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[88.21.201.17]) by smtp.gmail.com with ESMTPSA id j16sm35246581wmi.2.2021.03.15.02.10.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Mar 2021 02:10:39 -0700 (PDT) Subject: Re: [PATCH] utils: Use fma in qemu_strtosz To: Richard Henderson , qemu-devel@nongnu.org References: <20210314234821.1954428-1-richard.henderson@linaro.org> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <0697b6d1-0a64-3d71-2f7f-3c52a005b77b@redhat.com> Date: Mon, 15 Mar 2021 10:10:37 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210314234821.1954428-1-richard.henderson@linaro.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, 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: peter.maydell@linaro.org, berrange@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/15/21 12:48 AM, Richard Henderson wrote: > Use fma to simulatneously scale and round up fraction. "simultaneously" > The libm function will always return a properly rounded double precision > value, which will eliminate any extra precision the x87 co-processor may > give us, which will keep the output predictable vs other hosts. > > Adding DBL_EPSILON while scaling should help with fractions like > 12.345, where the closest representable number is actually 12.3449*. > > Signed-off-by: Richard Henderson > --- > util/cutils.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/util/cutils.c b/util/cutils.c > index d89a40a8c3..f7f8e48a68 100644 > --- a/util/cutils.c > +++ b/util/cutils.c > @@ -342,7 +342,7 @@ static int do_strtosz(const char *nptr, const char **end, > if (val > (UINT64_MAX - ((uint64_t) (fraction * mul))) / mul) { Shouldn't we use fma() here too? ^^^^^^^^^^^^^^^^^^^^^^^^^^ > retval = -ERANGE; > goto out; > } > - *result = val * mul + (uint64_t) (fraction * mul); > + *result = val * mul + (uint64_t)fma(fraction, mul, DBL_EPSILON); > retval = 0; > > out: >