Index: llvm/docs/LangRef.rst
===================================================================
--- llvm/docs/LangRef.rst
+++ llvm/docs/LangRef.rst
@@ -1532,6 +1532,18 @@
     If a function reads from a writeonly pointer argument, the behavior is
     undefined.
 
+``writable``
+    This attribute indicates that any memory location based on this pointer
+    argument that can be loaded from, can also be stored to without trapping.
+
+    If the argument is also ``dereferenceable(N)``, the first ``N`` bytes of
+    the pointer are (non-atomically) loaded and stored back to the pointer on
+    entry to the function.
+
+    The last rule is needed to ensure a well-defined interaction between
+    ``writable`` and ``noalias``, and to ensure that spurious stores can be
+    introduced without data races.
+
 .. _gc:
 
 Garbage Collector Strategy Names
Index: llvm/include/llvm/Bitcode/LLVMBitCodes.h
===================================================================
--- llvm/include/llvm/Bitcode/LLVMBitCodes.h
+++ llvm/include/llvm/Bitcode/LLVMBitCodes.h
@@ -713,6 +713,7 @@
   ATTR_KIND_SKIP_PROFILE = 85,
   ATTR_KIND_MEMORY = 86,
   ATTR_KIND_NOFPCLASS = 87,
+  ATTR_KIND_WRITABLE = 88,
 };
 
 enum ComdatSelectionKindCodes {
Index: llvm/include/llvm/IR/Attributes.td
===================================================================
--- llvm/include/llvm/IR/Attributes.td
+++ llvm/include/llvm/IR/Attributes.td
@@ -300,6 +300,9 @@
 /// Function always comes back to callsite.
 def WillReturn : EnumAttr<"willreturn", [FnAttr]>;
 
+/// Pointer argument is writable.
+def Writable : EnumAttr<"writable", [ParamAttr]>;
+
 /// Function only writes to memory.
 def WriteOnly : EnumAttr<"writeonly", [ParamAttr]>;
 
Index: llvm/lib/Analysis/AliasAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/AliasAnalysis.cpp
+++ llvm/lib/Analysis/AliasAnalysis.cpp
@@ -917,9 +917,8 @@
   if (isa<AllocaInst>(Object))
     return true;
 
-  // TODO: Also handle sret.
   if (auto *A = dyn_cast<Argument>(Object))
-    return A->hasByValAttr();
+    return A->hasByValAttr() || A->hasAttribute(Attribute::Writable);
 
   // TODO: Noalias shouldn't imply writability, this should check for an
   // allocator function instead.
Index: llvm/lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -2060,6 +2060,8 @@
     return Attribute::Hot;
   case bitc::ATTR_KIND_PRESPLIT_COROUTINE:
     return Attribute::PresplitCoroutine;
+  case bitc::ATTR_KIND_WRITABLE:
+    return Attribute::Writable;
   }
 }
 
Index: llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -809,6 +809,8 @@
     return bitc::ATTR_KIND_MUSTPROGRESS;
   case Attribute::PresplitCoroutine:
     return bitc::ATTR_KIND_PRESPLIT_COROUTINE;
+  case Attribute::Writable:
+    return bitc::ATTR_KIND_WRITABLE;
   case Attribute::EndAttrKinds:
     llvm_unreachable("Can not encode end-attribute kinds marker.");
   case Attribute::None:
Index: llvm/lib/IR/Attributes.cpp
===================================================================
--- llvm/lib/IR/Attributes.cpp
+++ llvm/lib/IR/Attributes.cpp
@@ -1961,7 +1961,8 @@
           .addAttribute(Attribute::ReadNone)
           .addAttribute(Attribute::ReadOnly)
           .addAttribute(Attribute::Dereferenceable)
-          .addAttribute(Attribute::DereferenceableOrNull);
+          .addAttribute(Attribute::DereferenceableOrNull)
+          .addAttribute(Attribute::Writable);
     if (ASK & ASK_UNSAFE_TO_DROP)
       Incompatible.addAttribute(Attribute::Nest)
           .addAttribute(Attribute::SwiftError)
Index: llvm/lib/Transforms/Utils/CodeExtractor.cpp
===================================================================
--- llvm/lib/Transforms/Utils/CodeExtractor.cpp
+++ llvm/lib/Transforms/Utils/CodeExtractor.cpp
@@ -990,6 +990,7 @@
       case Attribute::ImmArg:
       case Attribute::ByRef:
       case Attribute::WriteOnly:
+      case Attribute::Writable:
       //  These are not really attributes.
       case Attribute::None:
       case Attribute::EndAttrKinds:
Index: llvm/test/Bitcode/attributes.ll
===================================================================
--- llvm/test/Bitcode/attributes.ll
+++ llvm/test/Bitcode/attributes.ll
@@ -511,6 +511,11 @@
 ; CHECK: define void @f88() [[SKIPPROFILE:#[0-9]+]]
 define void @f88() skipprofile { ret void }
 
+; CHECK: define void @f89(ptr writable %p)
+define void @f89(ptr writable %p) {
+  ret void
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { memory(none) }
Index: llvm/test/Transforms/LICM/scalar-promote.ll
===================================================================
--- llvm/test/Transforms/LICM/scalar-promote.ll
+++ llvm/test/Transforms/LICM/scalar-promote.ll
@@ -885,9 +885,8 @@
   ret void
 }
 
-; TODO: The store can be promoted, as sret memory is writable.
-define void @sret_cond_store(ptr sret(i32) noalias %ptr) {
-; CHECK-LABEL: @sret_cond_store(
+define void @writable_cond_store(ptr sret(i32) noalias writable %ptr) {
+; CHECK-LABEL: @writable_cond_store(
 ; CHECK-NEXT:    [[PTR_PROMOTED:%.*]] = load i32, ptr [[PTR:%.*]], align 4
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
@@ -896,9 +895,10 @@
 ; CHECK-NEXT:    br i1 [[C]], label [[LOOP_LATCH]], label [[EXIT:%.*]]
 ; CHECK:       loop.latch:
 ; CHECK-NEXT:    [[V_INC]] = add i32 [[V_INC1]], 1
-; CHECK-NEXT:    store i32 [[V_INC]], ptr [[PTR]], align 4
 ; CHECK-NEXT:    br label [[LOOP]]
 ; CHECK:       exit:
+; CHECK-NEXT:    [[V_INC1_LCSSA:%.*]] = phi i32 [ [[V_INC1]], [[LOOP]] ]
+; CHECK-NEXT:    store i32 [[V_INC1_LCSSA]], ptr [[PTR]], align 4
 ; CHECK-NEXT:    ret void
 ;
   br label %loop