Re: is Vax addressing sane today

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Sujet : Re: is Vax addressing sane today
De : kegs (at) *nospam* provalid.com (Kent Dickey)
Groupes : comp.arch
Date : 07. Oct 2024, 19:55:26
Autres entêtes
Organisation : provalid.com
Message-ID : <ve1aqu$1qjrq$2@dont-email.me>
References : 1 2 3 4
User-Agent : trn 4.0-test76 (Apr 2, 2001)
In article <efXIO.169388$1m96.45507@fx15.iad>,
EricP  <ThatWouldBeTelling@thevillage.com> wrote:
Kent Dickey wrote:
In article <O2DHO.184073$kxD8.113118@fx11.iad>,
EricP  <ThatWouldBeTelling@thevillage.com> wrote:
Kent Dickey wrote:
In article <2024Sep10.094353@mips.complang.tuwien.ac.at>,
Anton Ertl <anton@mips.complang.tuwien.ac.at> wrote:
Brett <ggtgp@yahoo.com> writes:
Speaking of complex things, have you looked at Swift output, as it checks
all operations for overflow?
>
You could add an exception type for that, saving huge numbers of correctly
predicted branch instructions.
>
The future of programming languages is type safe with checks, you need to
get on that bandwagon early.
MIPS got on that bandwagon early.  It has, e.g., add (which traps on
signed overflow) in addition to addu (which performs modulo
arithmetic).  It has been abandoned and replaced by RISC-V several
years ago.
>
Alpha got on that bandwagon early.  It's a descendent of MIPS, but it
renamed add into addv, and addu into add.  It has been canceled around
the year 2000.
[ More details about architectures without trapping overflow instructions ]
>
Trapping on overflow is basically useless other than as a debug aid,
which clearly nobody values.  If you take Rust's approach, and only
detect overflow in debug builds, then you already don't care about
performance.
Those automatic software correctness checks, of which signed integer
overflow detection is one of many, went away because most code was
being written in C/C++ and those two languages don't require them.
>
That just makes it more expensive in code size and performance to effect
such checks. This overhead leads some to conclude it justifies eliminating
the error checks.
>
Eliminating the error event detectors doesn't make errors go away,
just your knowledge of them.
>
I gather portions of 16-bit Windows 3.1 were written in Pascal.
When Microsoft developed 32-bit WinNT, if instead of C it they had
switched their official development language from Pascal to Modula-2
which does require signed and unsigned, checked and modulo arithmetic,
and array bounds checks, the world would have been a much safer place.
>
But they didn't so it isn't.
>
The x86 designers might then have had an incentive to make all the
checks as efficient as possible, and rather than eliminate them,
they might have enhanced and more tightly integrated them.
 
OK, my post was about how having a hardware trap-on-overflow instruction
(or a mode for existing ALU instructions) is useless for anything OTHER
than as a debug aid where you crash the problem on overflow (you can
have a general exception handler to shut down gracefully, but "patching things
up and continuing" doesn't work).  I gave details of reasons folks might
want to try to use trap-on-overflow instructions, and show how the
other cases don't make sense.
>
For me error detection of all kinds is useful. It just happens
to not be conveniently supported in C so no one tries it in C.
>
GCC's -trapv option is not useful for a variety of reasons.
1) its slow, about 50% performance hit
2) its always on for a compilation unit which is not what programmers need
   as it triggers for many false positives so people turn it off.
>
In no way was I ever arguing that checking for overflow was a bad idea,
or a language issue, or anything else.  Just that CPUs should not bother
having trap-on-overflow instructions.
>
I understand, and I disagree with this conclusion.
I think all forms of software error detection are useful and
HW should make them simple and eliminate cost when possible.

I think I am not explaining the issue well.

I'm not arguing what you want to do with overflow.  I'm trying to show that
for all uses of detecting overflow other than crashing with no recovery,
hardware trapping on overflow is a poor approach.

