J E L L Y E N T
Suggestions to Ranking Fired The utilization of Swap Statements and Assertion Expressions (2016)
2016-10-27 – By Robert Elder

Updated Oct 27, 2016: Mounted sample code feedback in coroutine example as per advice, edited textual yell material to masks D’s same behaviour.

Updated Oct 28, 2016: Added lacking colon., corrected Duff’s instrument example.

     It can probably perhaps now no longer discipline whether or no longer or no longer it’s doubtlessly you are going to even be making an strive to discontinue job safety; impressing others by showing them how successfully-organized you are; or passive aggressively declaring your dominance over a code unhealthy, writing unmaintainable code has a special of wise functions.  One terribly unmaintainable and worm-ridden methodology for C programming entails the utilization of every swap statements and discipline expressions together.

     Listed right here, we are in a position to present attention to how to leverage swap statements and discipline expressions to make C code that is so delicate to engage pleasure in, it’s doubtlessly that you just would possibly well well also calm undercover agent on the meeting to unravel out what it does.  Many of the examples of syntax on this text don’t look like any longer requirements compliant or they’d presumably presumably moreover no longer certain even basically the most easy of static prognosis assessments.  That wants to be okay though, as a consequence of the writing barely a massive deal of these examples for your firm’s code unhealthy will doubtlessly discontinue up getting you fired anyway.

     Let’s delivery by reviewing the typical-or-garden-or-backyard swap discipline that everyone knows and interact pleasure in:

int i = ...;
swap(i){
	case 0:{
		...
		harm;
	}case 1:{
		...
		harm;
	}case 2:{
		...
		harm;
	}default:{
		...
	}
}

     The above is what most varied folks are worn to inquisitive about when ‘swap statements’ are talked about in C.  The cruel belief is that swap statements are a contain of more interesting varied to the utilization of a total bunch ‘else if’ statements when checking for some disjoint property.  Masses of of you are going to without doubt be in a misfortune to be deal surprised to learn that the following will likely be a sound swap discipline:

int i = ...;
swap(i){
        i++;
        default:{ }
        i++;
        case 0:{
                case 3: i;
        }
        if(i < 10){
                case 1:{
                        harm;
                }
                for(i=0; i < 10; i++){
                        case 2:;
                }
        }
}

     It is mark pointing out that with regards to about no varied languages toughen swap statements the arrangement that they work in C (though the D language is an example).  Most varied languages engage pleasure in a swap discipline that works honest just like the root of a more interesting varied to many ‘else if’ assessments.

     A swap discipline in C would presumably be more precisely known as a ‘goto self-discipline’.  This means that the swap(…) share merely makes a name about which trace to branch to.  After branching to that trace nothing special occurs associated to the very fact that it’s doubtlessly you are going to even be internal a swap discipline and the code will genuine join executing whatever machine instructions advance subsequent.  The one exception is, clearly, the harm discipline which is in a misfortune to jump to the level after the swap discipline physique.  Here is an an analogous version of the swap discipline written above the utilization of most environment pleasant ifs and gotos.

int i = ...;
if(i == 0)
	goto label_0;
if(i == 1)
	goto label_1;
if(i == 2)
	goto label_2;
if(i == 3)
	goto label_3;

goto label_default;

{
        i++;
        label_default:{ }
        i++;
        label_0:{
                label_3: i;
        }
        if(i < 10){
                label_1:{
                        goto break_from_switch;
                }
                for(i=0; i < 10; i++){
                        label_2:;
                }
        }
}
break_from_switch:

     Once you are already conversant in the well-liked Duff’s procedure the above this doubtlessly is now not any longer in actual fact files to you:


    int total_bytes = ...;
    int n = (total_bytes + 3) / 4;
    switch (total_bytes % 4) {
            case 0: attain { *to = *from++;
            case 3:      *to = *from++;
            case 2:      *to = *from++;
            case 1:      *to = *from++;
                    } while (--n > 0);
    }

     Drawing on Duff’s instrument as inspiration you are going to without doubt be in a misfortune to fritter away the outlandish behaviour of swap statements in C to establish in strength coroutines:

