[Bug c/91031] wrong code generated when using compound literal
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91031
--- Comment #6 from Andrew Pinski ---
(In reply to Alexey Makhalov from comment #5)
> (In reply to Andrew Pinski from comment #1)
> > In previous versions of gcc, the compound literal was put in the function
> > level scope rather than in the current scope. Which is why it worked
> > previously. But the code was undefined. This was added to the changes page
> > too.
>
> Hi Andrew, thanks for the update.
>
> There is an inconsistency which is really worried me.
>
> 1) The behavior of GCC is different (from user point of view). -O0 allocates
> anonymous variable per function, but -01 and higher allocate it per scope?
Because it is an optimization to be able to reuse the stack space. Undefined
code at runtime is just that undefined.
>
> 2) this sample will allocate anonymous (char *)"test" per function scope
> with any optimization
NO this example does not use compound literals so it does not have an anonymous
variable in play. In fact in below testcase, "test" is allocated in a static
read only region.
I think you want:
j = (char[]){"test"};
See PR 89113 for an example.
>
> #include
>
> int testme(char *j) {
> if (!j)
> j = (char *)"test";
>
> return strlen(j) == 4;
> }
>
> int main(void) {
> return testme(0) == 0;
> }
> --
> 3) Why GCC does not provide any warning/errors in that case?
Because escape analysis is "hard". This can be found at runtime though with
-fsanitize=address (which enables -fsanitize-address-use-after-scope).
>
> 4) Even if anonymous variable put only in current scope (with optimization),
> I still see space for it was allocated in function frame.
> I can give you bigger example where array of pointers was allocated on stack
> in prologue, but was not initialized.
It could have been shared with a different anonymous variable.
>
> Can you point to the commit which introduced this change, please?
r259641
[Bug c/91031] wrong code generated when using compound literal
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91031
--- Comment #5 from Alexey Makhalov ---
(In reply to Andrew Pinski from comment #1)
> In previous versions of gcc, the compound literal was put in the function
> level scope rather than in the current scope. Which is why it worked
> previously. But the code was undefined. This was added to the changes page
> too.
Hi Andrew, thanks for the update.
There is an inconsistency which is really worried me.
1) The behavior of GCC is different (from user point of view). -O0 allocates
anonymous variable per function, but -01 and higher allocate it per scope?
2) this sample will allocate anonymous (char *)"test" per function scope with
any optimization
#include
int testme(char *j) {
if (!j)
j = (char *)"test";
return strlen(j) == 4;
}
int main(void) {
return testme(0) == 0;
}
--
3) Why GCC does not provide any warning/errors in that case?
4) Even if anonymous variable put only in current scope (with optimization), I
still see space for it was allocated in function frame.
I can give you bigger example where array of pointers was allocated on stack in
prologue, but was not initialized.
Can you point to the commit which introduced this change, please?
PS: went to read C standard for anonymous variables.
[Bug c/91031] wrong code generated when using compound literal
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91031 Andrew Pinski changed: What|Removed |Added CC||makhaloff at gmail dot com --- Comment #4 from Andrew Pinski --- *** Bug 94979 has been marked as a duplicate of this bug. ***
[Bug c/91031] wrong code generated when using compound literal
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91031 Jakub Jelinek changed: What|Removed |Added CC||jakub at gcc dot gnu.org --- Comment #3 from Jakub Jelinek --- See https://gcc.gnu.org/gcc-9/porting_to.html#complit
[Bug c/91031] wrong code generated when using compound literal
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91031 David changed: What|Removed |Added CC||david at pgmasters dot net --- Comment #2 from David --- We figured out the issue, but were trying to figure out why it worked on any version of gcc. Thanks for the explanation.
[Bug c/91031] wrong code generated when using compound literal
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91031 Andrew Pinski changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #1 from Andrew Pinski --- The compound literal is consider an anonymous variable in the current scope. So in your case you take the address of it and then the scope ends but you access it afterwards. In previous versions of gcc, the compound literal was put in the function level scope rather than in the current scope. Which is why it worked previously. But the code was undefined. This was added to the changes page too.
