Re: naked switches

From	Manfred <noname@add.invalid>
Newsgroups	comp.lang.c
Subject	Re: naked switches
Date	2021-07-09 18:23 +0200
Organization	Aioe.org NNTP Server
Message-ID	<sc9t6i$1vbt$1@gioia.aioe.org> (permalink)
References	(5 earlier) <sc7aeg$10u$1@dont-email.me> <sc8von$8fo$1@dont-email.me> <sc979l$n65$1@dont-email.me> <sc9jam$1ahh$1@gioia.aioe.org> <sc9npu$9t3$1@dont-email.me>

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On 7/9/2021 4:51 PM, David Brown wrote:
> On 09/07/2021 15:35, Manfred wrote:
>> On 7/9/2021 12:09 PM, Bart wrote:
>>> On 09/07/2021 09:01, David Brown wrote:
>>>> On 08/07/2021 18:51, Bart wrote:
>>>>> On 08/07/2021 16:52, David Brown wrote:
>> [...]
>>>>>     It is not something I have seen on other compilers, and I've used
>>>>>> quite a large number over the years for far smaller and slower devices
>>>>>> than you are describing here.
>>>>>>
>>>>>> The way you handle this with gcc has already been covered - you use
>>>>>> __builtin_unreachable() to tell the compiler how to optimise for "this
>>>>>> can't happen" cases.  clang supports __builtin_unreachable() too, and
>>>>>> MSVC has "__assume(false)" that has the same effect.
>>>>>>
>>>>>
>>>>> How would that work in that case? There doesn't appear to be a bit of
>>>>> code to hang that onto, unless you specifically create an empty block
>>>>> for the purpose, guarded by the same sort of range check you want to
>>>>> avoid.
>>>>
>>>> The normal situation would be :
>>>>
>>>>      switch (x) {
>>>>          case 1 : handle1(); break;
>>>>          case 2 : handle2(); break;
>>>>          case 4 : handle4(); break;
>>>>          default : __builtin_unreachable();    // gcc
>>>>          default : __assume(0);            // msvc
>>>>      }
>>>
>>> As I said, you may want to reserve default: for x==3.
>>>
>>>>
>>>>
>>>>
>>>> If you want to say x == 3 is a "do nothing" situation, but you want to
>>>> tell the compiler it doesn't need to check for x < 1 or x > 4, then you
>>>> do so simply and clearly:
>>>>
>>>>      if ((x < 1) || (x > 4)) __builtin_unreachable();    // gcc
>>>>      __assume((x >= 1) && (x <= 4));                // msvc
>>>
>>> This is really ugly and may involve some compilers (eg. mine) adding
>>> all those extra checks.
>>>
>>
>> __builtin_unreachable() is a GCC extension (and similarly __assume() for
>> msvc) that is supposed to have no meaning other than for GCC. And that
>> meaning is an instruction for the compiler to consider that range for
>> 'x' impossible to happen, and optimize accordingly - not to add any
>> extra check.
>> The fact that your compiler may interpret this instruction differently
>> is irrelevant because, well, this is a GCC-specific extension - in fact
>> your compiler should just /refuse/ to compile the code because
>> __builtin_unreachable() should be undefined in your implementation.
>>
> 
> Presumably a compiler that does not implement __builtin_unreachable() as
> an extension will see it as a call to a function.  If the compiler is
> C90, rather than C99, then it will have to accept the code even though
> the function is not declared, and it will treat it as a normal function
> - thus it will include the range check on x, and a call to an external
> function "__builtin_unreachable()".  This is, of course, not remotely
> close to what you want to happen.

The compiler may generate the call, but the linker is not going to 
assemble an executable if the function does not exist. This still holds 
in C90.

> 
> If you are using features like this and want your code to be portable,
> you want something like :
> 
> #ifdef __GNUC__
> 	#define Unreachable() __builtin_unreachable()
> #elif defined(_MSC_VER)
> 	#define Unreachable() __assume(0)
> #else
> 	#define Unreachable()
> #endif
> 
> Then use Unreachable() macro in your code.
> 
> (And if the compiler still generates the extra range checks, then stop
> worrying about the insignificant inefficiencies in the switch statement,
> as you are not using an optimising compiler.)
> 
> 
>> Ugliness is an intentional feature of implementation-specific extensions.
>>
>

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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