[Bug c/39121] strange behavior in chained operations

[nospam at pamies dot cat]

--- Comment #2 from nospam at pamies dot cat  2009-02-06 21:07 ---
Is not the same bug as #15145. I agree with you that there is just one sequence
point, but the operation is not undefined.

void swap(int *a, int *b) {
*a ^= *b ^= *a ^= *b;
}

This code should be compiled to:

*a = *a ^ *b;
*b = *b ^ *a;
*a = *a ^ *b;

And not to something like (I think that is what happens):

int tmp;
tmp = *a ^ *b;
*b = *b ^ tmp;
//On that point *a should contain 5^8 instead of the original value 5.
//This happens because the temp variable generated by the compiler.
*a = *a ^ *b;  

I think that the compiler is not translating properly what was written in the
source code. Summarizing, I think that in:

y = 1;
x = (y += 1);

The execution order should be:

volatile_register <--- y + 1
y <--- volatile_register
x <--- volatile_register

instead of:

volatile_register <--- y + 1
x <--- volatile_register
y <--- volatile_register


-- 

nospam at pamies dot cat changed:

   What|Removed |Added

 Status|RESOLVED|UNCONFIRMED
 Resolution|DUPLICATE   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121

[Bug c/39121] New: strange behavior of a chain of operations.

[nospam at pamies dot cat]

#include 

/*
  Why the first swap operation works as expected but
  it does not happen the same with the second one ?
  I guess that it can be due operations within temp values,
  but IMHO swap operation should work in both cases.

  Thanks !
 */

void swap(int *a, int *b) {
*a ^= *b ^= *a ^= *b;
}

int main() {
int a = 5;
int b = 8;
printf("%d, %d\n", a, b);
a ^= b ^= a ^= b;
printf("%d, %d\n", a, b);
swap(&a, &b);
printf("%d, %d\n", a, b);
}


-- 
   Summary: strange behavior of a chain of operations.
   Product: gcc
   Version: unknown
Status: UNCONFIRMED
  Severity: major
  Priority: P3
 Component: c
AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: nospam at pamies dot cat


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121