CODEWITHSUNDEEP
cmacosgccllvm
zago
zago

Reputation: 532

LLVM and GCC, different output same code

This is a sample code just to show a different output from the LLVM compiler and the GCC. I wonder why? The answer should be very simple, but I can't see it. (Xcode 4.6.1)

The code:

#include <stdio.h>

#define MAX(a,b) ( (a) > (b) ? (a) : (b) )

int increment() {
    static int i = 42;
    i += 5;
    printf("increment returns %d\n",i);
    return i;
}

int main( int argc, char ** argv ) {
    int x = 50;
    printf("max of %d and %d is %d\n", x,increment(),MAX(x, increment()));
    printf("max of %d and %d is %d\n", x,increment(),MAX(x, increment()));
    return 0;
}

The LLVM Output:

increment returns 47
increment returns 52
increment returns 57
max of 50 and 47 is 57
increment returns 62
increment returns 67
increment returns 72
max of 50 and 62 is 72

GCC Output:

increment returns 47
increment returns 52
max of 50 and 52 is 50
increment returns 57
increment returns 62
increment returns 67
max of 50 and 67 is 62

Upvotes: 2

Views: 460

Answers (2)

John
John

Reputation: 1

The LLVM code produces the right results according to ANSI C. If you don't want increment to be called more than once per print statement, save the return value in a variable and use that. Mentally stepping through the code, the display should be increment returns 47 increment returns 52 increment returns 57 max of 50 and 47 is 57 increment returns 62 increment returns 67 max of 50 and 62 is 72 So my initial reaction and study of the code was wrong, and the LLVM output is right. The reason for the LLVM result is that increment is called three times for each print statement in main. The way to make this code print sensible output is to save the value returned by increment in a variable and print the variable, not another call to increment. The difference in results doesn't depend on undefined behavior as far as I know, but on correct versus incorrect conformance to ANSI C.

Upvotes: -2

Kiril Kirov
Kiril Kirov

Reputation: 38143

The order of evaluation of the parameters is not defined specified. So this:

printf("max of %d and %d is %d\n", x,increment(),MAX(x, increment()));

causes undefined unspecified behavior . That's why you have different results on both compilers.

Another (potential) problem is: MAX - it could cause two calls to increment. Avoid using such macros.

Upvotes: 13

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