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Thu, 21 Oct 2021 13:11:36 -0700 (PDT) Subject: Re: [PATCH v3 05/22] host-utils: move checks out of divu128/divs128 To: Luis Pires , qemu-devel@nongnu.org, qemu-ppc@nongnu.org References: <20210910112624.72748-1-luis.pires@eldorado.org.br> <20210910112624.72748-6-luis.pires@eldorado.org.br> From: Richard Henderson Message-ID: Date: Thu, 21 Oct 2021 13:11:35 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20210910112624.72748-6-luis.pires@eldorado.org.br> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::102f; envelope-from=richard.henderson@linaro.org; helo=mail-pj1-x102f.google.com X-Spam_score_int: -49 X-Spam_score: -5.0 X-Spam_bar: ----- X-Spam_report: (-5.0 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-2.867, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: =?UTF-8?B?RnLDqWTDqXJpYyBQw6l0cm90?= , groug@kaod.org, david@gibson.dropbear.id.au Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 9/10/21 4:26 AM, Luis Pires wrote: > 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 Reviewed-by: Richard Henderson Frederic, I had forgotten about this work that Luis has done here and in the next 3 patches. This overlaps with what you were doing in the RV128 patch set wrt getting a proper 128-bit divide. r~ > --- > include/hw/clock.h | 5 +++-- > include/qemu/host-utils.h | 36 +++++++++++++---------------------- > target/ppc/int_helper.c | 14 +++++++++----- > util/host-utils.c | 40 ++++++++++++++++++--------------------- > 4 files changed, 43 insertions(+), 52 deletions(-) > > diff --git a/include/hw/clock.h b/include/hw/clock.h > index a7187eab95..5a40a076aa 100644 > --- a/include/hw/clock.h > +++ b/include/hw/clock.h > @@ -320,8 +320,9 @@ 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 ca9f3f021b..e82e6239af 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 f5dac3aa87..510faf24cf 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 a789a11b46..701a371843 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 > >