Index: include/clang/StaticAnalyzer/Checkers/Checkers.td
===================================================================
--- include/clang/StaticAnalyzer/Checkers/Checkers.td
+++ include/clang/StaticAnalyzer/Checkers/Checkers.td
@@ -38,6 +38,11 @@
 // default. Such checkers belong in the alpha package.
 def OptIn : Package<"optin">;
 
+// In the Portability package reside checkers for finding code that relies on
+// implementation-defined behavior. Such checks are wanted for cross-platform
+// development, but unwanted for developers who target only a single platform.
+def PortabilityOptIn : Package<"portability">, InPackage<OptIn>;
+
 def Nullability : Package<"nullability">;
 
 def Cplusplus : Package<"cplusplus">;
@@ -57,6 +62,7 @@
 def Taint : Package<"taint">, InPackage<SecurityAlpha>, Hidden;
 
 def Unix : Package<"unix">;
+def UnixPortabilityOptIn : Package<"unix">, InPackage<PortabilityOptIn>;
 def UnixAlpha : Package<"unix">, InPackage<Alpha>, Hidden;
 def CString : Package<"cstring">, InPackage<Unix>, Hidden;
 def CStringAlpha : Package<"cstring">, InPackage<UnixAlpha>, Hidden;
@@ -416,7 +422,7 @@
 
 let ParentPackage = Unix in {
 
-def UnixAPIChecker : Checker<"API">,
+def UnixAPIMisuseChecker : Checker<"API">,
   HelpText<"Check calls to various UNIX/Posix functions">,
   DescFile<"UnixAPIChecker.cpp">;
 
@@ -442,6 +448,14 @@
 
 } // end "unix"
 
+let ParentPackage = UnixPortabilityOptIn in {
+
+def UnixAPIPortabilityChecker : Checker<"API">,
+  HelpText<"Finds implementation-defined behavior in UNIX/Posix functions">,
+  DescFile<"UnixAPIChecker.cpp">;
+
+} // end optin.portability.unix
+
 let ParentPackage = UnixAlpha in {
 
 def ChrootChecker : Checker<"Chroot">,
Index: lib/StaticAnalyzer/Checkers/UnixAPIChecker.cpp
===================================================================
--- lib/StaticAnalyzer/Checkers/UnixAPIChecker.cpp
+++ lib/StaticAnalyzer/Checkers/UnixAPIChecker.cpp
@@ -45,6 +45,8 @@
   mutable Optional<uint64_t> Val_O_CREAT;
 
 public:
+  DefaultBool CheckMisuse, CheckPortability;
+
   void checkPreStmt(const CallExpr *CE, CheckerContext &C) const;
 
   void CheckOpen(CheckerContext &C, const CallExpr *CE) const;
@@ -437,29 +439,42 @@
   if (FName.empty())
     return;
 
-  SubChecker SC =
-    llvm::StringSwitch<SubChecker>(FName)
-      .Case("open", &UnixAPIChecker::CheckOpen)
-      .Case("openat", &UnixAPIChecker::CheckOpenAt)
-      .Case("pthread_once", &UnixAPIChecker::CheckPthreadOnce)
-      .Case("calloc", &UnixAPIChecker::CheckCallocZero)
-      .Case("malloc", &UnixAPIChecker::CheckMallocZero)
-      .Case("realloc", &UnixAPIChecker::CheckReallocZero)
-      .Case("reallocf", &UnixAPIChecker::CheckReallocfZero)
-      .Cases("alloca", "__builtin_alloca", &UnixAPIChecker::CheckAllocaZero)
-      .Case("__builtin_alloca_with_align",
-            &UnixAPIChecker::CheckAllocaWithAlignZero)
-      .Case("valloc", &UnixAPIChecker::CheckVallocZero)
-      .Default(nullptr);
-
-  if (SC)
-    (this->*SC)(C, CE);
+  if (CheckMisuse) {
+    if (SubChecker SC =
+            llvm::StringSwitch<SubChecker>(FName)
+                .Case("open", &UnixAPIChecker::CheckOpen)
+                .Case("openat", &UnixAPIChecker::CheckOpenAt)
+                .Case("pthread_once", &UnixAPIChecker::CheckPthreadOnce)
+                .Default(nullptr)) {
+      (this->*SC)(C, CE);
+    }
+  }
+  if (CheckPortability) {
+    if (SubChecker SC =
+            llvm::StringSwitch<SubChecker>(FName)
+                .Case("calloc", &UnixAPIChecker::CheckCallocZero)
+                .Case("malloc", &UnixAPIChecker::CheckMallocZero)
+                .Case("realloc", &UnixAPIChecker::CheckReallocZero)
+                .Case("reallocf", &UnixAPIChecker::CheckReallocfZero)
+                .Cases("alloca", "__builtin_alloca",
+                       &UnixAPIChecker::CheckAllocaZero)
+                .Case("__builtin_alloca_with_align",
+                      &UnixAPIChecker::CheckAllocaWithAlignZero)
+                .Case("valloc", &UnixAPIChecker::CheckVallocZero)
+                .Default(nullptr)) {
+      (this->*SC)(C, CE);
+    }
+  }
 }
 
