[llvm-dev] [Clang] memory allocation

[Mohammad Norouzi via llvm-dev]

Thanks David for your reply!

However, OK is called inside nqueens. So, the same stack space cannot be
used/reused for both of them.

Best,
Mohammad

On Wed, Nov 20, 2019 at 11:43 PM David Blaikie <dblaikie at gmail.com> wrote:

> 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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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