Correctly combine alias.scope metadata by a union instead of intersecting

Summary:
The alias.scope metadata represents sets of things an instruction might
alias with. When generically combining the metadata from two
instructions the result must be the union of the original sets, because
the new instruction might alias with anything any of the original
instructions aliased with.

Reviewers: hfinkel

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7490

llvm-svn: 228525

1 files changed, 22 insertions, 0 deletions

diff --git a/llvm/lib/IR/Metadata.cpp b/llvm/lib/IR/Metadata.cpp
index 6227c209352..55dc96b10bf 100644
--- a/llvm/lib/IR/Metadata.cpp
+++ b/llvm/lib/IR/Metadata.cpp
@@ -782,6 +782,28 @@ MDNode *MDNode::intersect(MDNode *A, MDNode *B) {
   return getOrSelfReference(A->getContext(), MDs);
 }
 
+MDNode *MDNode::getMostGenericAliasScope(MDNode *A, MDNode *B) {
+  if (!A || !B)
+    return nullptr;
+
+  SmallVector<Metadata *, 4> MDs(B->op_begin(), B->op_end());
+  for (unsigned i = 0, ie = A->getNumOperands(); i != ie; ++i) {
+    Metadata *MD = A->getOperand(i);
+    bool insert = true;
+    for (unsigned j = 0, je = B->getNumOperands(); j != je; ++j)
+      if (MD == B->getOperand(j)) {
+        insert = false;
+        break;
+      }
+    if (insert)
+        MDs.push_back(MD);
+  }
+
+  // FIXME: This preserves long-standing behaviour, but is it really the right
+  // behaviour?  Or was that an unintended side-effect of node uniquing?
+  return getOrSelfReference(A->getContext(), MDs);
+}
+
 MDNode *MDNode::getMostGenericFPMath(MDNode *A, MDNode *B) {
   if (!A || !B)
     return nullptr;