[LLVMdev] builtin_apply?

[Patrik Hägglund H]

Indeed. However, I wouldn't have called variable number of arguments a "sophisticated feature".  :-)

/Patrik Hägglund

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Krzysztof Parzyszek
Sent: den 2 oktober 2013 19:12
To: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] builtin_apply?

I think varargs functions would be an example of where this builtin 
would not work.

-K


On 10/2/2013 2:32 AM, Patrik Hägglund H wrote:
> Interesting. Just having __builtin_apply_args() (and a correspoding
> __builtin_apply_args_size()), would make it possible to do a memcpy
> store of arguments to a (varargs) function, which sometimes is useful
> for our users (for debug/trace purposes).
>
> /Patrik Hägglund
>
> *From:*llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> *On Behalf Of *Eric Christopher
> *Sent:* den 2 oktober 2013 07:22
> *To:* reed kotler
> *Cc:* LLVMdev at cs.uiuc.edu
> *Subject:* Re: [LLVMdev] builtin_apply?
>
> __builtin_apply is an abomination. Even in GCC it is held to have been a
> bad idea.
>
> --eric
>
> On Oct 1, 2013 10:03 PM, "reed kotler" <rkotler at mips.com
> <mailto:rkotler at mips.com>> wrote:
>
> why does clang not support builtin_apply?
>
>
>       6.5 Constructing Function Calls
>
> Using the built-in functions described below, you can record the
> arguments a function received, and call another function with the same
> arguments, without knowing the number or types of the arguments.
>
> You can also record the return value of that function call, and later
> return that value, without knowing what data type the function tried to
> return (as long as your caller expects that data type).
>
> However, these built-in functions may interact badly with some
> sophisticated features or other extensions of the language. It is,
> therefore, not recommended to use them outside very simple functions
> acting as mere forwarders for their arguments.
>
> - Built-in Function: void * *__builtin_apply_args* ()
>
> This built-in function returns a pointer to data describing how to
> perform a call with the same arguments as are passed to the current
> function.
>
> The function saves the arg pointer register, structure value address,
> and all registers that might be used to pass arguments to a function
> into a block of memory allocated on the stack. Then it returns the
> address of that block.
>
> - Built-in Function: void * *__builtin_apply* (void (*function)(), void
> *arguments, size_t size)
>
> This built-in function invokes function with a copy of the parameters
> described by arguments and size.
>
> The value of arguments should be the value returned by
> |__builtin_apply_args|. The argument size specifies the size of the
> stack argument data, in bytes.
>
> This function returns a pointer to data describing how to return
> whatever value is returned by function. The data is saved in a block of
> memory allocated on the stack.
>
> It is not always simple to compute the proper value for size. The value
> is used by |__builtin_apply| to compute the amount of data that should
> be pushed on the stack and copied from the incoming argument area.
>
>
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