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Groups > comp.lang.c > #161728
| From | Bart <bc@freeuk.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: naked switches |
| Date | 2021-07-08 01:26 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <sc5gnf$v05$1@dont-email.me> (permalink) |
| References | <06a0049a-2d7d-40f0-899a-fb35c9a15d5fn@googlegroups.com> |
On 08/07/2021 00:39, Robert Finch wrote:
> I have been working on a C/C++ like compiler. One feature supported in the compiler is naked switches. A naked switch omits the range checking code that is normally associated with the switch statement. Omitting this code can improve performance at the risk of a crash if invalid cases are processed. I am wondering if there is a similar option in other C compilers? Or would this just be an automatic optimization at high levels?
>
> Normal Switch:
>
> ; switch(x) {
> ldo $t0,64[$fp]
> sge $t1,$t0,#1 ; x varies between 1 and 12
> sle $t2,$t0,#12
> and $t1,$t1,$t2
> beq $t1,TestSwitch_89
> sub $t0,$t0,#1
> sll $t0,$t0,#4
> ldo $t0,TestSwitch_116[$t0]
> jmp $t0
>
> Naked Switch
> ; switch(x; naked) {
> ldo $t0,64[$fp]
> sub $t0,$t0,#1
> sll $t0,$t0,#4
> ldo $t0,TestSwitch_144[$t0]
> jmp $t0
>
How much faster does this make it? Because I think that when I tried it
on x64, it made no measurable difference. (Maybe the branch predictor
dealt with it.)
(On x64 also, if there are fewer than about 8 jumptable entries, it was
faster just to do sequential tests.)
Note that I do a single test, not two, after adjusting the index
expression to be 0-based which is necessary as the jumptable is biased.
So for your 1-12 range, I adjust to 0-11 and do an unsigned comparison
with 11, which is two instructions.
As to what an optimising compiler could do, it could be anything at all,
including optimising the whole switch out of existence.
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naked switches Robert Finch <robfi680@gmail.com> - 2021-07-07 16:39 -0700
Re: naked switches Bart <bc@freeuk.com> - 2021-07-08 01:26 +0100
Re: naked switches Kaz Kylheku <563-365-8930@kylheku.com> - 2021-07-08 01:57 +0000
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-08 09:16 +0200
Re: naked switches Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-08 01:00 -0700
Re: naked switches Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-07-08 03:25 -0700
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-07-08 06:58 -0700
Re: naked switches Bart <bc@freeuk.com> - 2021-07-08 15:52 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-08 17:52 +0200
Re: naked switches Bart <bc@freeuk.com> - 2021-07-08 17:51 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 10:01 +0200
Re: naked switches Bart <bc@freeuk.com> - 2021-07-09 11:09 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 13:14 +0200
Re: naked switches Manfred <noname@add.invalid> - 2021-07-09 15:35 +0200
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 16:51 +0200
Re: naked switches Manfred <noname@add.invalid> - 2021-07-09 18:23 +0200
Re: naked switches Bart <bc@freeuk.com> - 2021-07-09 16:07 +0100
Re: naked switches Manfred <noname@add.invalid> - 2021-07-09 18:41 +0200
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-10 11:02 +0200
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 19:20 +0200
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-07-09 10:46 -0700
Re: naked switches Kaz Kylheku <563-365-8930@kylheku.com> - 2021-07-09 19:56 +0000
Re: naked switches Bart <bc@freeuk.com> - 2021-07-09 22:01 +0100
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-07-09 15:07 -0700
Re: naked switches Bart <bc@freeuk.com> - 2021-07-09 23:30 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-10 12:17 +0200
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-07-08 10:18 -0700
Re: naked switches Bart <bc@freeuk.com> - 2021-07-08 18:43 +0100
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-07-08 15:32 -0700
Re: naked switches Kaz Kylheku <563-365-8930@kylheku.com> - 2021-07-08 17:47 +0000
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-07-08 15:45 -0700
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 10:27 +0200
Re: naked switches Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-09 11:13 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 13:30 +0200
Re: naked switches Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-11 00:39 -0700
Re: naked switches scott@slp53.sl.home (Scott Lurndal) - 2021-07-11 14:06 +0000
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-07-11 08:01 -0700
Re: naked switches Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-16 03:53 -0700
Re: naked switches scott@slp53.sl.home (Scott Lurndal) - 2021-07-16 14:47 +0000
Re: naked switches Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-09-30 06:09 -0700
Re: naked switches Robert Finch <robfi680@gmail.com> - 2021-09-30 07:03 -0700
Re: naked switches Bart <bc@freeuk.com> - 2021-07-09 13:44 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-10 13:28 +0200
Re: naked switches Bart <bc@freeuk.com> - 2021-07-10 13:04 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-10 15:39 +0200
Re: naked switches Bart <bc@freeuk.com> - 2021-07-10 15:08 +0100
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-10 17:34 +0200
Re: naked switches Bart <bc@freeuk.com> - 2021-07-10 17:47 +0100
Re: naked switches Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-11 00:57 -0700
Re: naked switches Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-16 03:51 -0700
Re: naked switches Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-08 11:40 -0700
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 10:38 +0200
Re: naked switches Kaz Kylheku <563-365-8930@kylheku.com> - 2021-07-08 17:33 +0000
Re: naked switches scott@slp53.sl.home (Scott Lurndal) - 2021-07-08 17:47 +0000
Re: naked switches Kaz Kylheku <563-365-8930@kylheku.com> - 2021-07-08 17:58 +0000
Re: naked switches scott@slp53.sl.home (Scott Lurndal) - 2021-07-08 19:13 +0000
Re: naked switches Kaz Kylheku <563-365-8930@kylheku.com> - 2021-07-09 02:30 +0000
Re: naked switches David Brown <david.brown@hesbynett.no> - 2021-07-09 10:41 +0200
Re: naked switches Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-30 18:04 +0200
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