[AliasAnalysis] Teach BasicAA about memcpy.

[bryant]

Summary

From LangRef:

The '`llvm.memcpy.*`' intrinsics copy a block of memory from the
source location to the destination location, which are not allowed to overlap.

Quick demo with -aa-eval:

; opt -basicaa -aa-eval -print-all-alias-modref-info Transforms/Util/MemorySSA/basicaa-memcpy.ll

declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64,
                                        i32, i1) nounwind

define void @source_clobber(i8* %a, i8* %b) {
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
  %x = load i8, i8* %b
  ret void
}

; Function: source_clobber: 2 pointers, 1 call sites
;   MayAlias:     i8* %a, i8* %b
;   Just Mod:  Ptr: i8* %a        <->  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
;   Just Ref:  Ptr: i8* %b        <->  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)

Note the "Just {Mod,Ref}" results.

[bryant]

test/Analysis/BasicAA/cs-cs.ll

53

I'm uncertain about the correctness of this result. Since both operands are must-alias, shouldn't this remain "Both ModRef"?

221

Since %R and %Q may alias, should this remain "Both ModRef"?

[reames]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

This entire change should be unnecessary. We have readonly and writeonly attributes with these semantics and the intrinsic should already be declared with them.

Reading further, it looks like you're change is mainly aimed at the no alias property right? If so, take a look at the noalias parameter attribute. It might be we've forgotten to tag the intrinsic correctly.

[bryant]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

This entire change should be unnecessary. We have readonly and writeonly attributes with these semantics and the intrinsic should already be declared with them.

Including write-/readonly inside memcpy's declaration with my initial example doesn't work, at least not with -basicaa:

declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, 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
}

09:57:21 ~> opt -disable-output -basicaa -aa-eval -print-all-alias-modref-info memcpy-test.ll
Function: source_clobber: 2 pointers, 1 call sites
  MayAlias:     i8* %a, i8* %b
  Both ModRef:  Ptr: i8* %a     <->  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
  Both ModRef:  Ptr: i8* %b     <->  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)

Reading further, it looks like you're change is mainly aimed at the no alias property right? If so, take a look at the noalias parameter attribute. It might be we've forgotten to tag the intrinsic correctly.

It's not always the case that parameters can satisfy the noalias attribute. Suppose, for instance, that the previous function was instead

define void @f(i8* %a, i8* %b, i8* %c, i8* %d)

where a and b could alias c and d, respectively. Then neither a nor b fits noalias requirement. Yet, even in such a case, the LangRef constraint on memcpy should be sufficient to deduce that a no-alias b.

[hfinkel]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

I don't understand what your comment means. How are %c and %d accessed?

Just in case it is not clear, noalias's semantics are similar in this regard to C's restrict semantics. noalias means that memory accessed through the attributed pointer, or any pointers based on that pointer, are not accessed through a pointer not based on the attributed pointer. In C, memcpy certainly has restrict on its pointer arguments, and we could add noalias on ours as well.

[bryant]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

I don't understand what your comment means. How are %c and %d accessed?

I took reames' original suggestion to mean that in my original example, source_clobber's parameters %a and %b should have been tagged noalias, in which case the unpatched BasicAA indeed deduces the correct mod/ref info.

Just in case it is not clear, noalias's semantics are similar in this regard to C's restrict semantics. noalias means that memory accessed through the attributed pointer, or any pointers based on that pointer, are not accessed through a pointer not based on the attributed pointer.

So this implies that a parameter i8* %a noalias would no-alias every other pointer parameter, right? Because that was my point: Calling memcpy with %a and %b only implies that %a no-alias %b, whereas %a noalias implies a much stronger condition. Suppose, instead of source_clobber, we had the function:

define void @f(i8* %a, i8* %b, i8* %c, i8* %d) {
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %c, i8* %d, i64 128, i32 1, i1 false)
  %x = load i8, i8* %b
}

It's totally okay for %a to may-alias %c, and %b to may-alias %d, right? An i8* %a noalias would forbid this possibility.

In C, memcpy certainly has restrict on its pointer arguments, and we could add noalias on ours as well.