 //===----------------------------------------------------------------------===//
 // Registration.
 //===----------------------------------------------------------------------===//
 
-void ento::registerUnixAPIChecker(CheckerManager &mgr) {
-  mgr.registerChecker<UnixAPIChecker>();
-}
+#define REGISTER_CHECKER(Name)                                                 \
+  void ento::registerUnixAPI##Name##Checker(CheckerManager &mgr) {             \
+    mgr.registerChecker<UnixAPIChecker>()->Check##Name = true;                 \
+  }
+
+REGISTER_CHECKER(Misuse)
+REGISTER_CHECKER(Portability)
Index: test/Analysis/malloc-overflow2.c
===================================================================
--- test/Analysis/malloc-overflow2.c
+++ test/Analysis/malloc-overflow2.c
@@ -1,4 +1,5 @@
 // RUN: %clang_analyze_cc1 -triple x86_64-unknown-unknown -analyzer-checker=alpha.security.MallocOverflow,unix -verify %s
+// RUN: %clang_analyze_cc1 -triple x86_64-unknown-unknown -analyzer-checker=alpha.security.MallocOverflow,unix,optin.portability.unix -DPORTABILITY -verify %s
 
 typedef __typeof__(sizeof(int)) size_t;
 extern void *malloc(size_t);
@@ -32,5 +33,8 @@
 }
 
 void *f(int n) {
-  return malloc(n * 0 * sizeof(int)); // expected-warning {{Call to 'malloc' has an allocation size of 0 bytes}}
+  return malloc(n * 0 * sizeof(int));
+#ifdef PORTABILITY
+  // expected-warning@-2{{Call to 'malloc' has an allocation size of 0 bytes}}
+#endif
 }
Index: test/Analysis/unix-fns.c
===================================================================
--- test/Analysis/unix-fns.c
+++ test/Analysis/unix-fns.c
@@ -1,7 +1,7 @@
-// RUN: %clang_analyze_cc1 -triple x86_64-apple-darwin10 -analyzer-checker=core,unix.API,osx.API %s -analyzer-store=region -analyzer-output=plist -analyzer-eagerly-assume -analyzer-config faux-bodies=true -analyzer-config path-diagnostics-alternate=false -fblocks -verify -o %t.plist
+// RUN: %clang_analyze_cc1 -triple x86_64-apple-darwin10 -analyzer-checker=core,unix.API,osx.API,optin.portability.unix.API %s -analyzer-store=region -analyzer-output=plist -analyzer-eagerly-assume -analyzer-config faux-bodies=true -analyzer-config path-diagnostics-alternate=false -fblocks -verify -o %t.plist
 // RUN: FileCheck --input-file=%t.plist %s
 // RUN: mkdir -p %t.dir
-// RUN: %clang_analyze_cc1 -analyzer-checker=core,unix.API,osx.API -analyzer-output=html -analyzer-config faux-bodies=true -fblocks -o %t.dir %s
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,unix.API,osx.API,optin.portability.unix.API -analyzer-output=html -analyzer-config faux-bodies=true -fblocks -o %t.dir %s
 // RUN: rm -fR %t.dir
 struct _opaque_pthread_once_t {
   long __sig;