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, 10 deletions

diff --git a/src/vm/comdelegate.h b/src/vm/comdelegate.h
index cfb9afa783..ab8ca04338 100644
--- a/src/vm/comdelegate.h
+++ b/src/vm/comdelegate.h
@@ -211,10 +211,14 @@ void DistributeUnhandledExceptionReliably(OBJECTREF *pDelegate,
 //     signature.
 struct ShuffleEntry
 {
+    // Offset masks and special value
     enum {
-        REGMASK  = 0x8000,
-        OFSMASK  = 0x7fff,
-        SENTINEL = 0xffff,
+        REGMASK      = 0x8000, // Register offset bit
+        FPREGMASK    = 0x4000, // Floating point register bit
+        FPSINGLEMASK = 0x2000, // Single precising floating point register
+        OFSMASK      = 0x7fff, // Mask to get stack offset
+        OFSREGMASK   = 0x1fff, // Mask to get register index
+        SENTINEL     = 0xffff, // Indicates end of shuffle array
     };
 
 #if defined(_TARGET_AMD64_) && !defined(UNIX_AMD64_ABI)
@@ -224,17 +228,11 @@ struct ShuffleEntry
     };
 #else
 
-    // Special values:
-    //  -1       - indicates end of shuffle array: stacksizedelta
-    //             == difference in stack size between virtual and static sigs.
-    //  high bit - indicates a register argument: mask it off and
-    //             the result is an offset into ArgumentRegisters.
-
     UINT16    srcofs;
 
     union {
         UINT16    dstofs;           //if srcofs != SENTINEL
-        UINT16    stacksizedelta;   //if dstofs == SENTINEL
+        UINT16    stacksizedelta;   //if dstofs == SENTINEL, difference in stack size between virtual and static sigs
     };
 #endif // _TARGET_AMD64_
 };