[Embedded] Only reserve 4KB in embedded Darwin targets

Non-embedded Darwin targets reserve the first 4GB of address space. All
other targets reserve 4KB of address space. Make Embedded Darwin
targets only reserve the first 4KB (as with other targets), since they
aren't in userspace.

Fixes rdar://158981013.

Author: DougGregor

lib/IRGen/SwiftTargetInfo.cpp

@@ -49,7 +49,10 @@ static void configureARM64(IRGenModule &IGM, const llvm::Triple &triple,
   }
   setToMask(target.IsObjCPointerBit, 64, SWIFT_ABI_ARM64_IS_OBJC_BIT);
 
-  if (triple.isOSDarwin()) {
+  // Non-embedded Darwin reserves the low 4GB of address space.
+  if (triple.isOSDarwin() &&
+      !IGM.getSwiftModule()->getASTContext().LangOpts.hasFeature(
+          Feature::Embedded)) {
     target.LeastValidPointerValue =
       SWIFT_ABI_DARWIN_ARM64_LEAST_VALID_POINTER;
   }
@@ -104,7 +107,9 @@ static void configureX86_64(IRGenModule &IGM, const llvm::Triple &triple,
               SWIFT_ABI_X86_64_OBJC_RESERVED_BITS_MASK);
   }
 
-  if (triple.isOSDarwin()) {
+  if (triple.isOSDarwin() &&
+      !IGM.getSwiftModule()->getASTContext().LangOpts.hasFeature(
+          Feature::Embedded)) {
     target.LeastValidPointerValue =
       SWIFT_ABI_DARWIN_X86_64_LEAST_VALID_POINTER;
   }

test/embedded/min-pointer-value-default-darwin.swift

@@ -0,0 +1,22 @@
+
+// RUN: %target-swift-emit-ir %s -module-name main -parse-as-library -enable-experimental-feature Embedded -O -min-valid-pointer-value=0x1000  | %FileCheck %s
+// RUN: %target-swift-emit-ir %s -module-name main -parse-as-library -enable-experimental-feature Embedded -O | %FileCheck %s
+
+// REQUIRES: OS=macosx
+// REQUIRES: swift_feature_Embedded
+
+public func testit(_ s: S?) -> Bool {
+  return s != nil
+}
+
+class C {}
+
+public struct S {
+  var a = InlineArray<57, UInt8>(repeating: 0)
+  var b = C()
+  var c = InlineArray<49, UInt8>(repeating: 0)
+}
+
+// CHECK-LABEL: define {{.*}} @"$e4main1SVSgWOg"(ptr %0)
+// CHECK:         icmp {{.*}}, 4095
+// CHECK-LABEL: }