Commit ff7a1eb0 authored by Richard Henderson's avatar Richard Henderson Committed by Blue Swirl
Browse files

host-utils: Improve mulu64 and muls64 



The new formulation makes better use of add-with-carry type insns
that the host may have.  Use gcc's sign adjustment trick to avoid
having to perform a 128-bit negation.

Signed-off-by: default avatarRichard Henderson <rth@twiddle.net>
Signed-off-by: default avatarBlue Swirl <blauwirbel@gmail.com>
parent f540166b

util/host-utils.c

+38 −54
Original line number Diff line number Diff line
@@ -27,79 +27,63 @@ 
#include <stdint.h>

#include "qemu/host-utils.h"



//#define DEBUG_MULDIV



/* Long integer helpers */

#ifndef CONFIG_INT128

static void add128 (uint64_t *plow, uint64_t *phigh, uint64_t a, uint64_t b)

{

    *plow += a;

    /* carry test */

    if (*plow < a)

        (*phigh)++;

    *phigh += b;

}



static void neg128 (uint64_t *plow, uint64_t *phigh)

static inline void mul64(uint64_t *plow, uint64_t *phigh,

                         uint64_t a, uint64_t b)

{

    *plow = ~*plow;

    *phigh = ~*phigh;

    add128(plow, phigh, 1, 0);

}



static void mul64 (uint64_t *plow, uint64_t *phigh, uint64_t a, uint64_t b)

{

    uint32_t a0, a1, b0, b1;

    uint64_t v;



    a0 = a;

    a1 = a >> 32;



    b0 = b;

    b1 = b >> 32;

    typedef union {

        uint64_t ll;

        struct {

#ifdef HOST_WORDS_BIGENDIAN

            uint32_t high, low;

#else

            uint32_t low, high;

#endif

        } l;

    } LL;

    LL rl, rm, rn, rh, a0, b0;

    uint64_t c;



    v = (uint64_t)a0 * (uint64_t)b0;

    *plow = v;

    *phigh = 0;

    a0.ll = a;

    b0.ll = b;



    v = (uint64_t)a0 * (uint64_t)b1;

    add128(plow, phigh, v << 32, v >> 32);

    rl.ll = (uint64_t)a0.l.low * b0.l.low;

    rm.ll = (uint64_t)a0.l.low * b0.l.high;

    rn.ll = (uint64_t)a0.l.high * b0.l.low;

    rh.ll = (uint64_t)a0.l.high * b0.l.high;



    v = (uint64_t)a1 * (uint64_t)b0;

    add128(plow, phigh, v << 32, v >> 32);

    c = (uint64_t)rl.l.high + rm.l.low + rn.l.low;

    rl.l.high = c;

    c >>= 32;

    c = c + rm.l.high + rn.l.high + rh.l.low;

    rh.l.low = c;

    rh.l.high += (uint32_t)(c >> 32);



    v = (uint64_t)a1 * (uint64_t)b1;

    *phigh += v;

    *plow = rl.ll;

    *phigh = rh.ll;

}



/* Unsigned 64x64 -> 128 multiplication */

void mulu64 (uint64_t *plow, uint64_t *phigh, uint64_t a, uint64_t b)

{

    mul64(plow, phigh, a, b);

#if defined(DEBUG_MULDIV)

    printf("mulu64: 0x%016llx * 0x%016llx = 0x%016llx%016llx\n",

           a, b, *phigh, *plow);

#endif

}



/* Signed 64x64 -> 128 multiplication */

void muls64 (uint64_t *plow, uint64_t *phigh, int64_t a, int64_t b)

{

    int sa, sb;

    uint64_t rh;



    sa = (a < 0);

    if (sa)

        a = -a;

    sb = (b < 0);

    if (sb)

        b = -b;

    mul64(plow, phigh, a, b);

    if (sa ^ sb) {

        neg128(plow, phigh);

    mul64(plow, &rh, a, b);



    /* Adjust for signs.  */

    if (b < 0) {

        rh -= a;

    }

#if defined(DEBUG_MULDIV)

    printf("muls64: 0x%016llx * 0x%016llx = 0x%016llx%016llx\n",

           a, b, *phigh, *plow);

#endif

    if (a < 0) {

        rh -= b;

    }

    *phigh = rh;

}

#endif /* !CONFIG_INT128 */