host-utils: add 128-bit quotient support to divu128/divs128
These will be used to implement new decimal floating point
instructions from Power ISA 3.1.
The remainder is now returned directly by divu128/divs128,
freeing up phigh to receive the high 64 bits of the quotient.
Signed-off-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211025191154.350831-4-luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/util/host-utils.c b/util/host-utils.c
index 701a371..bcc772b 100644
--- a/util/host-utils.c
+++ b/util/host-utils.c
@@ -87,72 +87,117 @@
}
/*
- * Unsigned 128-by-64 division. Returns quotient via plow and
- * remainder via phigh.
- * The result must fit in 64 bits (plow) - otherwise, the result
- * is undefined.
- * This function will cause a division by zero if passed a zero divisor.
+ * Unsigned 128-by-64 division.
+ * Returns the remainder.
+ * Returns quotient via plow and phigh.
+ * Also returns the remainder via the function return value.
*/
-void divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor)
+uint64_t divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor)
{
uint64_t dhi = *phigh;
uint64_t dlo = *plow;
- unsigned i;
- uint64_t carry = 0;
+ uint64_t rem, dhighest;
+ int sh;
if (divisor == 0 || dhi == 0) {
*plow = dlo / divisor;
- *phigh = dlo % divisor;
+ *phigh = 0;
+ return dlo % divisor;
} else {
+ sh = clz64(divisor);
- for (i = 0; i < 64; i++) {
- carry = dhi >> 63;
- dhi = (dhi << 1) | (dlo >> 63);
- if (carry || (dhi >= divisor)) {
- dhi -= divisor;
- carry = 1;
- } else {
- carry = 0;
+ if (dhi < divisor) {
+ if (sh != 0) {
+ /* normalize the divisor, shifting the dividend accordingly */
+ divisor <<= sh;
+ dhi = (dhi << sh) | (dlo >> (64 - sh));
+ dlo <<= sh;
}
- dlo = (dlo << 1) | carry;
+
+ *phigh = 0;
+ *plow = udiv_qrnnd(&rem, dhi, dlo, divisor);
+ } else {
+ if (sh != 0) {
+ /* normalize the divisor, shifting the dividend accordingly */
+ divisor <<= sh;
+ dhighest = dhi >> (64 - sh);
+ dhi = (dhi << sh) | (dlo >> (64 - sh));
+ dlo <<= sh;
+
+ *phigh = udiv_qrnnd(&dhi, dhighest, dhi, divisor);
+ } else {
+ /**
+ * dhi >= divisor
+ * Since the MSB of divisor is set (sh == 0),
+ * (dhi - divisor) < divisor
+ *
+ * Thus, the high part of the quotient is 1, and we can
+ * calculate the low part with a single call to udiv_qrnnd
+ * after subtracting divisor from dhi
+ */
+ dhi -= divisor;
+ *phigh = 1;
+ }
+
+ *plow = udiv_qrnnd(&rem, dhi, dlo, divisor);
}
- *plow = dlo;
- *phigh = dhi;
+ /*
+ * since the dividend/divisor might have been normalized,
+ * the remainder might also have to be shifted back
+ */
+ return rem >> sh;
}
}
/*
- * Signed 128-by-64 division. Returns quotient via plow and
- * remainder via phigh.
- * The result must fit in 64 bits (plow) - otherwise, the result
- * is undefined.
- * This function will cause a division by zero if passed a zero divisor.
+ * Signed 128-by-64 division.
+ * Returns quotient via plow and phigh.
+ * Also returns the remainder via the function return value.
*/
-void divs128(int64_t *plow, int64_t *phigh, int64_t divisor)
+int64_t divs128(uint64_t *plow, int64_t *phigh, int64_t divisor)
{
- int sgn_dvdnd = *phigh < 0;
- int sgn_divsr = divisor < 0;
+ bool neg_quotient = false, neg_remainder = false;
+ uint64_t unsig_hi = *phigh, unsig_lo = *plow;
+ uint64_t rem;
- if (sgn_dvdnd) {
- *plow = ~(*plow);
- *phigh = ~(*phigh);
- if (*plow == (int64_t)-1) {
+ if (*phigh < 0) {
+ neg_quotient = !neg_quotient;
+ neg_remainder = !neg_remainder;
+
+ if (unsig_lo == 0) {
+ unsig_hi = -unsig_hi;
+ } else {
+ unsig_hi = ~unsig_hi;
+ unsig_lo = -unsig_lo;
+ }
+ }
+
+ if (divisor < 0) {
+ neg_quotient = !neg_quotient;
+
+ divisor = -divisor;
+ }
+
+ rem = divu128(&unsig_lo, &unsig_hi, (uint64_t)divisor);
+
+ if (neg_quotient) {
+ if (unsig_lo == 0) {
+ *phigh = -unsig_hi;
*plow = 0;
- (*phigh)++;
- } else {
- (*plow)++;
- }
+ } else {
+ *phigh = ~unsig_hi;
+ *plow = -unsig_lo;
+ }
+ } else {
+ *phigh = unsig_hi;
+ *plow = unsig_lo;
}
- if (sgn_divsr) {
- divisor = 0 - divisor;
- }
-
- divu128((uint64_t *)plow, (uint64_t *)phigh, (uint64_t)divisor);
-
- if (sgn_dvdnd ^ sgn_divsr) {
- *plow = 0 - *plow;
+ if (neg_remainder) {
+ return -rem;
+ } else {
+ return rem;
}
}
#endif