Workaround for mingw i686 compiler optimization bug. (#19)

32-bit llc.exe build using i686-w64-mingw32-gcc/i686-w64-mingw32-g++
at least on version 5.3.1 and 5.4.0 triggers an optimization bug crashing
the process. This was observed by Android team when running cross compiler
targeting armv7-linux-gnueabi using LLVM 6.0. The MatchPhiSet call from
findCommon triggered an optimization resulting in incorrect stack pointer
adjustment on return. That in turn caused an AV when reading the incorrect
pointer from stack representing NewPhiNodes stack variable.

The fix will reduce optimization around this function not triggering the
optimization rules hitting the bug.

Author: lateralusX

lib/CodeGen/CodeGenPrepare.cpp

@@ -2808,6 +2808,17 @@ class AddressingModeCombiner {
   // Request is
   //   <p, BB3> -> ?
   // The function tries to find or build phi [b1, BB1], [b2, BB2] in BB3
+  #if defined(__MINGW32__)
+  // This function triggers an optimization error in i686-w64-mingw32-gcc/i686-w64-mingw32-g++
+  // at least observed in 5.3.1 and 5.4.0. Since these mingw releases are the ones currently
+  // fetched by package managers building LLVM cross compilers targeting Windows, this make sure
+  // this method only gets compiled with an optimization level not triggering this bug. When the
+  // optimization error occurs it results in an incorrect stack pointer restore after the call to
+  // MatchPhiSet and since this method uses FPO next instruction loading NewPhiNodes relative stack pointer
+  // will get incorrect value triggering an access violation.
+  #pragma GCC push_options
+  #pragma GCC optimize ("O1")
+  #endif
   Value *findCommon(FoldAddrToValueMapping &Map) {
     // Tracks of new created Phi nodes.
     SmallPtrSet<PHINode *, 32> NewPhiNodes;
@@ -2849,6 +2860,9 @@ class AddressingModeCombiner {
     }
     return Result;
   }
+  #if defined(__MINGW32__)
+  #pragma GCC pop_options
+  #endif
 
   /// \brief Destroy nodes from a set.
   template <typename T> void DestroyNodes(SmallPtrSetImpl<T *> &Instructions) {