From 8963071a142368859fac215b68957a42286b3dd5 Mon Sep 17 00:00:00 2001
From: Arnold Schwaighofer <aschwaighofer@apple.com>
Date: Thu, 16 Jan 2014 04:53:18 +0000
Subject: [PATCH] BasicAA: We need to check both access sizes when comparing a
 gep and an underlying object of unknown size.

Fixes PR18460.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199351 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/BasicAliasAnalysis.cpp   | 10 +++++++-
 test/Analysis/BasicAA/noalias-bugs.ll | 33 +++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 test/Analysis/BasicAA/noalias-bugs.ll

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 404c5405933..6adeedb2448 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1009,7 +1009,15 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
         return NoAlias;
       }
     } else {
-      if (V1Size != UnknownSize) {
+      // We have the situation where:
+      // +                +
+      // | BaseOffset     |
+      // ---------------->|
+      // |-->V1Size       |-------> V2Size
+      // GEP1             V2
+      // We need to know that V2Size is not unknown, otherwise we might have
+      // stripped a gep with negative index ('gep <ptr>, -1, ...).
+      if (V1Size != UnknownSize && V2Size != UnknownSize) {
         if (-(uint64_t)GEP1BaseOffset < V1Size)
           return PartialAlias;
         return NoAlias;
diff --git a/test/Analysis/BasicAA/noalias-bugs.ll b/test/Analysis/BasicAA/noalias-bugs.ll
new file mode 100644
index 00000000000..2bcc14fd939
--- /dev/null
+++ b/test/Analysis/BasicAA/noalias-bugs.ll
@@ -0,0 +1,33 @@
+; RUN: opt -S -basicaa -dse < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; We incorrectly returned noalias in the example below for "ptr.64" and
+; "either_ptr.64".
+; PR18460
+
+%nested = type { %nested.i64 }
+%nested.i64 = type { i64 }
+
+define i64 @testcase(%nested * noalias %p1, %nested * noalias %p2,
+                     i32 %a, i32 %b) {
+  %ptr = getelementptr inbounds %nested* %p1, i64 -1, i32 0
+  %ptr.64 = getelementptr inbounds %nested.i64* %ptr, i64 0, i32 0
+  %ptr2= getelementptr inbounds %nested* %p2, i64 0, i32 0
+  %cmp = icmp ult i32 %a, %b
+  %either_ptr = select i1 %cmp, %nested.i64* %ptr2, %nested.i64* %ptr
+  %either_ptr.64 = getelementptr inbounds %nested.i64* %either_ptr, i64 0, i32 0
+
+; Because either_ptr.64 and ptr.64 can alias (we used to return noalias)
+; elimination of the first store is not valid.
+
+; CHECK: store i64 2
+; CHECK: load
+; CHECK; store i64 1
+
+  store i64 2, i64* %ptr.64, align 8
+  %r = load i64* %either_ptr.64, align 8
+  store i64 1, i64* %ptr.64, align 8
+  ret i64 %r
+}