Complex values handling in Clang

Clang Frontend
#1

Hi All,

Complex values stored in variable value mismatches if program compiled clang and gcc.

Below is the sample program printing byte by byte stored “complex long long int” value,

int main()
{
_Complex unsigned long long int lli_val= (5668231682443686620ULL + 7618395480654047911ULL*extension 1i);
// _Complex long double ld_val = (5668231682443686620ULL + 7618395480654047911ULL*extension 1i);
int size = sizeof(lli_val);
int index = 0;
printf(“long long int val:\n”);
while (size != index)
{
unsigned char cval = (unsigned char)(((unsigned char)&lli_val) + index);
printf("%x", cval);
index++;
}
}

Outputs of clang compiled, gcc compiled values are:
For Clang,
long long int val:
dcb2ab00000a76ad35e0000
For GCC,
long long int val:
dcb2ab0af9aa94ea76ad35e53fab969

Similar behaviour observed for “_Complex float, _Complex double, _Complex long double” types.

Complex values in clang are not handled in similar to GCC, Could someone comment about it.

-rajesh

#2

Hi all,

Could someone comment about it.

Thanks,
Rajesh

#3

_Complex integer types are a GCC extension (the standard specifies only float _Complex, double _Complex and long double _Complex. Apparently clang's support for this GCC extension doesn't perfectly match GCC. I suspect that clang and GCC differ in arithmetic promotion rank here; clang "promotes" the unsigned long long literals to int _Complex (which truncates them), whereas GCC promotes both to unsigned long long _Complex. I haven't actually dug into the issue at all, however.

A couple of work-arounds seem to be available:

  unsigned long long int _Complex lli_val = (unsigned long long int _Complex){5668231682443686620ULL, 7618395480654047911ULL};

  unsigned long long int _Complex lli_val;
  __real__ lli_val = 5668231682443686620ULL;
  __imag__ lli_val = 7618395480654047911ULL;

I believe that both of these will work with recent GCCs as well.

- Steve

#4

From: "Stephen Canon" <scanon@apple.com>
To: "rajesh viswabramana" <viswabramana.rajesh@gmail.com>
Cc: cfe-dev@cs.uiuc.edu
Sent: Friday, February 1, 2013 9:32:00 AM
Subject: Re: [cfe-dev] Complex values handling in Clang

_Complex integer types are a GCC extension (the standard specifies
only float _Complex, double _Complex and long double _Complex.
Apparently clang's support for this GCC extension doesn't perfectly
match GCC. I suspect that clang and GCC differ in arithmetic
promotion rank here; clang "promotes" the unsigned long long
literals to int _Complex (which truncates them), whereas GCC
promotes both to unsigned long long _Complex. I haven't actually
dug into the issue at all, however.

Did Bill just fix this in r174181?

-Hal

#5

Certainly sounds like it from his description.