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From: Damian McGuckin <damianm@esi.com.au>
To: musl@lists.openwall.com
Subject: Re: [musl] Considering x86-64 fenv.s to C
Date: Fri, 24 Jan 2020 17:08:54 +1100 (AEDT)	[thread overview]
Message-ID: <alpine.LRH.2.02.2001241604050.13947@key0.esi.com.au> (raw)
In-Reply-To: <20200124045554.GR30412@brightrain.aerifal.cx>

On Thu, 23 Jan 2020, Rich Felker wrote:

> Note that it's not necessary to write that logic in asm just because
> it's in the arch-defined part. It could simply be C that fixes up the
> value before/after the asm.

My rule was to have no logic in the embedded assembler. Just get or set 
the register and quit. Your skill levels may let you do more. I tried to 
keep these routine super simple.  Here they are for the PowerPC.

 	static inline double get_fpscr_f(void)
 	{
 		double fpscr;

 		__asm__ __volatile__ ("mffs %0" : "=d"(fpscr));
 		return fpscr;
 	}
 	static inline unsigned int get_csr(void)
 	{
 		return (unsigned int) (union {double f; long i;}) {get_fpscr_f()}.i;
 	}
 	static inline void set_fpscr_f(double fpscr)
 	{
 		__asm__ __volatile__ ("mtfsf 255, %0" : : "d"(fpscr));
 	}
 	static inline void set_csr(unsigned int fpscr)
 	{
 		set_fpscr_f((union {long i; double f;}) {(long) fpscr}.f);
 	}

> But indeed the following may be nicer anyway:
......
>> 	if (excepts & FE_INVALID)
>> 		excepts |= FE_ALL_INVALID;
>> and
>> 	if (excepts & FE_INVALID)
>> 		excepts |= FE_INVALID_SOFTWARE;
>>
>> An optimize should see the OR with zero for most architectures and
>> then ignore that whole 'if' statement because the action was a
>> NO-OP.
>
> Yes, this looks nice (although I would probably include FE_INVALID in 
> FE_ALL_INVALID just out of principle of least surprise; it should 
> generate the same code either way.

Do you mean

  	if (excepts & FE_INVALID)
  		excepts |= FE_ALL_INVALID | FE_INVALID;

If so, will it really be optimized away if FE_ALL_INVALID is zero (0)?

>>>>    case (FE_OVERFLOW | FE_INEXACT):
>
> One more style thing: no need for parens here.

Sorry. Too much cutting and pasting from the original 'if' that I realized 
(at the last minute) should be a switch.

> Indeed, I think if we make that change it would be nice to factor it
> as a separate change later.

Please. Sounds wise.

> I do think I want to make it, though,

I think the idea of a SOFT_FPSR is a really good idea.

> feclearexcept would do:
>
> 	soft_fpsr = (soft_fpsr | get_sr()) & ~except;
> 	clear_sr();
>
> At that point, FE_INVALID is clearable independent of the
> specific-reason flags and doesn't seem to need any special treatment.

??? - I need to wrap my head around that one over the weekend.

> Note that just using clear_sr() rather than set_sr() here is a huge 
> optimization on i386 too -- it avoids the need to read-modify-write the 
> full fenv.

Definitely. The Intel architectures are a world unto themselves.

> You should consider arch/*/bits/fenv.h as essentially immutable; the
> types and macros there are ABI or API conventions. The arch-specific
> definitions for fenv stuff should go in arch/$(ARCH)/fenv_arch.h.

OK.

Regards - Damian

Pacific Engineering Systems International, 277-279 Broadway, Glebe NSW 2037
Ph:+61-2-8571-0847 .. Fx:+61-2-9692-9623 | unsolicited email not wanted here
Views & opinions here are mine and not those of any past or present employer

  reply	other threads:[~2020-01-24  6:09 UTC|newest]

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-16  4:30 Damian McGuckin
2020-01-16 15:11 ` Markus Wichmann
2020-01-16 16:14 ` Rich Felker
2020-01-16 18:56   ` Damian McGuckin
2020-01-16 19:33     ` Rich Felker
2020-01-16 21:31       ` Damian McGuckin
2020-01-17  3:36       ` Damian McGuckin
2020-01-17  3:48         ` Damian McGuckin
2020-01-17  3:53         ` David Edelsohn
2020-01-17 14:13           ` Rich Felker
2020-01-17 14:19             ` David Edelsohn
2020-01-17 14:53         ` Rich Felker
2020-01-18  4:45           ` Damian McGuckin
2020-01-18  5:29             ` Rich Felker
2020-01-19  8:50               ` Damian McGuckin
2020-01-19  9:07           ` Damian McGuckin
2020-01-19 10:42             ` Szabolcs Nagy
2020-01-19 12:25               ` Damian McGuckin
2020-01-20  5:32           ` Damian McGuckin
2020-01-20 17:38             ` Rich Felker
2020-01-20 21:11               ` [musl] Triggering Overflow (or Underflow) without triggering Inexact on i386 Damian McGuckin
2020-01-20 22:32                 ` Szabolcs Nagy
2020-01-21  3:53           ` [musl] Considering x86-64 fenv.s to C Damian McGuckin
2020-01-21  4:22             ` Rich Felker
2020-01-21  4:46               ` Damian McGuckin
2020-01-21  7:26                 ` Damian McGuckin
2020-01-17 16:41         ` Markus Wichmann
2020-01-18  1:15           ` Szabolcs Nagy
2020-01-18  5:03             ` Damian McGuckin
2020-01-18  5:37               ` Rich Felker
2020-01-18  9:40                 ` Szabolcs Nagy
2020-01-24  0:42                   ` Damian McGuckin
2020-01-24  1:11                     ` Rich Felker
2020-01-24  4:13                       ` Damian McGuckin
2020-01-24  4:55                         ` Rich Felker
2020-01-24  6:08                           ` Damian McGuckin [this message]
2020-01-24 13:44                             ` Rich Felker
2020-01-24 14:45                               ` Damian McGuckin
2020-01-24 23:59                               ` Damian McGuckin
2020-01-25  0:11                                 ` Rich Felker
2020-01-26  3:28                                   ` Damian McGuckin
2020-01-26  3:28                                   ` Damian McGuckin
2020-01-26  3:30                                   ` Damian McGuckin
2020-01-26  3:32                                   ` Damian McGuckin
2020-01-26  5:25                                   ` Damian McGuckin
2020-01-27  3:32                                   ` Damian McGuckin
2020-01-27  5:39                                   ` Damian McGuckin
2020-02-02  0:47                                   ` Damian McGuckin
2020-02-03  0:54                                   ` Damian McGuckin
2020-02-03  2:09                                     ` Rich Felker
2020-02-03  2:12                                       ` Damian McGuckin
2020-02-22 20:17                                         ` Rich Felker
2020-02-22 20:53                                           ` Damian McGuckin
2020-02-23  5:41                                           ` Damian McGuckin
2020-02-23  6:25                                             ` Rich Felker
2020-02-23  8:35                                               ` Damian McGuckin
2020-02-07 21:25                                   ` Damian McGuckin
2020-02-07 21:38                                     ` Rich Felker
2020-02-07 23:53                                       ` Damian McGuckin
2020-02-09  5:04                                       ` Damian McGuckin
2020-01-24  7:10                           ` Damian McGuckin
2020-01-18  5:04             ` Markus Wichmann
2020-02-03 14:16 [musl] PPC64(LE) support in musl requires ALTIVEC Romain Naour
2020-02-03 14:50 ` Rich Felker
2020-02-05  1:32   ` [musl] Considering x86-64 fenv.s to C Damian McGuckin

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