Index: llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
+++ llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
@@ -543,7 +543,6 @@
         isa<IntegerType>(MemsetType->getParamType(2)))
       return true;
   }
-
   return false;
 }
 
@@ -583,9 +582,6 @@
   if (F->onlyAccessesArgMemory())
     Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
 
-  if (isMemsetPattern16(F, TLI))
-    Min = FMRB_OnlyAccessesArgumentPointees;
-
   // Otherwise be conservative.
   return FunctionModRefBehavior(AAResultBase::getModRefBehavior(F) & Min);
 }
@@ -609,14 +605,11 @@
   // We can bound the aliasing properties of memset_pattern16 just as we can
   // for memcpy/memset.  This is particularly important because the
   // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
-  // whenever possible.
-  if (CS.getCalledFunction() &&
-      isMemsetPattern16(CS.getCalledFunction(), TLI)) {
-    assert((ArgIdx == 0 || ArgIdx == 1) &&
-           "Invalid argument index for memset_pattern16");
-    return ArgIdx ? MRI_Ref : MRI_Mod;
-  }
-  // FIXME: Handle memset_pattern4 and memset_pattern8 also.
+  // whenever possible.  Note that all but the missing writeonly attribute are
+  // handled via InferFunctionAttr.
+  if (CS.getCalledFunction() && isMemsetPattern16(CS.getCalledFunction(), TLI))
+    if (ArgIdx == 0)
+      return MRI_Mod;
 
   if (CS.paramHasAttr(ArgIdx + 1, Attribute::ReadOnly))
     return MRI_Ref;
Index: llvm/trunk/lib/Transforms/IPO/InferFunctionAttrs.cpp
===================================================================
--- llvm/trunk/lib/Transforms/IPO/InferFunctionAttrs.cpp
+++ llvm/trunk/lib/Transforms/IPO/InferFunctionAttrs.cpp
@@ -22,6 +22,7 @@
 
 STATISTIC(NumReadNone, "Number of functions inferred as readnone");
 STATISTIC(NumReadOnly, "Number of functions inferred as readonly");
+STATISTIC(NumArgMemOnly, "Number of functions inferred as argmemonly");
 STATISTIC(NumNoUnwind, "Number of functions inferred as nounwind");
 STATISTIC(NumNoCapture, "Number of arguments inferred as nocapture");
 STATISTIC(NumReadOnlyArg, "Number of arguments inferred as readonly");
@@ -44,6 +45,15 @@
   return true;
 }
 
+static bool setOnlyAccessesArgMemory(Function &F) {
+  if (F.onlyAccessesArgMemory())
+    return false;
+  F.setOnlyAccessesArgMemory ();
+  ++NumArgMemOnly;
+  return true;
+}
+
+
 static bool setDoesNotThrow(Function &F) {
   if (F.doesNotThrow())
     return false;
@@ -900,6 +910,20 @@
     Changed |= setDoesNotAlias(F, AttributeSet::ReturnIndex);
     return Changed;
 
+  //TODO: add LibFunc entries for:
+  //case LibFunc::memset_pattern4:
+  //case LibFunc::memset_pattern8:
+  case LibFunc::memset_pattern16:
+    if (FTy->isVarArg() || FTy->getNumParams() != 3 ||
+        !isa<PointerType>(FTy->getParamType(0)) ||
+        !isa<PointerType>(FTy->getParamType(1)) ||
+        !isa<IntegerType>(FTy->getParamType(2)))
+      return false;
+
+    Changed |= setOnlyAccessesArgMemory(F);
+    Changed |= setOnlyReadsMemory(F, 2);
+    return Changed;
+
   default:
     // FIXME: It'd be really nice to cover all the library functions we're
     // aware of here.
Index: llvm/trunk/test/Analysis/BasicAA/memset_pattern.ll
===================================================================
--- llvm/trunk/test/Analysis/BasicAA/memset_pattern.ll
+++ llvm/trunk/test/Analysis/BasicAA/memset_pattern.ll
@@ -18,4 +18,4 @@
   ret i32 %l
 }
 
-declare void @memset_pattern16(i8*, i8*, i64)
+declare void @memset_pattern16(i8*, i8* readonly, i64) argmemonly
Index: llvm/trunk/test/Transforms/InferFunctionAttrs/annotate.ll
===================================================================
--- llvm/trunk/test/Transforms/InferFunctionAttrs/annotate.ll
+++ llvm/trunk/test/Transforms/InferFunctionAttrs/annotate.ll
@@ -22,9 +22,14 @@
 declare i8* @_Znwm(i64)
 ; CHECK: declare noalias nonnull i8* @_Znwm(i64)
 
+declare void @memset_pattern16(i8*, i8*, i64)
+; CHECK: declare void @memset_pattern16(i8*, i8*, i64)
+; CHECK-POSIX: declare void @memset_pattern16(i8*, i8* readonly, i64) [[G2:#[0-9]+]]
+
 declare i32 @gettimeofday(i8*, i8*)
 ; CHECK-POSIX: declare i32 @gettimeofday(i8* nocapture, i8* nocapture) [[G0:#[0-9]+]]
 
 ; CHECK: attributes [[G0]] = { nounwind }
 ; CHECK: attributes [[G1]] = { nounwind readonly }
 ; CHECK-POSIX: attributes [[G0]] = { nounwind }
+; CHECK-POSIX: attributes [[G2]] = { argmemonly }