Re: Infinity + Infinity (or NegInfinity - NegInfinity)

Date	2011-10-14 09:22 +0200
From	Terje Mathisen <"terje.mathisen at tmsw.no">
Newsgroups	comp.arch
Subject	Re: Infinity + Infinity (or NegInfinity - NegInfinity)
References	(7 earlier) <915bd9f1-ccd2-4061-a33e-115e1ea2509e@v11g2000vbm.googlegroups.com> <gkfim8-bf22.ln1@ntp6.tmsw.no> <4775c736-22ce-48af-b7e8-5587609f996c@g16g2000yqd.googlegroups.com> <a6Dlq.10949$GR4.8204@newsfe02.iad> <QyIlq.83$Ml3.0@newsfe17.iad>
Message-ID	<7i4lm8-bt52.ln1@ntp6.tmsw.no> (permalink)

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EricP wrote:
> EricP wrote:
>>
>> x86 does have overflow exceptions with the INTO instruction
>> (trap overflow flag set). INTO probably causes a pipeline stall too.
>> It doesn't have a VAX style global trap enable flag so one has
>> to test after every operation, but global flags have their own
>> problems with mixed checked and unchecked arithmetic.
>>
>> I think Alpha did it right with its separate
>> Add and AddTrapOverflow instructions,
>> though they didn't have multiple operand sizes as x86.
>
> One other thing I should mention is that Ada requires,
> and I think you'd want this in any language that includes checks,
> that the destination not be modified if there is an overflow
> to avoid storing invalid values into variables.
>
> In the case of the x86 that precludes the use all the
> various instructions that directly update memory,
> at least for checked operations, because an overflow is checked
> later by INTO and therefore does not abort the instruction.
> This forces you to use a load-reg-reg-store style code.

On the first hand, the x86 instructions that directly modify memory, 
basically load-op-store like SUB [EBX],EAX are rarely used in optimized 
code, since unrolling the operation makes it easier to schedule the 
three independent parts.

On the second hand, at least for the ADD/SUB + INTO style overflow 
detection, it is in fact possible for the trap handler to undo the 
operation.

The exception to this is when the operation is ADC/SBB, because it is 
impossible to determine what the previous content of the carry flag was.

On the gripping hand most compilers don't actually use INTO anyway, 
afaik even among those that do support/require overflow handling like Ada?

Terje

-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

Thread

Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-08 02:54 +0200
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-07 18:40 -0700
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-07 22:39 -0400
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) William Elliot <marsh@rdrop.com> - 2011-10-07 19:56 -0700
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) Jasen Betts <jasen@xnet.co.nz> - 2011-10-08 06:45 +0000
      Re: Infinity + Infinity (or NegInfinity - NegInfinity) gordonb.eug6r@burditt.org (Gordon Burditt) - 2011-10-08 02:37 -0500
        Re: Infinity + Infinity (or NegInfinity - NegInfinity) William Elliot <marsh@rdrop.com> - 2011-10-08 21:40 -0700
          Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-09 12:17 +0300
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) Jasen Betts <jasen@xnet.co.nz> - 2011-10-08 06:33 +0000
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-08 16:41 +0200
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) kenney@cix.compulink.co.uk - 2011-10-09 04:01 -0500
  Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-08 17:43 +0300
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-10-08 10:20 -0700
      Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-08 22:35 +0300
    Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-08 18:55 +0100
      Re: Infinity + Infinity (or NegInfinity - NegInfinity) Kaba <kaba@nowhere.com> - 2011-10-08 23:03 +0300
        Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-08 21:27 +0100
          Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-08 17:28 -0400
            Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-08 18:15 -0400
            Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-08 23:23 +0100
              Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-08 21:29 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-09 10:01 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-12 06:14 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-12 15:25 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-12 12:10 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-12 18:41 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-12 15:32 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-12 20:44 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-12 18:11 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Andrew Reilly <areilly---@bigpond.net.au> - 2011-10-13 03:48 +0000
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-12 23:15 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-13 09:12 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-13 00:49 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-13 10:37 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-13 11:04 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-13 17:15 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-14 09:22 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) EricP <ThatWouldBeTelling@thevillage.com> - 2011-10-14 13:37 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-10-13 11:11 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-14 01:04 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-13 09:07 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-13 07:00 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-14 10:54 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Andrew Reilly <areilly---@bigpond.net.au> - 2011-10-14 22:36 +0000
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-15 09:26 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-15 05:39 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-15 14:56 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-17 08:37 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-17 12:03 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-17 11:13 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-17 22:30 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-10-17 14:47 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-18 11:33 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-18 10:21 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-18 08:16 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-18 14:26 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-18 13:32 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-18 17:47 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-18 17:51 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-19 10:37 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-19 09:36 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-19 11:46 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-19 10:45 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-19 13:11 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-19 12:12 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-19 16:22 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-19 15:16 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Andrew Reilly <areilly---@bigpond.net.au> - 2011-10-15 20:16 +0000
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Robert Myers <rbmyersusa@gmail.com> - 2011-10-15 16:26 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-17 08:14 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) nmm1@cam.ac.uk - 2011-10-17 08:22 +0100
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-17 11:39 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) rpw3@rpw3.org (Rob Warnock) - 2011-10-17 21:00 -0500
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-18 05:44 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-10-17 21:35 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Jasen Betts <jasen@xnet.co.nz> - 2011-10-24 02:40 +0000
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-27 00:32 +0300
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-27 10:41 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-27 13:10 +0300
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-10-27 13:53 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Keith Thompson <kst-u@mib.org> - 2011-10-12 12:34 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-13 06:39 -0700
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) James Kuyper <jameskuyper@verizon.net> - 2011-10-13 10:34 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Al Grant <algrant@myrealbox.com> - 2011-10-14 00:54 -0700
            Re: Infinity + Infinity (or NegInfinity - NegInfinity) Larry Hewitt <larryhwtt@comporium.net> - 2011-10-09 21:35 -0400
              Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-10 04:37 +0200
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) Larry Hewitt <larryhwtt@comporium.net> - 2011-10-10 13:58 -0400
                Re: Infinity + Infinity (or NegInfinity - NegInfinity) "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-10-13 20:17 +0200
        Re: Infinity + Infinity (or NegInfinity - NegInfinity) Arivald <arivald_@AT_interia_DOT_.pl> - 2011-10-13 12:10 +0200

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