Begin adding docs and IR-level support for the inalloca attribute

The inalloca attribute is designed to support passing C++ objects by
value in the Microsoft C++ ABI.  It behaves the same as byval, except
that it always implies that the argument is in memory and that the bytes
are never copied.  This attribute allows the caller to take the address
of an outgoing argument's memory and execute arbitrary code to store
into it.

This patch adds basic IR support, docs, and verification.  It does not
attempt to implement any lowering or fix any possibly broken transforms.

When this patch lands, a complete description of this feature should
appear at http://llvm.org/docs/InAlloca.html .

Differential Revision: http://llvm-reviews.chandlerc.com/D2173

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197645 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/LLVMBitCodes.h

@@ -370,7 +370,8 @@ namespace bitc {
     ATTR_KIND_Z_EXT = 34,
     ATTR_KIND_BUILTIN = 35,
     ATTR_KIND_COLD = 36,
-    ATTR_KIND_OPTIMIZE_NONE = 37
+    ATTR_KIND_OPTIMIZE_NONE = 37,
+    ATTR_KIND_IN_ALLOCA = 38
   };
 
 } // End bitc namespace

include/llvm/IR/Argument.h

@@ -59,7 +59,7 @@ public:
   /// containing function.
   bool hasByValAttr() const;
 
-  /// \brief If this is a byval argument, return its alignment.
+  /// \brief If this is a byval or inalloca argument, return its alignment.
   unsigned getParamAlignment() const;
 
   /// \brief Return true if this argument has the nest attribute on it in its
@@ -86,6 +86,9 @@ public:
   /// on it in its containing function.
   bool onlyReadsMemory() const;
 
+  /// \brief Return true if this argument has the inalloca attribute on it in
+  /// its containing function.
+  bool hasInAllocaAttr() const;
 
   /// \brief Add a Attribute to an argument.
   void addAttr(AttributeSet AS);

include/llvm/IR/Attributes.h

@@ -71,6 +71,7 @@ public:
     Builtin,               ///< Callee is recognized as a builtin, despite
                            ///< nobuiltin attribute on its declaration.
     ByVal,                 ///< Pass structure by value
+    InAlloca,              ///< Pass structure in an alloca
     Cold,                  ///< Marks function as being in a cold path.
     InlineHint,            ///< Source said inlining was desirable
     InReg,                 ///< Force argument to be passed in register

include/llvm/Support/CallSite.h

@@ -257,6 +257,22 @@ public:
     return paramHasAttr(ArgNo + 1, Attribute::ByVal);
   }
 
+  /// @brief Determine whether this argument is passed in an alloca.
+  bool isInAllocaArgument(unsigned ArgNo) const {
+    return paramHasAttr(ArgNo + 1, Attribute::InAlloca);
+  }
+
+  /// @brief Determine whether this argument is passed by value or in an alloca.
+  bool isByValOrInAllocaArgument(unsigned ArgNo) const {
+    return paramHasAttr(ArgNo + 1, Attribute::ByVal) ||
+           paramHasAttr(ArgNo + 1, Attribute::InAlloca);
+  }
+
+  /// @brief Determine if there are any inalloca arguments.
+  bool hasInAllocaArgument() const {
+    return getAttributes().hasAttrSomewhere(Attribute::InAlloca);
+  }
+
   bool doesNotAccessMemory(unsigned ArgNo) const {
     return paramHasAttr(ArgNo + 1, Attribute::ReadNone);
   }