Groups > comp.lang.java.programmer > #6948

Re: higher precision doubles

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From	Joshua Cranmer <Pidgeot18@verizon.invalid>
Newsgroups	comp.lang.java.programmer
Subject	Re: higher precision doubles
Date	Tue, 09 Aug 2011 22:06:00 -0500
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On 8/9/2011 9:40 PM, Arne Vajhøj wrote:
> On 8/9/2011 11:11 AM, Joshua Cranmer wrote:
>> On 8/8/2011 11:42 PM,
>> supercalifragilisticexpialadiamaticonormalizeringelimatisticantations
>> wrote:
>>> How does that interact with JIT, though? On x86, the simplest way for
>>> JIT to make non-strictfp code use the FPU would be to just load the
>>> initial values into the (80-bit-wide!) registers and perform FADDs,
>>> FMULs, etc. on them. As long as the computation stayed in registers the
>>> higher precision then ought to remain in effect -- for JITted code.
>>> Adding extra code to mask off 16 of the register bits (or the mantissa
>>> subset of the extra bits) after every FP op would slow things down. Is
>>> the JLS interpreted to require the JIT do this (for non-strictfp code)?
>>> And, if not, what does the HotSpot JIT do in actuality?
>>
>> All modern x86 processors sport the SSE-style instructions, which can do
>> 32-bit and 64-bit instructions (also in a SIMD format) without touching
>> the FPU, and I suspect that these are slightly faster than using the x87
>> FPU instructions. I wouldn't be surprised if the JIT emitted SSE in the
>> vast majority of cases, so that JIT'd non-strictfp code would end up
>> returning the same results as JIT'd strictfp code.
>
> Is that possible in 32 bit mode?
>
> Practically all CPU's today are 64 bit capable, but many still run
> 32 bit desktop OS'es.

I don't recall x86's mode-switching semantics off the top of my head, 
but I do believe that it is possible to run 64-bit instructions in 
32-bit mode. The problem is the C ABI, particularly register saves and 
restores, don't handle 64-bit stuff properly when compiled with 32-bit 
targets for compilers.

SSE2 dates back to something like the Pentium II, so it's not a 64-bit 
mode thing. Although I think SSE2 itself may only be limited to single 
precision floats.

Poking around the hotspot source code for Java 7 does indicate that sse2 
support is in the JIT, although I don't know the entire set of 
circumstances that triggers it.

-- 
Beware of bugs in the above code; I have only proved it correct, not 
tried it. -- Donald E. Knuth

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higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-06 00:20 +0200
  Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-06 03:35 -0700
    Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-06 13:03 +0200
      Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-06 12:20 -0700
        Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-06 23:30 +0200
          Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-06 16:12 -0700
            Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 01:35 +0200
              Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-06 19:26 -0700
    Re: higher precision doubles supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-08-09 00:42 -0400
      Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-09 04:07 -0700
        Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-09 14:00 +0200
          Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-09 09:07 -0700
            Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-10 08:50 +0200
        Re: higher precision doubles supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-08-10 20:16 -0400
      Re: higher precision doubles Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-08-09 10:11 -0500
        Re: higher precision doubles Arne Vajhøj <arne@vajhoej.dk> - 2011-08-09 22:40 -0400
          Re: higher precision doubles Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-08-09 22:06 -0500
            Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-10 08:53 -0700
  Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-06 21:24 +0200
    Re: higher precision doubles markspace <-@.> - 2011-08-06 13:29 -0700
      Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-06 23:20 +0200
        Re: higher precision doubles markspace <-@.> - 2011-08-06 15:43 -0700
          Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 01:06 +0200
            Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-06 16:21 -0700
              Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 01:34 +0200
                Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-06 21:32 -0700
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 17:24 +0200
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 17:39 +0200
                Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-07 09:26 -0700
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 21:23 +0200
                Re: higher precision doubles Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-07 20:48 -0400
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-08 09:05 +0200
                Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-08 05:37 -0700
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-08 19:08 +0200
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-08 19:16 +0200
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-08 19:29 +0200
                Re: higher precision doubles Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-08-08 21:34 -0500
                Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-08 13:49 -0700
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-09 01:08 +0200
                Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-09 02:44 -0700
                Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-09 12:52 +0200
                Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-09 13:15 -0700
                Re: higher precision doubles BGB <cr88192@hotmail.com> - 2011-08-07 13:51 -0700
              Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 01:59 +0200
        Re: higher precision doubles Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-08-08 21:03 -0500
    Re: higher precision doubles Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-06 17:33 -0400
    Re: higher precision doubles Patricia Shanahan <pats@acm.org> - 2011-08-06 14:51 -0700
      Re: higher precision doubles Jan Burse <janburse@fastmail.fm> - 2011-08-07 00:57 +0200

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