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Groups > comp.lang.c > #161757
| From | Kaz Kylheku <563-365-8930@kylheku.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: naked switches |
| Date | 2021-07-08 17:33 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <20210708101340.323@kylheku.com> (permalink) |
| References | <06a0049a-2d7d-40f0-899a-fb35c9a15d5fn@googlegroups.com> |
On 2021-07-07, Robert Finch <robfi680@gmail.com> 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?
GNU C has the feature that labels can be treated as data; they
can be stored in variables/objects and used to perform a goto.
Thus GNU C has two computed-goto mechanisms: the regular switch
which branches to a fixed case label via a computed value,
and the literal computed goto, which branchews to a label pointer
retrieved from an object.
GNU C code can construct an array, which is populated with goto labels,
and then use it as a completely usafe, unchecked jump table.
It looks something like:
void *table[] = { &&L1, &&L2, &&L3, ... };
goto *table[x];
L1:
// code
goto somewhere;
L2;
// code
goto somewhere;
This not only ensures that there is no range check, but ensures that the
run-time representation of the control flow is a jump table in the first
place, as a matter of the program's expressed abstractc semantics rather
than optimization.
You could dress this up with some macros:
SWITCH(x, table, ENUM_FOO, ENUM_BAR, ENUM_XYZZY)
CASE(ENUM_FOO) statement;
CASE(ENUM_BAR) statement;
CASE(ENUM_XYZZY) { compound statement }
ENDSWITCH;
How to implement? Use some convoluted C99+ preprocessor tricks, which
I'm 95% certain are possible, to spin the first macro into the output:
void *table[] = { &&L_ENUM_FOO, &&L_ENUM_BAR, &&L_ENUM_XYZZY };
goto *table[x];
The CASE macros are easy:
#define CASE(LAB) if (0) L_ ## LAB:
The if (0) ensures that control falls through cases that are entered
from above without a goto. Thus when each case statement terminates, it
will fall through to the bottom, skipping all subsequent cases.
The ENDSWITCH macro does nothing; but it allows us to retarget
the macros to an ordinary switch statement if we are not on GCC/clang:
#define SWITCH(EXPR, TABLE, LAB ...) switch (x) {
#define CASE(LAB) if (0) case LAB:
#define ENDSWITCH }
Nothing tested in this article. :)
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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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