compiler thoughts

Tuesday January 16, 2007

enumerations

By working on gcc4/gccfss for OpenSolaris project I found one interesting difference between Studio and GCC compilers.
Studio treats all enumration types in C as signed integers, whereas GCC can put them in unsigned type depending on the values of enumerators.
At first it may look like a minor difference, but in Solaris there was this piece of code:

typedef enum {
  A = 0,
  B,
  C
} en_t;

en_t var;

for (;var >= A; var--);

From GCC point of view the expression "var>=A" is always true, so the loop is collapsed into for(;;) even at -O0 optimization level.
From Studio point of view this code is fine.
Both are correct, because the choice of type is implementation defined by C standard.