[LLVMdev] Proposing a new 'alloca' parameter attribute to implement the Microsoft C++ ABI

[Reid Kleckner]

Hi LLVM folks,

To properly implement pass-by-value in the Microsoft C++ ABI, we need to be
able
to take the address of an outgoing call argument slot.  This is
http://llvm.org/PR5064 .

Problem
-------

On Windows, C structs are pushed right onto the stack in line with the other
arguments.  In LLVM, we use byval to model this, and it works for C structs.
However, C++ records are also passed this way, and reusing byval for C++
records
breaks C++ object identity rules.

In order to implement the ABI properly, we need a way to get the address of
the
argument slot *before* we start the call, so that we can either construct
the
object in place on the stack or at least call its copy constructor.

This is further complicated by the possibility of nested calls passing
arguments by
value.  A good general case to think about is a binary tree of calls that
take
two arguments by value and return by value:

  struct A { int a; };
  A foo(A, A);
  foo(foo(A(), A()), foo(A(), A()));

To complete the outer call to foo, we have to adjust the stack for its
outgoing
arguments before the inner calls to foo, and arrange for the sret pointers
to
point to those slots.

To make this even more complicated, C++ methods are typically callee
cleanup (thiscall), but free functions are caller cleanup (cdecl).

Features
--------

A few weeks ago, I sat down with some folks at Google and we came up with
this
proposal, which tries to add the minimum set of LLVM IL features to make
this
possible.

1. Allow alloca instructions to use llvm.stacksave values to indicate
scoping.

This creates an SSA dependence between the alloca instruction and the
stackrestore instruction that prevents optimizers from accidentally
reordering
them in ways that don't verify.  llvm.stacksave in this case is taking on a
role
similar to CALLSEQ_START in the selection dag.

LLVM can also apply this to dynamic allocas from inline functions to ensure
that
optimizers don't move them.

2. Add an 'alloca' attribute for parameters.

Only an alloca value can be passed to a parameter with this attribute.  It
cannot be bitcasted or GEPed.  An alloca can only be passed in this way
once.
It can be passed as a normal pointer to any number of other functions.

Aside from allocas bounded by llvm.stacksave and llvm.stackrestore calls,
there
can be no allocas between the creation of an alloca passed with this
attribute
and its associated call.

3. Add a stackrestore field to call and invoke instructions.

This models calling conventions which do their own cleanup, and ensures that
even after optimizations have perturbed the IR, we don't consider the
allocas to
be live.  For caller cleanup conventions, while the callee may have called
destructors on its arguments, the allocas can be considered live until the
stack
restore.

Example
-------

A single call to foo, assuming it is stdcall, would be lowered something
like:

%res = alloca %struct.A
%base = llvm.stacksave()
%arg1 = alloca %struct.A, stackbase %base
%arg2 = alloca %struct.A, stackbase %base
call @A_ctor(%arg1)
call @A_ctor(%arg2)
call x86_stdcallcc @foo(%res sret, %arg1 alloca, %arg2 alloca),
stackrestore %base

If control does not flow through a call or invoke with a stackrestore field,
then manual calls to llvm.stackrestore must be emitted before another call
or
invoke can use an 'alloca' argument.  The manual stack restore call ends the
lifetime of the allocas.  This is necessary to handle unwind edges from
argument
expression evaluation as well as the case where foo is not callee cleanup.

Implementation
--------------

By starting out with the stack save and restore intrinsics, we can hopefully
approach a slow but working implementation sooner rather than later.  The
work
should mostly be in the verifier, the IR, its parser, and the x86 backend.

I don't plan to start working on this immediately, but over the long run
this will be really important to support well.

---

That's all!  Please send feedback!  This is admittedly a really complicated
feature and I'm sorry for inflicting it on the LLVM community, but it's
obviously beyond my control.
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