host-utils: move checks out of divu128/divs128

In preparation for changing the divu128/divs128 implementations
to allow for quotients larger than 64 bits, move the div-by-zero
and overflow checks to the callers.

Signed-off-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211025191154.350831-2-luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

4 files changed, 43 insertions, 52 deletions

diff --git a/include/hw/clock.h b/include/hw/clock.h
index 11f67fb..7443e6c 100644
--- a/include/hw/clock.h
+++ b/include/hw/clock.h
@@ -324,8 +324,9 @@ static inline uint64_t clock_ns_to_ticks(const Clock *clk, uint64_t ns)
         return 0;
     }
     /*
-     * Ignore divu128() return value as we've caught div-by-zero and don't
-     * need different behaviour for overflow.
+     * BUG: when CONFIG_INT128 is not defined, the current implementation of
+     * divu128 does not return a valid truncated quotient, so the result will
+     * be wrong.
      */
     divu128(&lo, &hi, clk->period);
     return lo;
diff --git a/include/qemu/host-utils.h b/include/qemu/host-utils.h
index ca9f3f0..e82e623 100644
--- a/include/qemu/host-utils.h
+++ b/include/qemu/host-utils.h
@@ -52,36 +52,26 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
     return (__int128_t)a * b / c;
 }
 
-static inline int divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor)
-{
-    if (divisor == 0) {
-        return 1;
-    } else {
-        __uint128_t dividend = ((__uint128_t)*phigh << 64) | *plow;
-        __uint128_t result = dividend / divisor;
-        *plow = result;
-        *phigh = dividend % divisor;
-        return result > UINT64_MAX;
-    }
+static inline void divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor)
+{
+    __uint128_t dividend = ((__uint128_t)*phigh << 64) | *plow;
+    __uint128_t result = dividend / divisor;
+    *plow = result;
+    *phigh = dividend % divisor;
 }
 
-static inline int divs128(int64_t *plow, int64_t *phigh, int64_t divisor)
+static inline void divs128(int64_t *plow, int64_t *phigh, int64_t divisor)
 {
-    if (divisor == 0) {
-        return 1;
-    } else {
-        __int128_t dividend = ((__int128_t)*phigh << 64) | (uint64_t)*plow;
-        __int128_t result = dividend / divisor;
-        *plow = result;
-        *phigh = dividend % divisor;
-        return result != *plow;
-    }
+    __int128_t dividend = ((__int128_t)*phigh << 64) | (uint64_t)*plow;
+    __int128_t result = dividend / divisor;
+    *plow = result;
+    *phigh = dividend % divisor;
 }
 #else
 void muls64(uint64_t *plow, uint64_t *phigh, int64_t a, int64_t b);
 void mulu64(uint64_t *plow, uint64_t *phigh, uint64_t a, uint64_t b);
-int divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor);
-int divs128(int64_t *plow, int64_t *phigh, int64_t divisor);
+void divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor);
+void divs128(int64_t *plow, int64_t *phigh, int64_t divisor);
 
 static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
 {
diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
index f5dac3a..510faf2 100644
--- a/target/ppc/int_helper.c
+++ b/target/ppc/int_helper.c
@@ -104,10 +104,11 @@ uint64_t helper_divdeu(CPUPPCState *env, uint64_t ra, uint64_t rb, uint32_t oe)
     uint64_t rt = 0;
     int overflow = 0;
 
-    overflow = divu128(&rt, &ra, rb);
-
-    if (unlikely(overflow)) {
+    if (unlikely(rb == 0 || ra >= rb)) {
+        overflow = 1;
         rt = 0; /* Undefined */
+    } else {
+        divu128(&rt, &ra, rb);
     }
 
     if (oe) {
@@ -122,10 +123,13 @@ uint64_t helper_divde(CPUPPCState *env, uint64_t rau, uint64_t rbu, uint32_t oe)
     int64_t rt = 0;
     int64_t ra = (int64_t)rau;
     int64_t rb = (int64_t)rbu;
-    int overflow = divs128(&rt, &ra, rb);
+    int overflow = 0;
 
-    if (unlikely(overflow)) {
+    if (unlikely(rb == 0 || uabs64(ra) >= uabs64(rb))) {
+        overflow = 1;
         rt = 0; /* Undefined */
+    } else {
+        divs128(&rt, &ra, rb);
     }
 
     if (oe) {
diff --git a/util/host-utils.c b/util/host-utils.c
index a789a11..701a371 100644
--- a/util/host-utils.c
+++ b/util/host-utils.c
@@ -86,24 +86,23 @@ void muls64 (uint64_t *plow, uint64_t *phigh, int64_t a, int64_t b)
     *phigh = rh;
 }
 
-/* Unsigned 128x64 division.  Returns 1 if overflow (divide by zero or */
-/* quotient exceeds 64 bits).  Otherwise returns quotient via plow and */
-/* remainder via phigh. */
-int divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor)
+/*
+ * 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.
+ */
+void divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor)
 {
     uint64_t dhi = *phigh;
     uint64_t dlo = *plow;
     unsigned i;
     uint64_t carry = 0;
 
-    if (divisor == 0) {
-        return 1;
-    } else if (dhi == 0) {
+    if (divisor == 0 || dhi == 0) {
         *plow  = dlo / divisor;
         *phigh = dlo % divisor;
-        return 0;
-    } else if (dhi >= divisor) {
-        return 1;
     } else {
 
         for (i = 0; i < 64; i++) {
@@ -120,15 +119,20 @@ int divu128(uint64_t *plow, uint64_t *phigh, uint64_t divisor)
 
         *plow = dlo;
         *phigh = dhi;
-        return 0;
     }
 }
 
-int divs128(int64_t *plow, int64_t *phigh, int64_t divisor)
+/*
+ * 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.
+ */
+void divs128(int64_t *plow, int64_t *phigh, int64_t divisor)
 {
     int sgn_dvdnd = *phigh < 0;
     int sgn_divsr = divisor < 0;
-    int overflow = 0;
 
     if (sgn_dvdnd) {
         *plow = ~(*plow);
@@ -145,19 +149,11 @@ int divs128(int64_t *plow, int64_t *phigh, int64_t divisor)
         divisor = 0 - divisor;
     }
 
-    overflow = divu128((uint64_t *)plow, (uint64_t *)phigh, (uint64_t)divisor);
+    divu128((uint64_t *)plow, (uint64_t *)phigh, (uint64_t)divisor);
 
     if (sgn_dvdnd  ^ sgn_divsr) {
         *plow = 0 - *plow;
     }
-
-    if (!overflow) {
-        if ((*plow < 0) ^ (sgn_dvdnd ^ sgn_divsr)) {
-            overflow = 1;
-        }
-    }
-
-    return overflow;
 }
 #endif