[Bug c/39121] strange behavior in chained operations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121 frankhb1989 at gmail dot com changed: What|Removed |Added CC||frankhb1989 at gmail dot com --- Comment #9 from frankhb1989 at gmail dot com --- This should work since C++11 because the rules of builtin assignment were modified (CWG 222; see also CWG 637). However, it is still undefined in C11, even if the new sequenced before wording has been copied from C++11 (WG21/N1944). Not sure if any diagnostics should be changed.
[Bug c/39121] strange behavior in chained operations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121 Manuel López-Ibáñez manu at gcc dot gnu.org changed: What|Removed |Added CC||manu at gcc dot gnu.org --- Comment #8 from Manuel López-Ibáñez manu at gcc dot gnu.org --- (In reply to joe.carnuccio from comment #7) *a ^= *b ^= *a ^= *b should work the same way, but it does not (unless you compile with -Os). https://gcc.gnu.org/wiki/FAQ#undefinedbut
[Bug c/39121] strange behavior in chained operations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121 --- Comment #7 from joe.carnuccio at qlogic dot com --- Ok, the sequence points are at each of the assignment operators. The crux of this is that doing the xor chain with dereferenced pointers fails (incorrect execution), whereas doing it with variables works... i.e. *a and *b are being treated differently than a and b; a ^= b ^= a ^= b is supposed to do the following: a = a ^ (b = b ^ (a = a ^ b)) from right-to-left each assignment is done in sequence (and has been verified to work correctly); *a ^= *b ^= *a ^= *b should work the same way, but it does not (unless you compile with -Os).
[Bug c/39121] strange behavior in chained operations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121 joe.carnuccio at qlogic dot com changed: What|Removed |Added CC||joe.carnuccio at qlogic dot com --- Comment #4 from joe.carnuccio at qlogic dot com --- I have found the following: This works: c ^= d ^= c ^= d (where c and d are not pointers) This fails: *a ^= *b ^= *a ^= *b (where a and b are pointers) When compiling using -Os then the failed case now works.
[Bug c/39121] strange behavior in chained operations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121 --- Comment #6 from Andrew Pinski pinskia at gcc dot gnu.org --- (In reply to joe.carnuccio from comment #5) Since using gcc -Os causes the correct execution, then sequence point does not have anything to do with it. And you are wrong about that. -Os causes what you think is the correct execution but there are multiple interpretations of the code because there are not sequence points there.
[Bug c/39121] strange behavior in chained operations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121 --- Comment #5 from joe.carnuccio at qlogic dot com --- Since using gcc -Os causes the correct execution, then sequence point does not have anything to do with it.
[Bug c/39121] strange behavior in chained operations
--- Comment #1 from pinskia at gcc dot gnu dot org 2009-02-06 20:09 --- This is undefined code as you are modifying *a twice without a sequence point inbetween the modifies. *** This bug has been marked as a duplicate of 15145 *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution||DUPLICATE http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121
[Bug c/39121] strange behavior in chained operations
--- 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] strange behavior in chained operations
--- Comment #3 from rguenth at gcc dot gnu dot org 2009-02-06 21:21 --- Evaluation order is undefined if there is no sequence point. -- rguenth at gcc dot gnu dot org changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution||INVALID http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39121
