https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91235
Bug ID: 91235
Summary: Array size expression is implicitly casted to unsigned
long type
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: bugzi...@poradnik-webmastera.com
Target Milestone: ---
[code]
void foo(char*);
inline void bar(int n)
{
if (__builtin_constant_p(n))
{
char a[(int)(n == 2 ? -1 : 0)];
foo(a);
}
}
void baz()
{
bar(2);
}
[/code]
When this is compiled with -O3 -Wall -Wextra -std=c++11 (tested via
godbolt.org), it produces following code:
[asm]
baz():
push rbp
mov rbp, rsp
mov rdi, rsp
call foo(char*)
leave
ret
[/asm]
During compilation gcc reported following warning:
[out]
<source>: In function 'void baz()':
<source>:7:14: warning: argument to variable-length array is too large
[-Wvla-larger-than=]
7 | char a[(int)(n == 2 ? -1 : 0)];
| ^
<source>:7:14: note: limit is 9223372036854775807 bytes, but argument is
18446744073709551615
Compiler returned: 0
[out]
This means that gcc saw that n is constant, and then expression specified as
array size was evaluated and implicitly casted to unsigned type.
When I removed "foo(a);" line, this warning is gone, and gcc warned about
unused variable.
When -1 is specified as array size, it correctly report error that array size
is negative. Looks that only expressions causes this issue.