Do you mean tagging the memcpy IR prototype's parameters noalias? I've tried that and it doesn't seem to work, either. Here's a quick patch that adds Attribute::NoAlias to the memcpy intrinsic:

diff
diff --git a/include/llvm/IR/Intrinsics.td b/include/llvm/IR/Intrinsics.td
index be0ae3e..4a240ae 100644
--- a/include/llvm/IR/Intrinsics.td
+++ b/include/llvm/IR/Intrinsics.td
@@ -78,6 +78,10 @@ class ReadNone<int argNo> : IntrinsicProperty {
   int ArgNo = argNo;
 }
 
+class NoAlias<int argNo> : IntrinsicProperty {
+  int ArgNo = argNo;
+}
+
 def IntrNoReturn : IntrinsicProperty;
 
 // IntrNoduplicate - Calls to this intrinsic cannot be duplicated.
@@ -368,7 +372,8 @@ def int_memcpy  : Intrinsic<[],
                              [llvm_anyptr_ty, llvm_anyptr_ty, llvm_anyint_ty,
                               llvm_i32_ty, llvm_i1_ty],
                             [IntrArgMemOnly, NoCapture<0>, NoCapture<1>,
-                             WriteOnly<0>, ReadOnly<1>]>;
+                             WriteOnly<0>, ReadOnly<1>, NoAlias<0>,
+                             NoAlias<1>]>;
 def int_memmove : Intrinsic<[],
                             [llvm_anyptr_ty, llvm_anyptr_ty, llvm_anyint_ty,
                              llvm_i32_ty, llvm_i1_ty],
diff --git a/utils/TableGen/CodeGenIntrinsics.h b/utils/TableGen/CodeGenIntrinsics.h
index ea3ec67..239e37d 100644
--- a/utils/TableGen/CodeGenIntrinsics.h
+++ b/utils/TableGen/CodeGenIntrinsics.h
@@ -108,7 +108,7 @@ struct CodeGenIntrinsic {
   /// True if the intrinsic is marked as convergent.
   bool isConvergent;
 
-  enum ArgAttribute { NoCapture, Returned, ReadOnly, WriteOnly, ReadNone };
+  enum ArgAttribute { NoCapture, Returned, ReadOnly, WriteOnly, ReadNone, NoAlias };
   std::vector<std::pair<unsigned, ArgAttribute>> ArgumentAttributes;
 
   CodeGenIntrinsic(Record *R);
diff --git a/utils/TableGen/CodeGenTarget.cpp b/utils/TableGen/CodeGenTarget.cpp
index 13c8c12..431611e 100644
--- a/utils/TableGen/CodeGenTarget.cpp
+++ b/utils/TableGen/CodeGenTarget.cpp
@@ -610,6 +610,9 @@ CodeGenIntrinsic::CodeGenIntrinsic(Record *R) {
     } else if (Property->isSubClassOf("ReadNone")) {
       unsigned ArgNo = Property->getValueAsInt("ArgNo");
       ArgumentAttributes.push_back(std::make_pair(ArgNo, ReadNone));
+    } else if (Property->isSubClassOf("NoAlias")) {
+      unsigned ArgNo = Property->getValueAsInt("ArgNo");
+      ArgumentAttributes.push_back(std::make_pair(ArgNo, NoAlias));
     } else
       llvm_unreachable("Unknown property!");
   }
diff --git a/utils/TableGen/IntrinsicEmitter.cpp b/utils/TableGen/IntrinsicEmitter.cpp
index 60cd2da..5fe75ad 100644
--- a/utils/TableGen/IntrinsicEmitter.cpp
+++ b/utils/TableGen/IntrinsicEmitter.cpp
@@ -587,6 +587,12 @@ void IntrinsicEmitter::EmitAttributes(const CodeGenIntrinsicTable &Ints,
             OS << "Attribute::ReadNone";
             addComma = true;
             break;
+          case CodeGenIntrinsic::NoAlias:
+            if (addComma)
+              OS << ",";
+            OS << "Attribute::NoAlias";
+            addComma = true;
+            break;
           }
 
           ++ai;

And re-running source_clobber through opt:

06:55:57 ~/3rd/llvm> opt -basicaa -aa-eval -print-all-alias-modref-info -S test.ll
Function: source_clobber: 2 pointers, 1 call sites
  MayAlias:     i8* %a, i8* %b
  Both ModRef:  Ptr: i8* %a     <->  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
  Both ModRef:  Ptr: i8* %b     <->  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
; ModuleID = 'test.ll'
source_filename = "test.ll"

; Function Attrs: argmemonly nounwind
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i32, i1) #0

define void @source_clobber(i8* %a, i8* %b) {
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %a, i8* %b, i64 128, i32 1, i1 false)
  %x = load i8, i8* %b
  ret void
}

attributes #0 = { argmemonly nounwind }

Notice that the intrinsic prototype now contains noalias, yet the -aa-eval results are still over-conservative.

[hfinkel]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

Because that was my point: Calling memcpy with %a and %b only implies that %a no-alias %b, whereas %a noalias implies a much stronger condition.