If you enable hardware traps on integer overflow, then to do anything other
than crash the program would require engineering a very complex set of
data structures, roughly approximately the complexity of adding debug
information to the executable, in order to make this work.  As far as I know,
no one in the history of computers has yet undertaken this task.

This is because each instruction which overflows would need special
handling, and the "debug" information would be needed.  It would be a huge
amount of compiler/linker/runtime complexity.

This is different than most "signal" handlers people have written, where
simple inspection of the instruction which failed and the address involved
allows it to be "handled".  But to do anything other than crash, each
instruction which overflows needs special handling unique to that instruction
and dependent on what the compiler was in the middle of doing when the
overflow happened.  This is why trapping just isn't a good idea.

I'm just explaining why trap-on-overflow has gone away, because it's
almost completely useless:  hardware trap on overflow is only good for the
case that you want to crash on integer overflow.  Branch-on-overflow is the
correct approach--the compiler can branch to either a trapping instruction
(if you just want to crash), or for all other cases of detecting overflow,
the compiler branches to "fixup" code.

And crash-on-overflow just isn't a popular use model, as I use the example
of x86 in 32-bit mode having a 1-byte INTO instruction which crashes,
and no compiler seems to use it.  Especially since branch-on-overflow
is almost as good in every way.

Kent

Date Sujet#  Auteur
5 Sep 24 * is Vax adressing sane today336Brett
5 Sep 24 +* Re: is Vax adressing sane today324John Dallman
6 Sep 24 i+- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 i`* Re: is Vax adressing sane today322Anton Ertl
6 Sep 24 i +- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 i +* Re: is Vax adressing sane today5MitchAlsup1
7 Sep 24 i i`* Re: is Vax adressing sane today4Anton Ertl
7 Sep 24 i i `* Re: is Vax adressing sane today3Anton Ertl
7 Sep 24 i i  `* Re: is Vax addressing sane today2John Dallman
7 Sep 24 i i   `- Re: is Vax addressing sane today1Anton Ertl
7 Sep 24 i +* Re: is Vax adressing sane today313John Levine
8 Sep 24 i i`* Re: is Vax adressing sane today312Anton Ertl
8 Sep 24 i i +* Re: is Vax adressing sane today304MitchAlsup1
8 Sep 24 i i i`* Re: is Vax addressing sane today303Lawrence D'Oliveiro
9 Sep 24 i i i +* Re: is Vax addressing sane today231MitchAlsup1
9 Sep 24 i i i i`* Re: is Vax addressing sane today230Brett
9 Sep 24 i i i i +* Re: is Vax addressing sane today7MitchAlsup1
10 Sep 24 i i i i i`* Re: is Vax addressing sane today6Niklas Holsti
11 Sep 24 i i i i i +- Re: is Vax addressing sane today1Lawrence D'Oliveiro
25 Sep 24 i i i i i `* Re: is Vax addressing sane today4Stephen Fuld
25 Sep 24 i i i i i  `* Re: is Vax addressing sane today3Michael S
25 Sep 24 i i i i i   `* Re: is Vax addressing sane today2MitchAlsup1
25 Sep 24 i i i i i    `- Re: is Vax addressing sane today1Niklas Holsti
10 Sep 24 i i i i `* Re: is Vax addressing sane today222Anton Ertl
10 Sep 24 i i i i  +* Re: is Vax addressing sane today4Michael S
