jslog2.c

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
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 * for the specific language governing rights and limitations under the
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 * The Original Code is Mozilla Communicator client code, released
 * March 31, 1998.
 *
 * The Initial Developer of the Original Code is
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#include "jsstddef.h"
#include "jsbit.h"
#include "jsutil.h"

/*
** Compute the log of the least power of 2 greater than or equal to n
*/
JS_PUBLIC_API(JSIntn) JS_CeilingLog2(JSUint32 n)
{
    JSIntn log2;

    JS_CEILING_LOG2(log2, n);
    return log2;
}

/*
** Compute the log of the greatest power of 2 less than or equal to n.
** This really just finds the highest set bit in the word.
*/
JS_PUBLIC_API(JSIntn) JS_FloorLog2(JSUint32 n)
{
    JSIntn log2;

    JS_FLOOR_LOG2(log2, n);
    return log2;
}

/*
 * js_FloorLog2wImpl has to be defined only for 64-bit non-GCC case.
 */
#if !defined(JS_HAS_GCC_BUILTIN_CLZ) && JS_BYTES_PER_WORD == 8

JSUword
js_FloorLog2wImpl(JSUword n)
{
    JSUword log2, m;

    JS_ASSERT(n != 0);

    log2 = 0;
    m = n >> 32;
    if (m != 0) { n = m; log2 = 32; }
    m = n >> 16;
    if (m != 0) { n = m; log2 |= 16; }
    m = n >> 8;
    if (m != 0) { n = m; log2 |= 8; }
    m = n >> 4;
    if (m != 0) { n = m; log2 |= 4; }
    m = n >> 2;
    if (m != 0) { n = m; log2 |= 2; }
    log2 |= (n >> 1);

    return log2;
}

#endif