why GCC does implicit promotion to unsigned char?

[Konstantin Vladimirov]

Hi,

Consider code:

char A;
char B;

char sum_A_B ( void )
{
  char sum = A + B;

  return sum;
}


Compile it on any backend (for example x86):
gcc (SUSE Linux) 4.5.0 20100604 [gcc-4_5-branch revision 160292]

gcc -m32 -O2 -S repro.c -fdump-tree-all-lineno-details

Now look at repro.c.004t.gimple:

sum_A_B ()
[repro.c : 5:1] {
  char A.0;
  unsigned char A.1;
  char B.2;
  unsigned char B.3;
  unsigned char D.1990;
  char D.1991;
  char sum;

  [repro.c : 6:8] A.0 = A;
  [repro.c : 6:8] A.1 = (unsigned char) A.0;
  [repro.c : 6:8] B.2 = B;
  [repro.c : 6:8] B.3 = (unsigned char) B.2;
  [repro.c : 6:8] D.1990 = A.1 + B.3;
  [repro.c : 6:8] sum = (char) D.1990;
  [repro.c : 8:3] D.1991 = sum;
  [repro.c : 8:3] return D.1991;
}

It looks really weird. Why gcc promotes char to unsigned char internally?

This case is simple repro, but in my production code it is a reason of
productivity overhead (vectorizer fails on such promotions).

Thanks in advance for any responses. Previous discussion about inline
assembler was really helpful.

---
With best regards, Konstantin