[llvm-dev] [Clang] memory allocation

[David Blaikie via llvm-dev]

You printed &j and &solutions - did you mean to print 'solutions' instead
of '&solutions' Because 'solutions' and 'j' are both local variables in
distinct function calls ('ok' is called, then 'nqueens' is called - so they
can both use/reuse the same stack space for their local variables).

On Wed, Nov 20, 2019 at 1:16 PM Mohammad Norouzi via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> Could anyone please help me understand why Clang reallocates the same
> memory address for different variables while their lifetime intersect?
>
> Here is an example code:
>
>
> #include <stdlib.h>
> #include <stdio.h>
> #include <memory.h>
> #include <alloca.h>
>
>
> /* Checking information */
>
> static int solutions[] = {
>         1,
>         0,
>         0,
>         2,
>         10, /* 5 */
>         4,
>         40,
>         92,
>         352,
>         724, /* 10 */
>         2680,
>         14200,
>         73712,
>         365596,
> };
> #define MAX_SOLUTIONS sizeof(solutions)/sizeof(int)
>
> int total_count;
> int sharedVar = 0;
>
> int ok(int n, char *a)
> {
>      int i, j;
>      char p, q;
>      printf("jjjjjjjjj: %d, %p\n", n,&j);
>      for (i = 0; i < n; i++) {
>            p = a[i];
>
>
>  for (j = i + 1; j < n; j++) {
>       q = a[j];
>       if (q == p || q == p - (j - i) || q == p + (j - i))
>          return 0;
>  }
>      }
>      return 1;
> }
>
> void nqueens (int n, int j, char *a, int *solutions)
> {
> int i,res;
>   sharedVar = sharedVar * j - n;
> if (n == j) {
> /* good solution, count it */
>      *solutions = 1;
>       return;
> }
> printf("solutions: %d, %p\n", j, &solutions);
> *solutions = 0;
>
>       /* try each possible position for queen <j> */
> for (i = 0; i < n; i++) {
>      a[j] = (char) i;
>      if (ok(j + 1, a)) {
>          nqueens(n, j + 1, a,&res);
>          *solutions += res;
>      }
>   }
> }
>
> int main()
> {
>   int size = 3;
> char *a;
> // printf("total_count: %p\n", &total_count);
> total_count=0;
> a = (char *)alloca(size * sizeof(char));
> printf("Computing N-Queens algorithm (n=%d) ", size);
>   sharedVar = -5;
>    nqueens(size, 0, a, &total_count);
>         printf("completed!\n");
>   printf("sharedVar: %d\n", sharedVar);
> }
>
>
>
> When I compile the program with clang -O0 variable j in function ok has
> the same memory address as th variable solutions in function nqueens.
>
> However, compiling it with gcc, they have different memory addresses.
>
> Any help is appreciated!
>
> Best Regards,
> Mohammad
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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