Index: llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
+++ llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
@@ -776,6 +776,31 @@
       return MRI_NoModRef;
   }
 
+  // The semantics of memcpy intrinsics forbid overlap between their respective
+  // operands, i.e., source and destination of any given memcpy must no-alias.
+  // If Loc must-aliases either one of these two locations, then it necessarily
+  // no-aliases the other.
+  if (auto *Inst = dyn_cast<MemCpyInst>(CS.getInstruction())) {
+    AliasResult SrcAA, DestAA;
+
+    if ((SrcAA = getBestAAResults().alias(MemoryLocation::getForSource(Inst),
+                                          Loc)) == MustAlias)
+      // Loc is exactly the memcpy source thus disjoint from memcpy dest.
+      return MRI_Ref;
+    if ((DestAA = getBestAAResults().alias(MemoryLocation::getForDest(Inst),
+                                           Loc)) == MustAlias)
+      // The converse case.
+      return MRI_Mod;
+
+    // It's also possible for Loc to alias both src and dest, or neither.
+    ModRefInfo rv = MRI_NoModRef;
+    if (SrcAA != NoAlias)
+      rv = static_cast<ModRefInfo>(rv | MRI_Ref);
+    if (DestAA != NoAlias)
+      rv = static_cast<ModRefInfo>(rv | MRI_Mod);
+    return rv;
+  }
+
   // While the assume intrinsic is marked as arbitrarily writing so that
   // proper control dependencies will be maintained, it never aliases any
   // particular memory location.
Index: llvm/trunk/test/Analysis/BasicAA/assume.ll
===================================================================
--- llvm/trunk/test/Analysis/BasicAA/assume.ll
+++ llvm/trunk/test/Analysis/BasicAA/assume.ll
@@ -14,8 +14,8 @@
 ; CHECK: MayAlias:	i8* %P, i8* %Q
 ; CHECK: NoModRef:  Ptr: i8* %P	<->  tail call void @llvm.assume(i1 true)
 ; CHECK: NoModRef:  Ptr: i8* %Q	<->  tail call void @llvm.assume(i1 true)
-; CHECK: Both ModRef:  Ptr: i8* %P	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:  Ptr: i8* %Q	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Mod:  Ptr: i8* %P	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Ref:  Ptr: i8* %Q	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
 ; CHECK: NoModRef:   tail call void @llvm.assume(i1 true) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
 ; CHECK: NoModRef:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.assume(i1 true)
 }
Index: llvm/trunk/test/Analysis/BasicAA/cs-cs.ll
===================================================================
--- llvm/trunk/test/Analysis/BasicAA/cs-cs.ll
+++ llvm/trunk/test/Analysis/BasicAA/cs-cs.ll
@@ -45,12 +45,12 @@
 ; CHECK-LABEL: Function: test2:
 
 ; CHECK:   MayAlias:     i8* %P, i8* %Q
-; CHECK:   Both ModRef:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK:   Both ModRef:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK:   Both ModRef:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK:   Both ModRef:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK:   Both ModRef:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK:   Both ModRef:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:   Just Mod:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:   Just Ref:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:   Just Mod:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:   Just Ref:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:   Just Mod:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:   Just Mod:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
 }
 
 define void @test2a(i8* noalias %P, i8* noalias %Q) nounwind ssp {
@@ -161,12 +161,12 @@
 ; CHECK-LABEL: Function: test3:
 
 ; CHECK: MayAlias:     i8* %P, i8* %Q
-; CHECK: Both ModRef:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false)
-; CHECK: Both ModRef:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false)
-; CHECK: Both ModRef:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false)
+; CHECK: Just Mod:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false)
+; CHECK: Just Ref:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false)
+; CHECK: Just Mod:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Ref:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Mod:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Mod:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 8, i32 1, i1 false)
 }
 
 define void @test3a(i8* noalias %P, i8* noalias %Q) nounwind ssp {
@@ -211,14 +211,14 @@
 ; CHECK: MayAlias:     i8* %P, i8* %Q
 ; CHECK: MayAlias:     i8* %P, i8* %R
 ; CHECK: MayAlias:     i8* %Q, i8* %R
-; CHECK: Both ModRef:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Mod:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Ref:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
 ; CHECK: Both ModRef:  Ptr: i8* %R     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
+; CHECK: Just Mod:  Ptr: i8* %P     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
 ; CHECK: Both ModRef:  Ptr: i8* %Q     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:  Ptr: i8* %R     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
-; CHECK: Both ModRef:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK: Just Ref:  Ptr: i8* %R     <->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
+; CHECK: Just Mod:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
+; CHECK: Just Mod:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false) <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
 }
 
 define void @test6(i8* %P) nounwind ssp {
Index: llvm/trunk/test/Analysis/BasicAA/guards.ll
===================================================================
--- llvm/trunk/test/Analysis/BasicAA/guards.ll
+++ llvm/trunk/test/Analysis/BasicAA/guards.ll
@@ -14,8 +14,8 @@
 
 ; CHECK:  Just Ref:  Ptr: i8* %P	<->  tail call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
 ; CHECK:  Just Ref:  Ptr: i8* %Q	<->  tail call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
-; CHECK:  Both ModRef:  Ptr: i8* %P	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
-; CHECK:  Both ModRef:  Ptr: i8* %Q	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:  Just Mod:  Ptr: i8* %P	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
+; CHECK:  Just Ref:  Ptr: i8* %Q	<->  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
 ; CHECK:  Just Ref:   tail call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ] <->   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false)
 ; CHECK:  Just Mod:   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %Q, i64 12, i32 1, i1 false) <->   tail call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
 }
Index: llvm/trunk/test/Transforms/Util/MemorySSA/basicaa-memcpy.ll
===================================================================
--- llvm/trunk/test/Transforms/Util/MemorySSA/basicaa-memcpy.ll
+++ llvm/trunk/test/Transforms/Util/MemorySSA/basicaa-memcpy.ll
@@ -0,0 +1,16 @@
+; RUN: opt -disable-output -basicaa -print-memoryssa %s 2>&1 | FileCheck %s
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind
+
+define void @source_clobber(i8* %a, i8* %b) {
+; CHECK-LABEL: @source_clobber(
+; CHECK-NEXT:  ; 1 = MemoryDef(liveOnEntry)
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
+; CHECK-NEXT:  ; MemoryUse(liveOnEntry)
+; CHECK-NEXT:    [[X:%.*]] = load i8, i8* %b
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
+  %x = load i8, i8* %b
+  ret void
+}