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Re: [ARM] Enable __fp16 as a function parameter and return type.

From: Tejas Belagod <tejas dot belagod at foss dot arm dot com>
To: Joseph Myers <joseph at codesourcery dot com>
Cc: Matthew Wahab <matthew dot wahab at foss dot arm dot com>, gcc-patches <gcc-patches at gcc dot gnu dot org>, Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>
Date: Mon, 16 May 2016 14:16:23 +0100
Subject: Re: [ARM] Enable __fp16 as a function parameter and return type.
Authentication-results: sourceware.org; auth=none
References: <5721D5C1 dot 3080000 at foss dot arm dot com> <alpine dot DEB dot 2 dot 20 dot 1604281528210 dot 13527 at digraph dot polyomino dot org dot uk> <5733499E dot 7050001 at foss dot arm dot com> <alpine dot DEB dot 2 dot 20 dot 1605111536270 dot 27272 at digraph dot polyomino dot org dot uk>

On 11/05/16 16:46, Joseph Myers wrote:

On Wed, 11 May 2016, Tejas Belagod wrote:

AFAICS, I don't think it mandates a double-rounding behavior for double to
__fp16 conversions and I don't see a change in stand between the two versions
of ACLE on the behavior of __fp16.


It's not a change between the two versions of ACLE.  It's a change
relative to the early (pre-ACLE) __fp16 specification (or, at least, a
clarification thereto in email on 12 Aug 2008) that was used as a basis
for the original implementation of __fp16 in GCC (and that thus is what's
currently implemented by GCC and tested for in the testsuite).


Hi Joseph,

Sorry for the delay in responding.

I've had a conversation with Al and I now have some context. You're right - the2008 mail you are referring to is the pre-ACLE behavior. By the time the firstdraft of the first version of ACLE was reviewed by CodeSourcery(circa 2011), italready mandated single rounding. No published ACLE has ever allowed doublerounding.

This meant that when the first draft of ACLE was published in 2011, its pre-ACLEimplementations in gcc and armcc were already non-conformant, in other words,'bug-compatible'.

We do have plans to fix pre-ACLE behavior of fp16 to conform to current ACLEspec, but can't say when exactly.


Thanks,
Tejas.

Follow-Ups:
- Re: [ARM] Enable __fp16 as a function parameter and return type.
  - From: Ramana Radhakrishnan

References:
- Re: Re: [ARM] Enable __fp16 as a function parameter and return type.
  - From: Tejas Belagod
- Re: Re: [ARM] Enable __fp16 as a function parameter and return type.
  - From: Joseph Myers

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