llvm dev - Jul 2013 - [LLVMdev] Proposing a new 'alloca' parameter attribute to implement the Microsoft C++ ABI

>> object in place on the stack or at least call its copy constructor.
>
>
> what does GCC do?
Nothing. It does not support MSVC ABI.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

On 29/07/13 15:30, Anton Korobeynikov wrote:
>>> object in place on the stack or at least call its copy constructor.
>>
>>
>> what does GCC do?
> Nothing. It does not support MSVC ABI.
Maybe we shouldn't either :)  So the ABI requires the struct to be pushed on
the
stack by the caller along with the other parameters, and what's more it
requires
the caller to execute some code on that copy, in place on the stack, before
performing the call.  Is that right?

Ciao, Duncan.

On Mon, Jul 29, 2013 at 9:40 AM, Duncan Sands <baldrick at free.fr> wrote:
> On 29/07/13 15:30, Anton Korobeynikov wrote:
>
>> object in place on the stack or at least call its copy constructor.
>>>>
>>>
>>>
>>> what does GCC do?
>>>
>> Nothing. It does not support MSVC ABI.
>>
>
> Maybe we shouldn't either :)  So the ABI requires the struct to be
pushed
> on the
> stack by the caller along with the other parameters, and what's more it
> requires
> the caller to execute some code on that copy, in place on the stack, before
> performing the call.  Is that right?
>
Just calling the copy ctor is the bare minimum needed to support the ABI.
 cl.exe uses a more efficient lowering that evaluates the arguments right
to left directly into the outgoing argument stack slots.  It lowers
following call to bar to something like:

struct A {int x; int y;};
A foo();
void bar(int, A, int);
...
bar(0xdead, foo(), 0xbeef);

x86_32 pseudo intel asm:

push 0xdead
sub esp, 8
push esp  # sret arg for foo
call foo
add esp, 4 # clear sret arg
push 0xbeef
call bar
add 16 # clear args

With the current proposal, LLVM would generate code that adjusts the stack
once to begin the call, and once more to end it.  Initially, it would do a
blind SP restore, but that can be optimized to a constant SP adjustment.
 Arguments will still be evaluated left to right and will be stored into
the special alloca outgoing argument slots.
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Duncan Sands <baldrick at free.fr> writes:
> On 29/07/13 15:30, Anton Korobeynikov wrote:
>>>> object in place on the stack or at least call its copy
constructor.
>>>
>>>
>>> what does GCC do?
>> Nothing. It does not support MSVC ABI.
>
> Maybe we shouldn't either :)
Right. What's the point of all the effort devoted to MSVC++ ABI
compatibility when Clang doesn't need it for being a top-notch C++
compiler on Windows?