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

test/Bitcode/attributes.ll

@@ -213,6 +213,11 @@ define void @f35() optnone noinline
         ret void;
 }
 
+define void @f36(i8* inalloca) {
+; CHECK: define void @f36(i8* inalloca) {
+        ret void
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }

test/CodeGen/CPP/attributes.ll

@@ -0,0 +1,7 @@
+; RUN: llc < %s -march=cpp | FileCheck %s
+
+define void @f1(i8* byval, i8* inalloca) {
+; CHECK: ByVal
+; CHECK: InAlloca
+  ret void
+}

test/Verifier/inalloca1.ll

@@ -0,0 +1,19 @@
+; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+
+declare void @a(i64* byval inalloca %p)
+; CHECK: Attributes {{.*}} are incompatible
+
+declare void @b(i64* inreg inalloca %p)
+; CHECK: Attributes {{.*}} are incompatible
+
+declare void @c(i64* sret inalloca %p)
+; CHECK: Attributes {{.*}} are incompatible
+
+declare void @d(i64* nest inalloca %p)
+; CHECK: Attributes {{.*}} are incompatible
+
+declare void @e(i64* readonly inalloca %p)
+; CHECK: Attributes {{.*}} are incompatible
+
+declare void @f(void ()* inalloca %p)
+; CHECK: do not support unsized types

test/Verifier/inalloca2.ll

@@ -0,0 +1,21 @@
+; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+
+declare void @doit(i64* inalloca %a)
+
+define void @a() {
+entry:
+  %a = alloca [2 x i32]
+  %b = bitcast [2 x i32]* %a to i64*
+  call void @doit(i64* inalloca %b)
+; CHECK: Inalloca argument is not an alloca!
+  ret void
+}
+
+define void @b() {
+entry:
+  %a = alloca i64
+  call void @doit(i64* inalloca %a)
+  call void @doit(i64* inalloca %a)
+; CHECK: Allocas can be used at most once with inalloca!
+  ret void
+}