#encompass 

#elaborate coroutine_begin() static int repeat=0; swap(repeat) { case 0: 
#elaborate coroutine_return(x) { repeat=__LINE__; return x; case __LINE__:; }
#elaborate coroutine_finish() }

int get_next(void) {
        static int i = 0;
        coroutine_begin();
        whereas (1){
                coroutine_return(++i);
                coroutine_return(100);
        }
        coroutine_finish();
}

int main(void){
        printf("i is %dn", get_next()); 1
        printf("i is %dn", get_next()); 100
        printf("i is %dn", get_next()); 2
        printf("i is %dn", get_next()); 100
        return 0;
}

     The occasion on this share attracts on one masks in Coroutines in C by Simon Tatham.  The long-established provide describes genuine a couple of caveats of swap basically based fully completely mostly coroutines that I would presumably moreover no longer give attention to on this text.

     If we unravel the macros, and indent the code a minute better, then join away some superfluous brackets and semicolons, the ‘get_next’ operate turns into:

#encompass 

12344567

int get_next(void) {
        static int i = 0;
        static int repeat = 0;
	swap(repeat) {
		case 0:;
		whereas(1){
			repeat = 1234;
			return ++i;
			case 1234:;
			repeat = 4567;
			return 100;
			case 4567:;
		}
        }
}

     The static variables are key right here, as a consequence of the they allow us to succor in contact files between calls to the operate ‘get_next’.  The swap discipline efficiently genuine offers us a helpful technique to establish in strength the goto statements that is in possibility of be main to both jump to the delivery of the coroutine (repeat = 0), or the level genuine after the coroutine returns (repeat = 1234, and repeat = 4567).  Once that you just would possibly well well wish to make more return/resume parts you are going to without doubt be in a misfortune to without misfortune add more calls to the ‘coroutine_return’ macro.  It is correct to also lisp that that that these calls seem between ‘coroutine_begin’ and ‘coroutine_end’.

     If the syntax of the case statements is throwing you off, genuine join into myth that

case 0:;

     does same share as

case 0: {
}

     The predominant represents a ‘discipline’ with out a expression, and the 2nd represents a ‘compound discipline’ with out a declarations or expressions.  Neither of these kinds has a harm discipline, so execution will at all instances genuine proceed doing whatever is after the case trace in the same arrangement that a goto would work.

     Now that now we engage pleasure in considered how swap statements can bag barely irregular, let’s to find genuine a couple of absorbing examples of legit swap discipline syntax that (taking a undercover agent on how genuine of a 1337 hacker you are) it’s doubtlessly you are going to also engage pleasure in by no manner considered forward of:


swap(0);


swap(0)
	i++;


swap(0)
	swap(0)
		swap(0)
			swap(0)
				swap(0)
					swap(0);


swap(0)
	case 0:;

swap(0)
	case 0:
		for(i = 0; i < 10; i++)
			case 1:
				for(j = 0; j < 10; j++)
					case 2:
						for(okay = 0; okay < 10; okay++);


swap(0){
	case 0:{
		for(i = 0; i < 10; i++){
			case 1:{
				whereas(j){
					case 2:{
						for(okay = 0; okay < 10; okay++){
						}
					}
					j++;
				}
			}
		}
	}case 3:{
		
	}default:{
		
	}
}




swap(i)
	default: case 0: case 1: case 2: case 3:;





swap(0){
	case UNIQUE_CASE_A:{
		harm;
	}case UNIQUE_CASE_B:{
		harm;
	}case SIMILAR_CASE1: case SIMILAR_CASE2: case SIMILAR_CASE3:{
		harm;
	}
}

     Assertion expressions are a GNU extension that is now not any longer supported by the C same earlier, but they’re supported by default in gcc and clang.  They’ll make it more uncomplicated to embed a compound discipline internal an expression.  The price returned by the final expression is the worth returned by the total discipline expression:


int i = 0;

int j = ({int okay; okay = i + 1; i;});

     That you would possibly well well presumably presumably presumably also establish a requirement to « Why would you ever are making an strive to achieve this kind of part? »  There are a special of quite quite rather a lot of solutions and barely a massive deal of them are associated to convenience:  One fritter away case is anxious with guaranteeing that expression discipline part-outcomes are most environment pleasant evaluated as soon as in the case of a operate macro that might perhaps cause the expression to appear more than one instances in the operate macro physique.

