does it matter how I construct an aggregate struct type if its size is the same?

[Andrew Haley]

On 18/12/15 22:15, Hayden Livingston wrote:
> Sorry, I should have been clearer -- I'm absolutely talking about
> passing by value. But isn't it on a per argument basis and on its
> size? If you have a data structure that is 9 bytes, after alignment
> let's say it's 12, isn't that all that matters?

No.

> I mean I'm new to this but it seems that if you have a function
> compiled by compiler 1, and let's say it is "exported", i.e. some body
> else can call into this code via dlopen/loadlibrary you can't
> arbitrarily decide how things should be passed right? It has to be on
> an ABI-level, and the ABI I'm guessing says parameter 1 if it's size <
> X can be passed on the stack, or use some registers, but does an ABI
> also specify that struct member 1 if is less than X size can be passed
> in register?

Yes, it often does exactly that.  So if you have an arg which is 2
ints then the 2 ints get passed in 2 registers.  Float members get
passed in float registers, etc.  These days that's the way things are
usually done, with some exceptions such as legacy 32-bit x86.

The ABI really does need to know what is inside the struct.

> This would mean you have to use only 1 compiler.

Huh?  Its the ABI.

> While typing this email, a thought occurred to me that probably we
> programmers know this, and therefore never make our public
> dlopen/loadlibrary APIs take structs that are not completely opaque?
> 
> Sorry I'm formulating some of my thoughts .. but I'm new to this.

Fair enough.

Andrew.