Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
Groups > comp.lang.c > #161733
| From | Kaz Kylheku <563-365-8930@kylheku.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: naked switches |
| Date | 2021-07-08 01:57 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <20210707175722.828@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?
I've not heard of such an option. Some modern compilers aggressively
optimize on the assumption that there is no undefined behavior.
Now, I will tell you a fantasy.
A conforming C implementation is free to assume that no undefined
behavior occurs because in that case no requirements apply.
For instance, if it is obvious that some pieces of code can only be
reached if the program has invoked undefined behavior, then that code
can be removed as if it were unreachable. There is no requirement that
anything work at all after undefined behavior.
This concept allows you to encode assertions about properties like this:
#define undefined_behavior (0/0)
if (x < 1 || x > 12)
perpetrate_undefined_behavior();
switch (x)
Because undefined behavior happens if x is outside the range 1 to 12,
the assumption that the program is free of undefined behavior logically
implies that x is not outside of that range.
Therefore, the if statement can be optimized away, and the switch
statement.
A simpler way to express the above situation is this:
switch (x) {
// ... cases 1 to 12 ..
break;
// out of range cases:
default: 0/0; // division by zero: undefined
}
Basically we divert the out-of-range cases to an obvious, explicitly
defined undefined behavior. Then we hope that the compiler infers that,
since undefined does not happen, x can be assumed not to be outside of
the 1 to 12 range, and so why bother checking.
How well that works in practice, you have to determine experimentally.
Experimentally, I'm not able to get GCC to eliminate the check for x
being above 9 in this code:
switch (x) {
case 0: return "zero";
case 1: return "one";
case 2: return "two";
case 3: return "three";
case 4: return "four";
case 5: return "five";
case 6: return "six";
case 7: return "seven";
case 8: return "eight";
case 9: return "nine";
default: exit(x/0);
}
The best that happens is that x is compared to 9, and if it is above,
a branch takes place to a label, where the "ud2" instruction is executed.
There is no division and no call to exit.
Basically, this "optimize based on defined behavior" business is the
programmer's enemy. It will not do the obvious optimizations you want,
but it will shoot you in the foot when you accidentally introduce some
undefined behavior, or use some formally undefined "classic" idiom that
has nevertheless worked on pretty much every compiler over forty yeras.
Now, GCC even has a way to explicitly assert "if we reach this point
in the program, the behavior is undefined". Even that is not getting rid
of the range test for me:
const char *fun(int x)
{
switch (x) {
case 0: return "zero";
case 1: return "one";
case 2: return "two";
case 3: return "three";
case 4: return "four";
case 5: return "five";
case 6: return "six";
case 7: return "seven";
case 8: return "eight";
case 9: return "nine";
default: __builtin_unreachable();
}
}
Worse, gcc (version 7.5.0 on Ubuntu 18) is not emitting the ud2
instruction in this case, as it does for a null pointer dereference or
division by zero. It just generates code that returns from the function.
Maybe there is some option you're supposed to use for this, but I can't
find it.
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll 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
csiph-web