Re: m68k: Simple loop compiles into boundless recursion with -O2

[Fredrik Noring <noring@xxxxxxxxxx>]

Hi Hao,

> I used to run into the same issue around CRT code on x86. Use of
> `-ffreestanding` disables a number of optimizations, for example, the
> compiler cannot optimize
> 
>     int data[4];
>     memset(&data, 0, sizeof(data));
> 
> to a series of store operations, but leave it as a function call, which
> is rather overkill.

With an array initialiser GCC (having -ffreestanding) will optimise

	void g(int *data);
	void f()
	{
		int data[N] = { };
		g(data);
	}

to stores, for N < 15, as observed at <http://franke.ms/cex/z/WEse7e>.
Structures have the nice property in GCC that one can do assignments like

	struct s { int data[N]; } d;
	void f()
	{
		d = (struct s) { };
	}

and once again GCC will store for N < 15 <http://franke.ms/cex/z/KM4ced>.

> The issue in the original post can be resolved by writing through a
> pointer to `volatile char` like this:
> 
>     void *memset2(void *s, int c, unsigned int n)
>     {
>         volatile char *b = s;
>         for (unsigned int i = 0; i < n; i++)
>             b[i] = c;
>         return s;
>     }

The GCC optimiser will also yield with a small amount of loop unrolling,

	void *memset2(void *s, int c, unsigned int n)
	{
		unsigned int i = 0;
		char *b = s;
	
		while (i + 1 < n) {
			b[i++] = c;
			b[i++] = c;
		}
	
		if (i < n)
			b[i] = c;
	
		return s;
	}

and I suppose I will implement something like this in assembly, eventually,
for speed reasons. Storing 32-bit longs rather than bytes is faster still,
but one complication is that the 68000 has address alignment restrictions
for 16-bit and 32-bit load/stores. And then the 68000 has the MOVEM.L
instruction, if one wants to max out on larger sizes. :)

Fredrik