     That you would possibly well well presumably presumably presumably stick discipline expressions barely genuine wherever it’s doubtlessly you are going to also establish a variable:

#encompass 

int get_zero(void){ return 0; }

int main(void){
        09
        for(int i = ({get_zero();}); i < 10; i++)
                printf("%dn", i);
        return 0;
}

     That you would possibly well well presumably presumably presumably personal barely genuine one part else internal a discipline expression that it’s doubtlessly you are going to also personal in a worn compound discipline:

#encompass 

int main(void){
        0Ninety nine
        int i = ({int j = 0; for(int i = 0; i < 100; i++) j+=i; j;});
        printf("Sum is %dn", i);
        return 0;
}

     Sadly, a majority of these examples will most environment pleasant elevate together in clang, since gcc dis-enables branching to a trace that is internal a discipline expression (which might perhaps well well be for the best).

     Let’s strive to embed a swap discipline magnificent real into a swap discipline expression:


swap(({swap(0);3;}));

     Yup, this compiles in gcc and clang!  This code would now no longer personal one part else absorbing, but it indubitably illustrates one procedure to writing C code that is delicate to read.

     Now let’s strive developing a more devoted example that mixes swap and discipline expressions.  Here is an example the join you make an strive to conditionally substitute the bounds of a loop.  That you would possibly well well presumably presumably presumably also fritter away a variable or a operate but it indubitably is presumably you are going to also fritter away a discipline expression with embedded case labels!

#encompass 

void print_stuff(int kind){
        int i = 0;
        int r = 0;
        swap(kind){
                for(i = 0; i < ({if(0){ case 1:r+=2; case 0:r+=3;}r;}); i++){
                        printf("i is %dn", i);
                }
        }
}

int main(void){
        printf("First acceleraten");
        print_stuff(0);
        printf("2nd acceleraten");
        print_stuff(1);
        return 0;
}

      The above example offers me the following output:

First accelerate
i is 0
i is 1
i is 2
2nd accelerate
i is 0
i is 1
i is 2
i is 3
i is 4

      However, whilst you occur to accelerate this program by strategy of valgrind you are going to without doubt be in a misfortune to be triumphant in that this or no longer it’s doing uninitialized reads:

==16228== Conditional jump or certain is relying on uninitialised worth(s)
==16228==    at 0x4005AB: print_stuff (main.c:7)
==16228==    by 0x400619: main (main.c: 15)
...
==16228== Conditional jump or certain is relying on uninitialised worth(s)
==16228==    at 0x4005AB: print_stuff (main.c:7)
==16228==    by 0x400637: main (main.c: 17)

      That in all fairness spectacular as a consequence of the we’re in a misfortune to make uninitialized reads in a program the join all variables are initialized, and there would presumably be no pointer or array magic!  Let’s undercover agent at a smaller example that has the same map abet:

#encompass 

int main(void){
        int i = 0;
        swap(i){
                for(i = 0; i < ({case 0:; 10;}); i++){
                        (void)i;
                }
        }
        return 0;
}

      And generating the meeting on my machine you bag:

	pushq   %rbp
.Ltmp0:
        .cfi_def_cfa_offset 16
.Ltmp1:
        .cfi_offset %rbp, -16
        movq    %rsp, %rbp
.Ltmp2:
        .cfi_def_cfa_register %rbp
        movl    $0, -4(%rbp)
        .loc    1 4 13 prologue_end     
.Ltmp3:
        movl    $0, -8(%rbp)
        .loc    1 5 2                   
        movb    $1, %al
        testb   %al, %al
        jne     .LBB0_2
        jmp     .LBB0_6
.LBB0_1:                                
        .loc    1 6 14 discriminator 1  
.Ltmp4:
        movl    -8(%rbp), %eax
        movl    %eax, -16(%rbp)         
.LBB0_2:                                
        .loc    1 6 19 is_stmt 0 discriminator 2 
        movl    $10, -12(%rbp)
        .loc    1 6 16 discriminator 2  
        movl    -16(%rbp), %eax         
        cmpl    -12(%rbp), %eax
        .loc    1 6 3 discriminator 2   
        jge     .LBB0_5