10 Sep 24 i i i i  i`* Re: is Vax addressing sane today3Anton Ertl
10 Sep 24 i i i i  i +- Re: is Vax addressing sane today1Niklas Holsti
11 Sep 24 i i i i  i `- Re: is Vax addressing sane today1Michael S
11 Sep 24 i i i i  +* Re: is Vax addressing sane today7Lawrence D'Oliveiro
11 Sep 24 i i i i  i`* Re: is Vax addressing sane today6Michael S
11 Sep 24 i i i i  i `* Re: is Vax addressing sane today5David Brown
11 Sep 24 i i i i  i  +* Re: is Vax addressing sane today2Thomas Koenig
11 Sep 24 i i i i  i  i`- Re: is Vax addressing sane today1David Brown
11 Sep 24 i i i i  i  `* Re: is Vax addressing sane today2David Schultz
13 Sep 24 i i i i  i   `- Re: is Vax addressing sane today1David Brown
11 Sep 24 i i i i  +* Re: is Vax addressing sane today5John Levine
11 Sep 24 i i i i  i`* Re: is Vax addressing sane today4Thomas Koenig
11 Sep 24 i i i i  i +* Re: is Vax addressing sane today2Anton Ertl
11 Sep 24 i i i i  i i`- Re: is Vax addressing sane today1jseigh
11 Sep 24 i i i i  i `- Re: is Vax addressing sane today1John Levine
20 Sep 24 i i i i  `* Re: is Vax addressing sane today205Kent Dickey
21 Sep 24 i i i i   +* Re: is Vax addressing sane today4MitchAlsup1
21 Sep 24 i i i i   i`* Re: is Vax addressing sane today3Lawrence D'Oliveiro
21 Sep 24 i i i i   i `* Re: is Vax addressing sane today2MitchAlsup1
21 Sep 24 i i i i   i  `- Re: is Vax addressing sane today1Lawrence D'Oliveiro
21 Sep 24 i i i i   +* Re: is Vax addressing sane today4Lawrence D'Oliveiro
21 Sep 24 i i i i   i`* Re: is Vax addressing sane today3MitchAlsup1
21 Sep 24 i i i i   i +- Re: is Vax addressing sane today1Niklas Holsti
21 Sep 24 i i i i   i `- Re: is Vax addressing sane today1Lawrence D'Oliveiro
21 Sep 24 i i i i   +* Re: is Vax addressing sane today30MitchAlsup1
22 Sep 24 i i i i   i+* Re: except what, is Vax addressing sane today18John Levine
22 Sep 24 i i i i   ii+- Re: except what, is Vax addressing sane today1Michael S
22 Sep 24 i i i i   ii+* Re: except what, is Vax addressing sane today8Lawrence D'Oliveiro
22 Sep 24 i i i i   iii`* Re: except what, is Vax addressing sane today7Chris M. Thomasson
22 Sep 24 i i i i   iii `* Re: except what, is Vax addressing sane today6MitchAlsup1
22 Sep 24 i i i i   iii  +* Re: except what, is Vax addressing sane today2Lawrence D'Oliveiro
22 Sep 24 i i i i   iii  i`- Re: except what, is Vax addressing sane today1MitchAlsup1
22 Sep 24 i i i i   iii  +- Re: except what, is Vax addressing sane today1Chris M. Thomasson
22 Sep 24 i i i i   iii  `* Re: except what, is Vax addressing sane today2John Dallman
22 Sep 24 i i i i   iii   `- Re: except what, is Vax addressing sane today1MitchAlsup1
22 Sep 24 i i i i   ii+* Re: except what, is Vax addressing sane today5Terje Mathisen
24 Sep 24 i i i i   iii`* Re: except what, is Vax addressing sane today4Lawrence D'Oliveiro
24 Sep 24 i i i i   iii `* Re: except what, is Vax addressing sane today3Chris M. Thomasson
24 Sep 24 i i i i   iii  `* Re: except what, is Vax addressing sane today2MitchAlsup1
16 Oct 24 i i i i   iii   `- Re: except what, is Vax addressing sane today1Chris M. Thomasson
22 Sep 24 i i i i   ii`* Re: except what, is Vax addressing sane today3Lars Poulsen
