Subtraction + comparison in one asm instruction?

Several processors (e.g. ARM, Sparc) can do a subtraction and the
associated comparison in only one instruction. Basically, this is done
by performing the subtraction and setting the flags.


However, it seems that some/most C compilers don't take advantage of
this and do both a subtraction and a comparison. This can be seen on
the code


int d;
int f(int c)
{
    d = c - 1;
    return c > 1;
}


or when writing c - 1 > 0 instead of c > 1. Each time, a comparison
is performed separately. A practical use would be a loop of the form:


    while (--c > 0)
        { ... }


(where c is a signed integer).


Are there compilers that do the subtraction and the comparison in
the same asm instruction (for the processors for which there is a
difference)? For the other compilers, have there been discussions,
plans to add such an optimization, etc?

Return to the comp.compilers page.
Search the comp.compilers archives again.