[ILUG] gcc optimisation weirdness?

[Kenn Humborg]

> gcc seems to do weird things to the following:
>
> int input, frequency[9];
>
> //frequency[] initialised to 0.
>
> do {
>                 scanf("%d", &input);
>                 if (input > 0 && input < 11) {
> 			//point 1
>                         (frequency[input-1])++;
> 			//point 2
>                         }
>         } while (input);
>
> if i compile with -O2 or -O3 it works as i expect it to, but if i compile
> with "gcc -o test test.c" then:
>
> it works correctly for digits 0 thru 9, however if i enter 10 it goes
> wrong: at point 1 input == 10, at point 2 input == 11!

Very odd...  I'd expect that the statement

   (f[i])++

would not be legal.  Is (var) a legal lvalue?  Aha... info gcc
(under the C Extensions section) says that

   File: gcc.info,  Node: Lvalues,  Next: Conditionals,  Prev: Typeof,  Up:
C Extensions

   Generalized Lvalues
   ===================

      Compound expressions, conditional expressions and casts are allowed
   as lvalues provided their operands are lvalues.  This means that you
   can take their addresses or store values into them.

      Standard C++ allows compound expressions and conditional expressions
   as lvalues, and permits casts to reference type, so use of this
   extension is deprecated for C++ code.

      For example, a compound expression can be assigned, provided the last
   expression in the sequence is an lvalue.  These two expressions are
   equivalent:

        (a, b) += 5
        a, (b += 5)

      Similarly, the address of the compound expression can be taken.
   These two expressions are equivalent:

        &(a, b)
        a, &b

      A conditional expression is a valid lvalue if its type is not void
   and the true and false branches are both valid lvalues.  For example,
   these two expressions are equivalent:

        (a ? b : c) = 5
        (a ? b = 5 : (c = 5))

Hmmm...

> PS: is there anyway to initialise an array at declaration time, rather
> than iterate thru the array and set each element manually? eg something
> like: int array[9]=0;

You mean like:

   int array[9]={0, 1, 2, 3, 4, 3, 2, 1, 0};

That is standard C.  GCC also has some extensions that allow
you to do fancied stuff with initializers.  Take a look at
the C and C++ Extensions sections in the GCC info pages.

Later,
Kenn