Index: lib/Basic/Targets/ARM.cpp
===================================================================
--- lib/Basic/Targets/ARM.cpp
+++ lib/Basic/Targets/ARM.cpp
@@ -28,14 +28,6 @@
   DoubleAlign = LongLongAlign = LongDoubleAlign = SuitableAlign = 64;
   const llvm::Triple &T = getTriple();
 
-  // size_t is unsigned long on MachO-derived environments, NetBSD, and
-  // OpenBSD.
-  if (T.isOSBinFormatMachO() || T.getOS() == llvm::Triple::NetBSD ||
-      T.getOS() == llvm::Triple::OpenBSD)
-    SizeType = UnsignedLong;
-  else
-    SizeType = UnsignedInt;
-
   bool IsNetBSD = T.getOS() == llvm::Triple::NetBSD;
   bool IsOpenBSD = T.getOS() == llvm::Triple::OpenBSD;
   if (!T.isOSWindows() && !IsNetBSD && !IsOpenBSD)
@@ -83,12 +75,6 @@
   else
     DoubleAlign = LongLongAlign = LongDoubleAlign = SuitableAlign = 32;
 
-  // size_t is unsigned int on FreeBSD.
-  if (T.getOS() == llvm::Triple::FreeBSD)
-    SizeType = UnsignedInt;
-  else
-    SizeType = UnsignedLong;
-
   WCharType = SignedInt;
 
   // Do not respect the alignment of bit-field types when laying out
@@ -225,22 +211,25 @@
                              const TargetOptions &Opts)
     : TargetInfo(Triple), FPMath(FP_Default), IsAAPCS(true), LDREX(0),
       HW_FP(0) {
-
-  switch (getTriple().getOS()) {
-  case llvm::Triple::NetBSD:
-  case llvm::Triple::OpenBSD:
-    PtrDiffType = SignedLong;
-    break;
-  default:
-    PtrDiffType = SignedInt;
-    break;
-  }
-
   bool IsOpenBSD = Triple.getOS() == llvm::Triple::OpenBSD;
   bool IsNetBSD = Triple.getOS() == llvm::Triple::NetBSD;
-  IntPtrType =
+
+  PtrDiffType = IntPtrType =
       (Triple.isOSDarwin() || IsOpenBSD || IsNetBSD) ? SignedLong : SignedInt;
 
+  // FIXME: the isOSBinFormatMachO is a workaround for identifying a Darwin-like
+  // environment where size_t is `unsigned long` rather than `unsigned int`
+  SizeType = (Triple.isOSDarwin() || Triple.isOSBinFormatMachO() || IsOpenBSD ||
+              IsNetBSD)
+                 ? UnsignedLong
+                 : UnsignedInt;
+
+  // ptrdiff_t is inconsistent on Darwin
+  if (Triple.isOSDarwin())
+    PtrDiffType = SignedInt;
+  else if (Triple.isWatchABI())
+    PtrDiffType = SignedLong;
+
   // Cache arch related info.
   setArchInfo();
 
@@ -927,7 +916,6 @@
 WindowsARMTargetInfo::WindowsARMTargetInfo(const llvm::Triple &Triple,
                                            const TargetOptions &Opts)
     : WindowsTargetInfo<ARMleTargetInfo>(Triple, Opts), Triple(Triple) {
-  SizeType = UnsignedInt;
 }
 
 void WindowsARMTargetInfo::getVisualStudioDefines(const LangOptions &Opts,
@@ -1047,10 +1035,6 @@
     // Darwin on iOS uses a variant of the ARM C++ ABI.
     TheCXXABI.set(TargetCXXABI::WatchOS);
 
-    // The 32-bit ABI is silent on what ptrdiff_t should be, but given that
-    // size_t is long, it's a bit weird for it to be int.
-    PtrDiffType = SignedLong;
-
     // BOOL should be a real boolean on the new ABI
     UseSignedCharForObjCBool = false;
   } else
Index: test/Preprocessor/init.c
===================================================================
--- test/Preprocessor/init.c
+++ test/Preprocessor/init.c
@@ -1791,6 +1791,11 @@
 // ARM:#define __arm 1
 // ARM:#define __arm__ 1
 
+// RUN: %clang_cc1 -dM -ffreestanding -triple arm-none-none -target-abi apcs-gnu -E /dev/null -o - | FileCheck -match-full-lines -check-prefix ARM-APCS-GNU %s
+// ARM-APCS-GNU: #define __INTPTR_TYPE__ int
+// ARM-APCS-GNU: #define __PTRDIFF_TYPE__ int
+// ARM-APCS-GNU: #define __SIZE_TYPE__ unsigned int
+
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple=armeb-none-none < /dev/null | FileCheck -match-full-lines -check-prefix ARM-BE %s
 //
 // ARM-BE-NOT:#define _LP64