[Restructuring] Transform Guarded do-while loops

[ebehner]

Proposal

Depending on the optimization level, easy while loops are sometimes transformed into an if-condition that has only a true branch consisting of a do-while loop with the same condition, i.e., while(cond){...} is transformed into if(cond){do{...}while(cond)}.

An example of this problem is function main in a.zip. The decompiled code is

int main(int arg1, char ** arg2, char ** arg3) {
    int var_0;
    int var_1;
    unsigned long var_2;
    int * var_3;
    var_3 = &var_0;
    __isoc99_scanf(/* format */ "%d", var_3);
    var_3 = &var_1;
    __isoc99_scanf(/* format */ "%d", var_3);
    printf(/* format */ "b = %d\n", (unsigned int)var_1);
    var_2 = (unsigned int)var_0;
    if ((unsigned int)var_0 < var_1) { // <---------------- guarded condition
        do {
            printf(/* format */ "a = %d\n", (unsigned int)var_0, var_2);
            var_0++;
            var_2 = (unsigned int)var_0;
        }
        while ((unsigned int)var_0 < var_1); // <---------------- same condition as guarded if-condition
    }
    puts(/* str */ "done");
    return 0;
}

Approach

The task could be done by the loop structure. It has to be done before the for-loop restructuring such that we can transform these loops to for-loops if we needed.