From: Andrew Haley <aph@redhat.com>
To: libffi-discuss@sourceware.org
Subject: Re: does it matter how I construct an aggregate struct type if its size is the same?
Date: Fri, 18 Dec 2015 22:24:00 -0000 [thread overview]
Message-ID: <567487A1.4040803@redhat.com> (raw)
In-Reply-To: <CAMxMwyKxCONEgw4Lo7w8VPicPnn+57D49YepEJYH-J2PrGD=kA@mail.gmail.com>
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.
next prev parent reply other threads:[~2015-12-18 22:24 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-12-18 6:47 Hayden Livingston
2015-12-18 6:50 ` Hayden Livingston
2015-12-18 11:08 ` Andrew Haley
2015-12-18 15:16 ` Hayden Livingston
2015-12-18 20:04 ` Andrew Haley
2015-12-18 21:42 ` Jay
2015-12-18 22:16 ` Hayden Livingston
2015-12-18 22:24 ` Andrew Haley [this message]
2015-12-18 23:10 ` Hayden Livingston
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