No, it is actually a weaker condition. That's the problem. noalias does not say anything about the aliasing properties of the function arguments themselves. It only has a meaning if you know how the pointers are used within the function. So, yes, we could add noalias to memcpy. No, we'd not be able to optimize the caller of memcpy based only on that information. In the case of memcpy, we know how the memory is accessed so we can say more about the aliasing properties.

[bryant]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

No, we'd not be able to optimize the caller of memcpy based only on that information. In the case of memcpy, we know how the memory is accessed so we can say more about the aliasing properties.

Why not? Why can't we assume that two pointers don't alias as soon as they're fed into memcpy, and that if they do, it's undefined behavior?

No, it is actually a weaker condition. That's the problem. noalias does not say anything about the aliasing properties of the function arguments themselves. It only has a meaning if you know how the pointers are used within the function.

LangRef says that "objects accessed via pointer values based on the argument or return value are not also accessed, during the execution of the function, via pointer values not based on the argument or return value." "During execution of the function" tells me that regardless of how the function uses a noalias pointer argument, it's not allowed to alias any other pointer argument. Correct?

And it's UB if I pass in aliased pointers to any function whose arguments are noalias because "the caller shares the responsibility with the callee for ensuring that these requirements are met."

[hfinkel]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

Why not? Why can't we assume that two pointers don't alias as soon as they're fed into memcpy, and that if they do, it's undefined behavior?

For memcpy, we can. In general for functions with restrict-qualified arguments, we can't. For example, the following is a fine function:

void foo(int * restrict x, int * restrict y) {

x[0] = y[1];

}

and can be called with foo(p, p) without any UB.

LangRef says that "objects accessed via pointer values based on the argument or return value are not also accessed, during the execution of the function, via pointer values not based on the argument or return value." "During execution of the function" tells me that regardless of how the function uses a noalias pointer argument, it's not allowed to alias any other pointer argument. Correct?

I think the thing that you're missing is that, in general, we don't know what offsets of which pointers are actually used for accesses. Hopefully, my example above makes it clear what I mean. If not, please let me know. For memcpy, we obviously know how the relative offsets work, so we have more information than for a generic function with restrict-qualified pointer arguments.

[bryant]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

"Object accessed via pointer values..." presumes that there are objects pointed to by those pointer arguments. So noalias means that those objects don't overlap. If you choose some astronomically large offset that, added to int * restrict x, brings you into int * restrict y's territory, then either 1) you're making an out-of-bounds access to a pointer or 2) the objects pointed to by x and y do in fact overlap. Either way, it's UB.

Also, as a side note, restricted pointers are allowed to alias if they're never written to: "If the object is never modified, it may be aliased and accessed through different restrict-qualified pointers..." from http://en.cppreference.com/w/c/language/restrict . I think this differs from noalias, whose non-overlap condition is qualified by "access," which to me means both reads and writes.

In any case: If memcpy is special, what should the correct fix to get the right answers out of -aa-eval, since the tagging noalias to its arguments doesn't seem to work as indicated previously?

[hfinkel]

lib/Analysis/BasicAliasAnalysis.cpp

773–798

"Object accessed via pointer values..." presumes that there are objects pointed to by those pointer arguments. So noalias means that those objects don't overlap. If you choose some astronomically large offset that, added to int * restrict x, brings you into int * restrict y's territory, then either 1) you're making an out-of-bounds access to a pointer or 2) the objects pointed to by x and y do in fact overlap. Either way, it's UB

x and y can point into the same array so long as the elements of the array accessed through each is distinct. The array is not itself an object, in this sense, but the objects are the array elements.

In any case: If memcpy is special, what should the correct fix to get the right answers out of -aa-eval, since the tagging noalias to its arguments doesn't seem to work as indicated previously?

I think that we need to add special logic (which is what I presume you're doing in this patch -- I'll take a look at the specifics, but I've not done so yet).

[hfinkel]

lib/Analysis/BasicAliasAnalysis.cpp

777

Please add a comment here explaining why you can return MRI_Ref without even checking the destination (i.e. that you're relying on the memcpy semantics which forbid overlapping memory). With that, this LGTM.

We really should also add similar logic here for memmove and memset. Would you do this as follow-up?

[bryant]

lib/Analysis/BasicAliasAnalysis.cpp

777

We really should also add similar logic here for memmove and memset. Would you do this as follow-up?

Sure, but memmove permits source and destination to overlap, and memset splats a constant onto a destination. What sort of similar logic did you have in mind?

[hfinkel]

lib/Analysis/BasicAliasAnalysis.cpp

777

Never mind. MemoryLocation::getForArgument already has special handling for memmove, etc. You're right, memcpy is the one that benefits from special handling.

