[LLVMdev] Accessing arguments in a caller

[David Blaikie]

Variadic templates themselves don't avoid the copy, but combined with
perfect forwarding you should be doing well (see, for example,
make_shared)
From: Jordy Rose
Sent: 8/21/2011 10:34 AM
To: Carlo Alberto Ferraris
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Accessing arguments in a caller
What I can think of:
1. Anonymous struct, to avoid a copy.
2. Use stdarg.h...dangerous but accomplishes what you're looking for.
3. Split up F: giant switch statements often indicate that your
function is doing several different things. (IMHO...)
4. Don't worry about it, it's probably not a bottleneck. (Or rather,
profile first...)

But no, there's no standard way to do this. Even C++11's variadic
templates don't avoid the copy, and initializer lists are pretty much
equivalent to an anonymous struct / const struct reference.

(Why not? Because calling conventions differ from platform to
platform; often the first few arguments of a function are passed in
registers, not on the stack.)

Jordy


On Aug 21, 2011, at 6:23, Carlo Alberto Ferraris wrote:

> Nella citazione giovedì 18 agosto 2011 09:11:36, Carlo Alberto Ferraris ha scritto:
>> I need some advice on "forwarding" arguments to a callee. Suppose I have a function F that is called at the beginning of all other functions in the module. From F I need to access (read) the arguments passed to its immediate caller. Right now I do something like boxing all arguments in the caller inside a struct and passing a pointer to the struct to F, alongside an identifier telling which caller F is being called from. F has then a giant switch that branches to the appropriate unboxing code. This is obviously suboptimal because I need to allocate the struct on the stack, copy the arguments inside of it and then passing an additional pointer to F.
>> I was wondering if there's a better way to do this, e.g. a way to access the stack frame of the caller (knowing, thanks to the identifier, which caller F was called from) or, more in general, arbitrary values d
> efined
>> in the caller. Any suggestions?
>
> Are there really no better ways of doing this?
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