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Re: RFC: Should x86-64 support arbitrary calling conventions?
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: GNU C Library <libc-alpha at sourceware dot org>, "Kreitzer, David L" <david dot l dot kreitzer at intel dot com>
- Date: Fri, 17 Mar 2017 08:51:17 -0700
- Subject: Re: RFC: Should x86-64 support arbitrary calling conventions?
- Authentication-results: sourceware.org; auth=none
- References: <CAMe9rOoLSU=zfhdS1DqKQVr-5Hr9jqBjoR_CMUyFHQ8JKJfwZw@mail.gmail.com> <20170316232737.GD24205@vapier>
On Thu, Mar 16, 2017 at 4:27 PM, Mike Frysinger <vapier@gentoo.org> wrote:
> On 16 Mar 2017 15:07, H.J. Lu wrote:
>> According to x86-64 psABI, xmm0-xmm7 can be used to pass function
>> parameters. But ICC also uses xmm8-xmm15 to pass function parameters.
>> This doesn't work with newer glibc since _dl_runtime_resolve only preserves
>> the first 8 vector registers:
>>
>> https://sourceware.org/bugzilla/show_bug.cgi?id=21236
>>
>> We can use xsave/xrstor to preserve all vector registers to support arbitrary
>> calling conventions. But xsave/restor is about 8X slower, comparing against
>> saving/restoring the first 8 vector registers.
>
> seems like an obvious "no" then ? :)
>
> where does the line stop with "arbitrary calling conventions" ? do we
> save every single register/state in case someone might try to use it to
> pass values ? or do we only do it for projects that are "big enough" ?
> seems like the whole point of having a psABI is so that everyone agrees
> in one place and we don't have to waste time (buth in development and in
> runtime) supporting non-standard stuff.
> -mike
Arbitrary calling conventions may be too strong. Support all vector and mask
registers is sufficient. Intel C++ __regcall calling convention for
x86-64 uses
the first 16 vector registers:
https://sourceware.org/bugzilla/show_bug.cgi?id=21265
David can provide more details.
--
H.J.