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From: Richard Biener <richard.guenther@gmail.com>
To: Marc Glisse <marc.glisse@inria.fr>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: builtin fenv functions
Date: Fri, 26 May 2017 10:58:00 -0000	[thread overview]
Message-ID: <CAFiYyc0OzqzfhW_rbqjJTCEPrMdo8wmGQBE9gaaENvUWKtw=7g@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1705261230480.2384@stedding.saclay.inria.fr>

On Fri, May 26, 2017 at 12:39 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Fri, 26 May 2017, Richard Biener wrote:
>
>> On Fri, May 26, 2017 at 11:52 AM, Marc Glisse <marc.glisse@inria.fr>
>> wrote:
>>>
>>> Hello,
>>>
>>> coming back to https://gcc.gnu.org/ml/gcc-patches/2014-02/msg01378.html ,
>>> Richard wanted a complete set of fenv functions instead of just the 2 I
>>> was
>>> interested in. However, most functions in fenv.h handle a struch fenv_t
>>> or
>>> fexcept_t, which makes them inconvenient to handle as builtins.
>>
>>
>> We do have stdio functions having FILE * arguments.  There's
>> 'fileptr_type_node'
>> (pointer to incomplete struct) to handle that.  I suppose rth was looking
>> for
>> sth similar for fenv_t * and fexcept_t *.  I guess it's hard to implement
>> a
>> dependency scheme other than "memory" without seeing the full set of
>> functions.
>
>
> I'll see how hard it is to imitate FILE*...
>
>>> Similarly, I
>>> don't see div as a builtin in that file, only FILE* has special code, but
>>> that doesn't seem worth the trouble here. So I am only declaring the 5
>>> "simple" functions, with minimal properties: leaf, nothrow, and for
>>> fegetround pure (glibc already declares it that way). We can then discuss
>>> the safety of future optimizations on a case by case basis.
>>
>>
>> +DEF_C99_BUILTIN        (BUILT_IN_FERAISEEXCEPT, "feraiseexcept",
>> BT_FN_INT_INT, ATTR_NOTHROW_LEAF_LIST)
>>
>> I think feraiseexcept shouldn't be nothrow?
>
>
> glibc marks it as nothrow. I can remove the nothrow flag for now, for
> safety. It may trap, but it does not throw a C++ exception AFAIU.

Also with -fnon-call-exceptions?

>> But it may be pure.
>
>
> It writes to the exception register (aka memory for now), so I would hardly
> call it pure.

But it doesn't have to be ordered with control word writes/reads, no?

>> Likewise fetestexcept may be pure?
>
>
> Too unsafe for now, since any FP operation can write to the memory that
> fetestexcept reads.

Ah...  but then FP operations are not ordered with the builtins anyway,
only FP loads/stores would be.

After all having builtins is only the first easiest step of properly modeling
dependences between FP ops and the FP control/exception registers.

Richard.

>
> --
> Marc Glisse

  reply	other threads:[~2017-05-26 10:49 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-26 10:27 Marc Glisse
2017-05-26 10:39 ` Richard Biener
2017-05-26 10:49   ` Marc Glisse
2017-05-26 10:58     ` Richard Biener [this message]
2017-05-28 22:26       ` Marc Glisse
2017-05-29  8:46         ` Richard Biener

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