[9fans] 6c bug

[9fans] 6c bug

[erik quanstrom]

uncomment the prints, and the function will return the
correct value.

it looks like the compiler is aliasing n and m, from 6c -S:

TEXT	log2ceil+0(SB),0,$16
	MOVQ	BP,SI
	MOVQ	BP,AX
	DECQ	,AX
	MOVQ	BP,CX
	ANDQ	AX,CX
	CMPQ	CX,$0
	JEQ	,3(PC)
	MOVL	$1,AX
	JMP	,2(PC)
	MOVL	$0,AX
	MOVL	AX,BX
	MOVL	SI,DX
	JMP	,2(PC)
	JMP	,3(PC)
>>	CMPQ	SI,SI
	JNE	,2(PC)
	JMP	,5(PC)
	MOVQ	SI,AX
	SHRQ	$32,AX
	MOVL	AX,DX
	ADDL	$32,BX
[etc].

- erik

#include <u.h>
#include <libc.h>
typedef u64int	uintmem;

static	uchar	log2v[256];

void
log2init(void)
{
	int i;

	for(i=2; i<nelem(log2v); i++)
		log2v[i] = log2v[i/2] + 1;
}

/* ceil(log₂ m) */
int
log2ceil(uintmem m)
{
	uint n;
	int r;

	r = (m & (m-1)) != 0;	/* not a power of two => round up */
	n = (uint)m;
//print("%llux %ux\n", m, n);
	if(sizeof(uintmem)>sizeof(uint) && n != m){
		n = (u64int)m>>32;
		r += 32;
	}
//print("r = %d\n", r);
	if((n>>8) == 0)
		return log2v[n] + r;
	if((n>>16) == 0)
		return 8 + log2v[n>>8] + r;
	if((n>>24) == 0)
		return 16 + log2v[n>>16] + r;
	return 24 + log2v[n>>24] + r;
}

void
main(void)
{
	log2init();
	print("%d\n", log2ceil(0x430000000ull));
	exits("");
}