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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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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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 >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191126/b0fc5ef9/attachment.html>