24 Sep 24 i i i i   ii `* Re: except what, is Vax addressing sane today2Lawrence D'Oliveiro
24 Sep 24 i i i i   ii  `- Re: except what, is Vax addressing sane today1Michael S
22 Sep 24 i i i i   i+* Re: is Vax addressing sane today7Lawrence D'Oliveiro
22 Sep 24 i i i i   ii`* Re: is Vax addressing sane today6MitchAlsup1
22 Sep 24 i i i i   ii +* Re: is Vax addressing sane today3Lawrence D'Oliveiro
22 Sep 24 i i i i   ii i`* Re: is Vax addressing sane today2MitchAlsup1
22 Sep 24 i i i i   ii i `- Re: is Vax addressing sane today1Lawrence D'Oliveiro
22 Sep 24 i i i i   ii `* Re: is Vax addressing sane today2Anton Ertl
24 Sep 24 i i i i   ii  `- Re: is Vax addressing sane today1MitchAlsup1
24 Sep 24 i i i i   i+* Re: is Vax addressing sane today2Stefan Monnier
24 Sep 24 i i i i   ii`- Re: is Vax addressing sane today1MitchAlsup1
25 Sep 24 i i i i   i+- Re: is Vax addressing sane today1MitchAlsup1
12 Oct 24 i i i i   i`- Re: is Vax addressing sane today1Anton Ertl
22 Sep 24 i i i i   +* Re: is Vax addressing sane today4Thomas Koenig
24 Sep 24 i i i i   i`* Re: is Vax addressing sane today3Kent Dickey
24 Sep 24 i i i i   i +- Re: is Vax addressing sane today1Bill Findlay
24 Sep 24 i i i i   i `- Re: is Vax addressing sane today1Thomas Koenig
23 Sep 24 i i i i   +- Re: is Vax addressing sane today1Stefan Monnier
23 Sep 24 i i i i   `* Re: is Vax addressing sane today161Kent Dickey
24 Sep 24 i i i i    +* Re: is Vax addressing sane today11MitchAlsup1
24 Sep 24 i i i i    i+- Re: is Vax addressing sane today1Lawrence D'Oliveiro
24 Sep 24 i i i i    i`* Re: is Vax addressing sane today9Terje Mathisen
24 Sep 24 i i i i    i +* Re: is Vax addressing sane today4Michael S
24 Sep 24 i i i i    i i`* Re: is Vax addressing sane today3Terje Mathisen
24 Sep 24 i i i i    i i `* Re: is Vax addressing sane today2Michael S
24 Sep 24 i i i i    i i  `- Re: is Vax addressing sane today1MitchAlsup1
24 Sep 24 i i i i    i +* Re: is Vax addressing sane today3Stephen Fuld
24 Sep 24 i i i i    i i`* Re: is Vax addressing sane today2MitchAlsup1
25 Sep 24 i i i i    i i `- Re: is Vax addressing sane today1Stephen Fuld
12 Oct 24 i i i i    i `- Re: is Vax addressing sane today1Anton Ertl
24 Sep 24 i i i i    +* Re: is Vax addressing sane today3Terje Mathisen
29 Sep 24 i i i i    i+- Re: is Vax addressing sane today1Michael S
7 Oct 24 i i i i    i`- Re: is Vax addressing sane today1Kent Dickey
25 Sep 24 i i i i    +* Re: is Vax addressing sane today133MitchAlsup1
26 Sep 24 i i i i    i+- Re: is Vax addressing sane today1MitchAlsup1
28 Sep 24 i i i i    i`* Re: is Vax addressing sane today131Lawrence D'Oliveiro
28 Sep 24 i i i i    +* Re: is Vax addressing sane today3George Neuner
7 Oct 24 i i i i    +* Re: is Vax addressing sane today8Kent Dickey
12 Oct 24 i i i i    `* Re: is Vax addressing sane today2Anton Ertl
9 Sep 24 i i i `* Re: is Vax addressing sane today71Anton Ertl
8 Sep 24 i i `* Re: is Vax adressing sane today7Brett
8 Sep 24 i `* Re: is Vax adressing sane today2MitchAlsup1
6 Sep 24 +* Re: is Vax adressing sane today2MitchAlsup1
6 Sep 24 +- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 `* Re: is Vax adressing sane today8Anton Ertl

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