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From: "isanbard at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/113514] New: Wrong __builtin_dynamic_object_size when using a
 set local variable
Date: Fri, 19 Jan 2024 23:59:50 +0000
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113514

            Bug ID: 113514
           Summary: Wrong __builtin_dynamic_object_size when using a set
                    local variable
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: isanbard at gmail dot com
  Target Milestone: ---

There appears to be an issue with __builtin_dynamic_object_size() when usin=
g a
local variable. The following code should output 40 for each __bdos call:

$ cat test.c
#include <stdio.h>

struct s {
    int a, b, c, d;
    int  foo;
#define IDX 2
    int blue;
    char bar[IDX][40];
    int  baz;
    int  qux;
};

int main(int argc, char **argv) {
    struct s f;

    #define report(x) fprintf(stderr, #x ": %zu\n", x)

    report(__builtin_dynamic_object_size(f.bar[1], 1));
    report(__builtin_dynamic_object_size(f.bar[1], 3));

    report(__builtin_dynamic_object_size(f.bar[argc], 1));
    report(__builtin_dynamic_object_size(f.bar[argc], 3));

    argc =3D 1;
    report(__builtin_dynamic_object_size(f.bar[argc], 1));
    report(__builtin_dynamic_object_size(f.bar[argc], 3));

    return 0;
}

However, the last two print 48:

$ gcc -O2 test.c && a.out
__builtin_dynamic_object_size(f.bar[1], 1): 40
__builtin_dynamic_object_size(f.bar[1], 3): 40
__builtin_dynamic_object_size(f.bar[argc], 1): 40
__builtin_dynamic_object_size(f.bar[argc], 3): 40
__builtin_dynamic_object_size(f.bar[argc], 1): 48
__builtin_dynamic_object_size(f.bar[argc], 3): 48=