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[187.189.51.144]) by smtp.gmail.com with ESMTPSA id f12sm10227664qti.63.2021.03.15.06.20.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Mar 2021 06:20:07 -0700 (PDT) Subject: Re: [PATCH] utils: Use fma in qemu_strtosz To: Thomas Huth , qemu-devel@nongnu.org References: <20210314234821.1954428-1-richard.henderson@linaro.org> <8422f58c-743b-e028-a116-b7c3b507e3c9@redhat.com> From: Richard Henderson Message-ID: Date: Mon, 15 Mar 2021 07:20:04 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <8422f58c-743b-e028-a116-b7c3b507e3c9@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::82d; envelope-from=richard.henderson@linaro.org; helo=mail-qt1-x82d.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, 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 3/14/21 11:32 PM, Thomas Huth wrote: > On 15/03/2021 00.48, Richard Henderson wrote: >> Use fma to simulatneously scale and round up fraction. >> >> 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, >>           retval = -ERANGE; >>           goto out; >>       } >> -    *result = val * mul + (uint64_t) (fraction * mul); >> +    *result = val * mul + (uint64_t)fma(fraction, mul, DBL_EPSILON); >>       retval = 0; >>   out: > > Will this fix the failure that we're currently seeing with 32-bit builds? Yes. https://gitlab.com/rth7680/qemu/-/pipelines/270311986