Re: naked switches

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: naked switches
Date	2021-07-08 11:40 -0700
Organization	None to speak of
Message-ID	<87y2ag7ip9.fsf@nosuchdomain.example.com> (permalink)
References	<06a0049a-2d7d-40f0-899a-fb35c9a15d5fn@googlegroups.com> <8735sp8cbm.fsf@nosuchdomain.example.com> <255c09e7-2365-4983-ad4d-3bfc83cf04e7n@googlegroups.com> <31342574-cb69-4ad6-8576-387dcf9caf70n@googlegroups.com>

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Robert Finch <robfi680@gmail.com> writes:
> On Thursday, July 8, 2021 at 6:25:26 AM UTC-4, Malcolm McLean wrote:
>> On Thursday, 8 July 2021 at 09:00:40 UTC+1, Keith Thompson wrote: 
>> > Robert Finch <robf...@gmail.com> writes: 
>> > > 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? 
>> > What range checking code are you referring to? Can you give an example 
>> > in C that demonstrates the change in behavior? How does a default: 
>> >
>> Normal switch 
>> 
>> foo(int x) 
>> { 
>> switch(x) 
>> { 
>> case 1: printf("one\n"); break; 
>> case 2: printf("two\n"); break; 
>> case 3:: printf("three\n"); break; 
>> case 4: printf("four\n"): break; 
>> default: printf("x is in error\n"); break; 
>> } 
>> } 
>> 
>> That will compile to something like the following 
>> if(x < 1 || x > 4) x -= 5; 
>> x -= 1; 
>> goto jumptable[x] 

That's a normal switch, not a naked one, and the if/goto code is not
equivalent to the switch.  If x is 42, the switch statement will print
"x is in error"; the if/goto code (assuming "goto jumptable[x]" has the
obvious meaning) has undefined behavior.  Or did you intend this to
be a naked switch?

>> Namke switch 
>> 
>> foo(int x) 
>> { 
>> nakedswitch(x) 
>> { 
>> case 1: printf("one\n"); break; 
>> case 2: printf("two\n"); break; 
>> case 3:: printf("three\n"); break; 
>> case 4: printf("four\n"): break; 
>> } 
>> } 
>> 
>> would compile to the following 
>> /* jumptable[0] = NULL; */ 
>> goto jumptable[x]; 
>> 
>> so x is unchecked. If it is out of range, the program crashes.
>
> That is basically how it is working. There is still a default
> statement for unimplemented values between the min and max. The table
> entry may as well point somewhere useful.  There were two goals with
> this, a) a performance optimization and b) code size optimization. I
> am dealing with small roms in an FPGA so bytes count. The processor is
> also rather slow <40MHz.

So here's my understanding of how nakedswitch might be defined:

If there is no default: label, then it acts like a normal switch *except
that* if the value of x is less than the smallest case value or greater
than the largest case value, the behavior is undefined.  Is that the intent?

If there is a default: value, then ... what?

And both examples use contiguous ranges of values.  How would this
behave?

    int x = 15;
    nakedswitch(x) {
        case 10: puts("ten"); break;
        case 20: puts("twenty"); break;
    }

There are no "range checks" in the defined behavior of a C switch
statement, and no requirement that they be implemented using a jump
table.

(What I'm looking for is a definition of the *behavior* of a
nakedswitch, including the exact situations where the behavior is
undefined.)

Many years ago, I used a Pascal compiler that always compiled case
statements to a jump table.  When I wrote a case statement with cases 1,
10, 100, 1000, 10000, it crashed the compiler.  gcc, for example, will
generate the equivalent of either a jump table or an if/else chain
depending on the values.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */

Thread

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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