Merging r199351:

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r199351 | aschwaighofer | 2014-01-15 23:53:18 -0500 (Wed, 15 Jan 2014) | 5 lines

BasicAA: We need to check both access sizes when comparing a gep and an
underlying object of unknown size.

Fixes PR18460.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_34@206053 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 42 insertions, 1 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index f1a9dd991c..d0e186c5f2 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 0000000000..c02a302c19
--- /dev/null
+++ b/test/Analysis/BasicAA/noalias-bugs.ll
@@ -0,0 +1,33 @@
+; RUN: opt -S -basicaa -dse < %s | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
+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
+}