[LLVMdev] builtin_apply?

[Patrik Hägglund H]

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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