[IRGen] Mark inout ptrs noalias.

Thanks to exclusivity checking, inout arguments can be marked noalias.
In the fullness of time, this should be applied in general.

As an incremental step towards marking all such arguments noalias, start
by marking inout arguments of pointer type noalias.  Take this
conservative step rather than always applying the attribute to ward
against issues with LLVM's alias analysis.

rdar://76540030

Author: nate-chandler

lib/IRGen/GenCall.cpp

@@ -265,14 +265,19 @@ static void addIndirectValueParameterAttributes(IRGenModule &IGM,
   attrs = attrs.addParamAttributes(IGM.getLLVMContext(), argIndex, b);
 }
 
-static void addInoutParameterAttributes(IRGenModule &IGM,
+static void addInoutParameterAttributes(IRGenModule &IGM, SILType paramSILType,
                                         llvm::AttributeList &attrs,
                                         const TypeInfo &ti, unsigned argIndex,
                                         bool aliasable) {
   llvm::AttrBuilder b;
-  // Aliasing inouts is unspecified, but we still want aliasing to be memory-
-  // safe, so we can't mark inouts as noalias at the LLVM level.
-  // They still can't be captured without doing unsafe stuff, though.
+  // Thanks to exclusivity checking, it is not possible to alias inouts except
+  // those that are inout_aliasable.
+  if (!aliasable && paramSILType.getASTType()->getAnyPointerElementType()) {
+    // To ward against issues with LLVM's alias analysis, for now, only add the
+    // attribute if it's a pointer being passed inout.
+    b.addAttribute(llvm::Attribute::NoAlias);
+  }
+  // Aliasing inouts can't be captured without doing unsafe stuff.
   b.addAttribute(llvm::Attribute::NoCapture);
   // The inout must reference dereferenceable memory of the type.
   addDereferenceableAttributeToBuilder(IGM, b, ti);
@@ -1510,8 +1515,9 @@ void SignatureExpansion::expand(SILParameterInfo param) {
 
   case ParameterConvention::Indirect_Inout:
   case ParameterConvention::Indirect_InoutAliasable:
-    addInoutParameterAttributes(IGM, Attrs, ti, ParamIRTypes.size(),
-                          conv == ParameterConvention::Indirect_InoutAliasable);
+    addInoutParameterAttributes(
+        IGM, paramSILType, Attrs, ti, ParamIRTypes.size(),
+        conv == ParameterConvention::Indirect_InoutAliasable);
     addPointerParameter(IGM.getStorageType(getSILFuncConventions().getSILType(
         param, IGM.getMaximalTypeExpansionContext())));
     return;

test/IRGen/inout_noalias.sil

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -parse-sil %s -emit-ir | %FileCheck %s
+
+import Swift
+
+// CHECK: define{{.*}}swiftcc void @takeInoutAliasable(%TSP* nocapture dereferenceable({{[0-9]+}}) %0, %swift.type* %T)
+sil @takeInoutAliasable : $<T> (@inout_aliasable UnsafePointer<T>) -> () {
+entry(%ptr : $*UnsafePointer<T>):
+  %retval = tuple ()
+  return %retval : $()
+}

test/IRGen/inout_noalias.swift

@@ -0,0 +1,5 @@
+// RUN: %target-swift-frontend %s -emit-ir | %FileCheck %s
+
+// CHECK: define{{.*}}swiftcc void @swapPointers({{.*}}noalias{{.*}},{{.*}}noalias{{.*}})
+@_silgen_name("swapPointers")
+public func swapPointers<T>(_ lhs: inout UnsafePointer<T>, _ rhs: inout UnsafePointer<T>) {}