From mboxrd@z Thu Jan  1 00:00:00 1970
From: erik quanstrom <quanstro@quanstro.net>
Date: Thu, 21 Feb 2013 10:37:19 -0500
To: 9fans@9fans.net
Message-ID: <bf865712c818f9c3e91d72798b0a48c2@brasstown.quanstro.net>
MIME-Version: 1.0
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Subject: [9fans] 6c bug
Topicbox-Message-UUID: 1c8a84bc-ead8-11e9-9d60-3106f5b1d025

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=3D2; i<nelem(log2v); i++)
		log2v[i] =3D log2v[i/2] + 1;
}

/* ceil(log=E2=82=82 m) */
int
log2ceil(uintmem m)
{
	uint n;
	int r;

	r =3D (m & (m-1)) !=3D 0;	/* not a power of two =3D> round up */
	n =3D (uint)m;
//print("%llux %ux\n", m, n);
	if(sizeof(uintmem)>sizeof(uint) && n !=3D m){
		n =3D (u64int)m>>32;
		r +=3D 32;
	}
//print("r =3D %d\n", r);
	if((n>>8) =3D=3D 0)
		return log2v[n] + r;
	if((n>>16) =3D=3D 0)
		return 8 + log2v[n>>8] + r;
	if((n>>24) =3D=3D 0)
		return 16 + log2v[n>>16] + r;
	return 24 + log2v[n>>24] + r;
}

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