http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
Bug #: 55425
Summary: constexpr does not work in many situations (both
built-in and user supplied)
Classification: Unclassified
Product: gcc
Version: 4.7.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: [email protected]
ReportedBy: [email protected]
constexpr expressions/functions do not work in many cases because of compiler
wrongly interpreting the code. Seen in gcc-MinGW 4.7.0 and 4.7.1.
Situation 1: __func__
--------------------------
A return statement is not a return statement if the returned value is __func__
(also true for non-standard identifiers like __PRETTY_FUNCTION__).
// good
//static const char func[] = "function-name";
//constexpr const char* x() { return func; }
// bad
constexpr const char* x() { return __func__;}
int main() { __builtin_puts(x()); return 0; }
Compiler output:
----------------
error: body of constexpr function 'constexpr const char* x()' not a
return-statement
Cross-reference to corresponding MinGW ticket:
http://sourceforge.net/tracker/index.php?func=detail&aid=3471328&group_id=2435&atid=102435
Situation 2: user literals
--------------------------
The (obviously constant) string that the compiler builds from the literal is
not constant according to the compiler:
#include <stdio.h>
constexpr int valid_bin_number(const char* c) { return *c ? ((*c == '1' || *c
== '0') ? valid_bin_number(c+1) : false ) : true; }
unsigned int operator"" _bin(const char* str)
{
static_assert(valid_bin_number(str), "not a binary number");
unsigned int ret = 0;
for(unsigned int i = 0; str[i] != '\0'; ++i)
{
char digit = str[i];
ret = ret * 2 + (digit - '0');
}
return ret;
}
int main()
{
unsigned int a = 10000_bin;
(void) a;
return 0;
}
Compiler output:
----------------
In function 'unsigned int operator"" _bin(const char*)':
error: non-constant condition for static assertion
error: 'str' is not a constant expression
Cross-reference to corresponding MinGW ticket:
http://sourceforge.net/tracker/?func=detail&atid=102435&aid=3582841&group_id=2435
Situation 3: __m128i type
--------------------------
Assigning a literal value to a constexpr __m128 fails because the literal is
not a literal.
#include <emmintrin.h>
constexpr unsigned int a[] = { 5, 3}; // works (of course)
constexpr float b[] = { 1.1, 3.7 }; // works, and no warning?!
__m128i c[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } };
const __m128i d[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } };
static const __m128i e[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } };
namespace { const __m128i f[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } }; }
constexpr __m128i g[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } }; // <---- fails
int main() { return 0; }
Compiler output:
----------------
error: the type 'const __m128i [] {aka const __vector(2) long long int []}' of
constexpr variable 'g' is not literal
