Implementation of System V ABI struct passing.

This PR adds support for System V x86_64 ABI classification and calling
convention to the VM and the Jit, including, but not limited to Ubuntu
Linux and Mac OS X.
The general rules outlined in the System V x86_64 ABI (described at
http://www.x86-64.org/documentation/abi.pdf) are followed with a few
little exceptions, described below:
1.	The hidden argument for by-value passed structs is always after
the ÎéÎíthisÎéÎí parameter (if there is one.). This is a difference with
the Sysetem V ABI and affects only the internal jit calling conventions.
For PInvoke calls the hidden argument is always the first parameter since
there is no ÎéÎíthisÎéÎí parameter in this case.
2.	Managed structs that have no fields are always passed by-value on
the stack.
3.	The jit proactively generates frame register frames (with RBP as a
frame register) in order to aid the native OS tooling for stack unwinding
and the like.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/vm/callhelpers.cpp b/src/vm/callhelpers.cpp
index a910c0ea30..137dbb8656 100644
--- a/src/vm/callhelpers.cpp
+++ b/src/vm/callhelpers.cpp
@@ -401,7 +401,7 @@ ARG_SLOT MethodDescCallSite::CallTargetWorker(const ARG_SLOT *pArguments)
         // Record this call if required
         g_IBCLogger.LogMethodDescAccess(m_pMD);
 
-        // 
+        //  
         // All types must already be loaded. This macro also sets up a FAULT_FORBID region which is
         // also required for critical calls since we cannot inject any failure points between the 
         // caller of MethodDesc::CallDescr and the actual transition to managed code.
@@ -537,9 +537,12 @@ ARG_SLOT MethodDescCallSite::CallTargetWorker(const ARG_SLOT *pArguments)
             // have at least one such argument we point the call worker at the floating point area of the
             // frame (we leave it null otherwise since the worker can perform a useful optimization if it
             // knows no floating point registers need to be set up).
-            if ((ofs < 0) && (pFloatArgumentRegisters == NULL))
+            if (TransitionBlock::HasFloatRegister(ofs, m_argIt.GetArgLocDescForStructInRegs()) && 
+                (pFloatArgumentRegisters == NULL))
+            {
                 pFloatArgumentRegisters = (FloatArgumentRegisters*)(pTransitionBlock +
                                                                     TransitionBlock::GetOffsetOfFloatArgumentRegisters());
+            }
 #endif
 
 #if CHECK_APP_DOMAIN_LEAKS
@@ -553,6 +556,9 @@ ARG_SLOT MethodDescCallSite::CallTargetWorker(const ARG_SLOT *pArguments)
             }
 #endif // CHECK_APP_DOMAIN_LEAKS
 
+#if defined(UNIX_AMD64_ABI) && defined(FEATURE_UNIX_AMD64_STRUCT_PASSING)
+            _ASSERTE(ofs != TransitionBlock::StructInRegsOffset);
+#endif
             PVOID pDest = pTransitionBlock + ofs;
 
             UINT32 stackSize = m_argIt.GetArgSize();