[cmake] Enable reverse iteration by default through build macro

[mgrang]

Summary

Reverse iteration can be turned on, by default, by setting -DLLVM_REVERSE_ITERATION:BOOL=ON during cmake.
With this enabled, we can uncover lots of cases of non-determinism in codegen by simply running our tests (without any other change).
We can then setup a buildbot which will have this turned on by default. Initially, a lot of unit tests will fail in this configuration.
Once we start fixing non-determinism issues, we can gradually make this a blocker for patches.

[dblaikie]

Seems like a good idea to me.

[probinson]

Does this make the two new tests from D33795 superfluous? i.e. can they be deleted?

[mgrang]

In D33908#773290, @probinson wrote:

Does this make the two new tests from D33795 superfluous? i.e. can they be deleted?

We need to have a builder with LLVM_ENABLE_REVERSE_ITERATION enabled by default. Once that is in place we can delete the two tests.
I was reading through http://llvm.org/docs/HowToAddABuilder.html but I do not have a machine to spare. Is there any way we can setup the builder on existing machines?

--Mandeep

cmake/modules/HandleLLVMOptions.cmake

elseif( uppercase_LLVM_ABI_BREAKING_CHECKS STREQUAL "FORCE_OFF" )
   # We don't need to do anything special to turn off ABI breaking checks.
 elseif( NOT DEFINED LLVM_ABI_BREAKING_CHECKS )
   # Treat LLVM_ABI_BREAKING_CHECKS like "FORCE_OFF" when it has not been
   # defined.
 else()
   message(FATAL_ERROR "Unknown value for LLVM_ABI_BREAKING_CHECKS: \"${LLVM_ABI_BREAKING_CHECKS}\"!")
 endif()
 
+if( LLVM_REVERSE_ITERATION )
+  set( LLVM_ENABLE_REVERSE_ITERATION 1 )
+endif()
+
 if(WIN32)
   set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
   if(CYGWIN)
     set(LLVM_ON_WIN32 0)
     set(LLVM_ON_UNIX 1)
   else(CYGWIN)
     set(LLVM_ON_WIN32 1)
     set(LLVM_ON_UNIX 0)

include/llvm/ADT/SmallPtrSet.h

 #include <initializer_list>
 #include <iterator>
 #include <utility>
 
 namespace llvm {
 
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
 template <class T = void> struct ReverseIterate { static bool value; };
+#if LLVM_ENABLE_REVERSE_ITERATION
+template <class T> bool ReverseIterate<T>::value = true;
+#else
 template <class T> bool ReverseIterate<T>::value = false;
 #endif
+#endif
 
 /// SmallPtrSetImplBase - This is the common code shared among all the
 /// SmallPtrSet<>'s, which is almost everything.  SmallPtrSet has two modes, one
 /// for small and one for large sets.
 ///
 /// Small sets use an array of pointers allocated in the SmallPtrSet object,
 /// which is treated as a simple array of pointers.  When a pointer is added to
 /// the set, the array is scanned to see if the element already exists, if not

include/llvm/Config/abi-breaking.h.cmake

 /* This file controls the C++ ABI break introduced in LLVM public header. */
 
 #ifndef LLVM_ABI_BREAKING_CHECKS_H
 #define LLVM_ABI_BREAKING_CHECKS_H
 
 /* Define to enable checks that alter the LLVM C++ ABI */
 #cmakedefine01 LLVM_ENABLE_ABI_BREAKING_CHECKS
 
+/* Define to enable reverse iteration of unordered llvm containers */
+#cmakedefine01 LLVM_ENABLE_REVERSE_ITERATION
+
 /* Allow selectively disabling link-time mismatch checking so that header-only
    ADT content from LLVM can be used without linking libSupport. */
 #if !LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING
 
 // ABI_BREAKING_CHECKS protection: provides link-time failure when clients build
 // mismatch with LLVM
 #if defined(_MSC_VER)
 // Use pragma with MSVC