Re: RFC: Should x86-64 support arbitrary calling conventions?

[Mike Frysinger <vapier at gentoo dot org>]

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

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