https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914
Bug ID: 86914
Summary: gcc 8.1 -O2 generates wrong code with strlen() of
pointers within one-element arrays of structures
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: even.rouault at spatialys dot com
Target Milestone: ---
The following code
{{{
#include <string.h>
struct s
{
int i;
char c[1];
};
size_t foo(struct s* p)
{
return strlen(p->c+1);
}
}}}
compiled with gcc 8.1 -O2 generates the following code
{{{
0000000000000000 <foo>:
0: 31 c0 xor %eax,%eax
2: c3 retq
}}}
returning 0
Previous gcc versions generate the "correct" code
{{{
0000000000000000 <foo>:
0: 48 83 c7 05 add $0x5,%rdi
4: e9 00 00 00 00 jmpq 9 <foo+0x9> 5: R_X86_64_PC32
strlen-0x4
}}}
According to https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html , """Although
using one-element arrays this way is discouraged, GCC handles accesses to
trailing one-element array members analogously to zero-length arrays. """
If modifying c[1] to be c[0], gcc 8.1 -O2 generates the correct code. If
changing strlen(p->c+1) to strlen(p->c), the correct code is although generated
So it looks to be a too agressive optimization.
