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Re: RFC: Enabling fixed point modes on x86
- From: Joseph Myers <joseph at codesourcery dot com>
- To: Martin Ling <martin-gcc at earth dot li>
- Cc: <gcc at gcc dot gnu dot org>
- Date: Mon, 30 Sep 2019 16:31:34 +0000
- Subject: Re: RFC: Enabling fixed point modes on x86
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- References: <firstname.lastname@example.org>
On Mon, 30 Sep 2019, Martin Ling wrote:
> For starters, by doing this I have implicitly invented an ABI for fixed
> point types on x86 (they get passed exactly like correspondingly-sized
> integers, I suppose). This ABI would be unique to gcc.
> I have also implicitly set the numbers of integral and fractional bits
> for each mode to the gcc defaults (which are specified in the docs at
Yes - and we had the discussion at the Cauldron recently about trying to
coordinate ABIs for new features with anyone who might wish to support
those in other implementations.
In the case of x86, there are ABI mailing lists email@example.com
and firstname.lastname@example.org that can be consulted (though they don't
always come to a conclusion when issues are raised, it's probably
necessary to be quite active about pushing things to a conclusion and
getting it into the ABI repositories).
I should also mention the concern I previously expressed that the present
implementation approach for fixed-point support in GCC is questionable and
it might be better to have GIMPLE lowering of fixed-point types to normal
integer ones, with some extra GIMPLE operations but without separate
machine modes <https://gcc.gnu.org/ml/gcc-patches/2011-05/msg00846.html>.
This is not however intended to block addition of fixed-point support to
any architecture based on the existing approach, given that we don't have
an implementation based on such lowering (and the same ABI issues would
still apply this this approach).
Joseph S. Myers