        .loc    1 6 37 discriminator 3  
        movl    -8(%rbp), %eax
        addl    $1, %eax
        movl    %eax, -8(%rbp)
        .loc    1 6 3 discriminator 3   
        jmp     .LBB0_1
.Ltmp5:
.LBB0_5:
        .loc    1 9 2 is_stmt 1         
        jmp     .LBB0_6
.Ltmp6:
.LBB0_6:
        xorl    %eax, %eax
        .loc    1 10 9                  
        popq    %rbp
        retq
.Ltmp7:
.Lfunc_end0:

     The uninitialized read occurs as a consequence of the these lines bag skipped on the belief loop iteration:

.Ltmp4:
        movl    -8(%rbp), %eax
        movl    %eax, -16(%rbp)         

     These instructions make a reproduction of the ‘i’ variable and retailer it at -16(%rbp) to be worn in the comparability ‘i < ({case 0:; 10;})'.  Within the belief iteration the utilization of the jump from the swap discipline jumps over these instructions, though they're accomplished on later loop iterations.  It looks barely low-mark that the compiler would personal this, in the conclude you are telling it to branch into the center of a for loop comparability.

     Here is every varied virtually wise example the join we are in a position to fritter away a case trace to jump into the center of a operate parameter evaluation:

#encompass 

void f(int kind){ }

int main(void){
        int i = 0;
        swap(i){
                f(i + ({case 0:; 1;}) + i);
        }
        return 0;
}

     Once you elevate together this with clang 3.8.0-2ubuntu4 you bag the following:

...
clang: error: unable to admire itemizing: Segmentation fault (core dumped)
clang: error: clang frontend itemizing failed as a consequence of signal (fritter away -v to explore invocation)
clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/closing)
Form: x86_64-deepest computer-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
clang: masks: diagnostic msg: PLEASE put up a worm fantasy to http://llvm.org/bugs/ and encompass the ruin backtrace, preprocessed provide, and associated accelerate script.
clang: masks: diagnostic msg:
PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed provide(s) and associated accelerate script(s) might perhaps well well be found at:
clang: masks: diagnostic msg: /tmp/main-df5301.c
clang: masks: diagnostic msg: /tmp/main-df5301.sh
...

     Here is every varied same case that can ship my version of clang into (what seem love) a wide loop:

int main(void){
        int i = 0;
        swap(i){
                i = i + ({case 0:; 0;});
        }
        return 0;
}

     Whereas it’s doubtlessly you are going to even be at it, why no longer sneak a case discipline trace into the internal of a bitfield width calculation.  This occasion will elevate together in clang, but it indubitably would presumably presumably presumably moreover no longer output one part else (the case 0 looks to bag fully handed over and nil will without doubt be caught by a default case whilst you occur to add it):

#encompass 

3.8.0
int main(void){
        int i = 0;
        swap(i){
                int j = sizeof(struct {int i:({case 0:; 1;});});
                printf("Fin %d.n", j);
        }
        return 0;
}

     You bag same behaviour whilst you occur to establish a case trace internal every varied case trace:

#encompass 

int main(void){
        int i = 1;
        swap(i){
                case ({case 1:; 0;}): printf("right heren");
        }
        return 0;
}

     As you engage pleasure in considered on this text, you are going to without doubt be in a misfortune to fritter away swap statements to make a special of legit C programs which might perhaps well well be terribly delicate to engage pleasure in.  That you would possibly well well presumably presumably presumably even join this extra by embedding case labels internal discipline expressions which make indubitably subsequent-level tense to engage pleasure in code that might perhaps cause a unfold of delicate complications.  As now we engage pleasure in considered above, this entails compiler crashes, compiler hangs, and subtly broken executable code.  Once you commit wide of these to your code unhealthy, it’s efficient to bag you fired!

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