[bryant]

Add explanation about memcpy operand ovelap semantics.

[bryant]

Ping.

@hfinkel: Do the adjustments look good to you?

@reames: Given that marking the memcpy operands as noalias didn't work, do you see any alternatives to this approach? Do you think there might be problems with the approach itself?

[hfinkel]

In D27034#623500, @bryant wrote:

Ping.

@hfinkel: Do the adjustments look good to you?

Yes, please proceed. Thanks!

@reames: Given that marking the memcpy operands as noalias didn't work, do you see any alternatives to this approach? Do you think there might be problems with the approach itself?

[hfinkel]

LGTM

[sanjoy]

I added some minor stylistic comments inline.

llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp

783 ↗

(On Diff #82484)

This shadows the Inst variable in the outer scope, which can potentially be confusing. How about auto *MCI = dyn_cast<...>(...))?

786 ↗

(On Diff #82484)

How about

auto SrcAA = getBestAAResults().alias(...)
if (SrcAA == MustAlias)
  ...
auto DestAA = getBestAAResults().alias(...)
if (DestAA == MustAlias)
  ...

That seems less cluttered.

796 ↗

(On Diff #82484)

LLVM coding style forbids variables with lower case names. ModRefInfo MRI would be more idiomatic.

lib/Analysis/BasicAliasAnalysis.cpp

ModRefInfo BasicAAResult::getModRefInfo(ImmutableCallSite CS,
   // that's implemented fully. 
   auto *Inst = CS.getInstruction();
   if (isMallocLikeFn(Inst, &TLI) || isCallocLikeFn(Inst, &TLI)) {
     // Be conservative if the accessed pointer may alias the allocation -
     // fallback to the generic handling below.
     if (getBestAAResults().alias(MemoryLocation(Inst), Loc) == NoAlias)
       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.
   if (isIntrinsicCall(CS, Intrinsic::assume))
     return MRI_NoModRef;
 
   // Like assumes, guard intrinsics are also marked as arbitrarily writing so
   // that proper control dependencies are maintained but they never mods any

test/Analysis/BasicAA/assume.ll

 ; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
 target datalayout = "e-p:32:32:32-i1:8:32-i8:8:32-i16:16:32-i32:32:32-i64:32:32-f32:32:32-f64:32:32-v64:32:64-v128:32:128-a0:0:32-n32"
 
 declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) #0
 declare void @llvm.assume(i1) #0
 
 define void @test1(i8* %P, i8* %Q) nounwind ssp {
   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)
   ret void
 
 ; CHECK-LABEL: Function: test1:
 
 ; 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)
 }
 
 attributes #0 = { nounwind }

test/Analysis/BasicAA/cs-cs.ll

 define void @test2(i8* %P, i8* %Q) nounwind ssp {
   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)
   ret void
 
 ; 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 {
   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)
   ret void
 
 ; CHECK-LABEL: Function: test2a:

 define void @test3(i8* %P, i8* %Q) nounwind ssp {
   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)
   ret void
 
 ; 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 {
   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)
   ret void
 
 ; CHECK-LABEL: Function: test3a:
define void @test5(i8* %P, i8* %Q, i8* %R) nounwind ssp {
   tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %P, i8* %R, i64 12, i32 1, i1 false)
   ret void
 
 ; CHECK-LABEL: Function: test5:
 
 ; 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 {
   call void @llvm.memset.p0i8.i64(i8* %P, i8 -51, i64 32, i32 8, i1 false)
   call void @a_readonly_func(i8* %P)
   ret void
 
 ; CHECK-LABEL: Function: test6:

test/Analysis/BasicAA/guards.ll

 ; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
 target datalayout = "e-p:32:32:32-i1:8:32-i8:8:32-i16:16:32-i32:32:32-i64:32:32-f32:32:32-f64:32:32-v64:32:64-v128:32:128-a0:0:32-n32"
 
 declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) #0
 declare void @llvm.experimental.guard(i1, ...)
 declare void @unknown_but_readonly() readonly
 
 define void @test1(i8* %P, i8* %Q) {
   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)
   ret void
 
 ; CHECK-LABEL: Function: test1:
 
 ; 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"() ]
 }
 
 define void @test2() {
   tail call void(i1,...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
   tail call void @unknown_but_readonly()
   ret void
 ; CHECK-LABEL: Function: test2:
 ; CHECK:  NoModRef:   tail call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ] <->   tail call void @unknown_but_readonly()
 ; CHECK:  NoModRef:   tail call void @unknown_but_readonly() <->   tail call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
 }

test/Transforms/Util/MemorySSA/basicaa-memcpy.ll

+; 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
+}