This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: [std-discussion] Is this union aliasing code well-defined?
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Yubin Ruan <ablacktshirt at gmail dot com>
- Cc: std-discussion at isocpp dot org, gcc-list <gcc at gcc dot gnu dot org>
- Date: Fri, 27 Oct 2017 10:54:48 +0200
- Subject: Re: [std-discussion] Is this union aliasing code well-defined?
- Authentication-results: sourceware.org; auth=none
- References: <128124e9-3df8-41bf-99da-3675429fcb42@isocpp.org> <20171027130038.GB11592@HP>
On Fri, Oct 27, 2017 at 3:00 PM, Yubin Ruan <ablacktshirt@gmail.com> wrote:
> +Cc gcc-list.
>
> Does any gcc developer have any comments?
See PR82224. The code is valid.
> On Mon, Sep 25, 2017 at 01:41:55PM -0700, Myriachan wrote:
>> This question that "supercat" posted on Stack Overflow ran into an
>> interesting problem:
>>
>> https://stackoverflow.com/questions/46205744/is-this-use-of-unions-strictly-conforming/
>>
>> A copy of the code involved is as follows:
>>
>> struct s1 {unsigned short x;};
>> struct s2 {unsigned short x;};
>> union s1s2 { struct s1 v1; struct s2 v2; };
>>
>> static int read_s1x(struct s1 *p) { return p->x; }
>> static void write_s2x(struct s2 *p, int v) { p->x=v;}
>>
>> int test(union s1s2 *p1, union s1s2 *p2, union s1s2 *p3)
>> {
>> if (read_s1x(&p1->v1))
>> {
>> unsigned short temp;
>> temp = p3->v1.x;
>> p3->v2.x = temp;
>> write_s2x(&p2->v2,1234);
>> temp = p3->v2.x;
>> p3->v1.x = temp;
>> }
>> return read_s1x(&p1->v1);
>> }
>> int test2(int x)
>> {
>> union s1s2 q[2];
>> q->v1.x = 4321;
>> return test(q,q+x,q+x);
>> }
>> #include <stdio.h>
>> int main(void)
>> {
>> printf("%d\n",test2(0));
>> }
>>
>>
>> Both GCC and Clang in -fstrict-aliasing mode with optimizations are acting
>> as if they ran into undefined behavior, and return 4321 instead of the
>> expected 1234. This happens in both C and C++ mode. Intel C++ and Visual
>> C++ return the expected 1234. All four compilers hardwire the result as a
>> constant parameter to printf rather than call test2 or modify memory at
>> runtime.
>>
>> From my reading of the C++ Standard, particularly [class.union]/5,
>> assignment expressions through a union member access changes the active
>> member of the union (if the union member has a trivial default constructor,
>> which it does here, being C code). Taking the address of p2->v2 and p1->v1
>> ought to be legal because those are the active members of the union at the
>> time their pointers are taken.
>>
>> Is this a well-defined program, or is there subtle undefined behavior
>> happening here?
>>
>> Melissa
>>
>> --
>>
>> ---
>> You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Discussion" group.
>> To unsubscribe from this group and stop receiving emails from it, send an email to std-discussion+unsubscribe@isocpp.org.
>> To post to this group, send email to std-discussion@isocpp.org.
>> Visit this group at https://groups.google.com/a/isocpp.org/group/std-discussion/.
>