; CHECK-LABEL: @test15(
+; CHECK:  %res = icmp ule i1 %known, %to.prove
+; CHECK:  ret i1 %res
+
+  %large = add nuw i32 %x, 100
+  %small = add nuw i32 %x, 110
+  %known = icmp ult i32 %large, %length.i
+  %to.prove = icmp ult i32 %small, %length.i
+  %res = icmp ule i1 %known, %to.prove
+  ret i1 %res
+}
+
 ; X >=(s) Y == X ==> Y (i1 1 becomes -1 for reasoning)
 define i1 @test_sge(i32 %length.i, i32 %i) {
 ; CHECK-LABEL: @test_sge