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[66.27.223.101]) by smtp.gmail.com with ESMTPSA id e16-20020a170902cf5000b001d77a0e1374sm147972plg.151.2024.02.08.12.07.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 Feb 2024 12:07:29 -0800 (PST) Message-ID: <099ba824-7e79-4c2b-a548-5922d69a5720@linaro.org> Date: Thu, 8 Feb 2024 10:07:26 -1000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/6] util/bufferiszero: introduce an inline wrapper Content-Language: en-US To: Alexander Monakov Cc: qemu-devel@nongnu.org, Mikhail Romanov , Paolo Bonzini References: <20240206204809.9859-1-amonakov@ispras.ru> <20240206204809.9859-3-amonakov@ispras.ru> From: Richard Henderson In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::631; envelope-from=richard.henderson@linaro.org; helo=mail-pl1-x631.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, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2/6/24 21:13, Alexander Monakov wrote: > Thank you. I agree we shouldn't penalize non-x86 hosts here, but to be honest > I'd really like to keep this optimization because so many places in Qemu invoke > buffer_is_zero with a constant length, allowing the compiler to optimize out > the length test. Hmm. True, both migration and image copy use large blocks frequently. > Would you be open to testing availability of optimized variants > in the inline wrapper like this: > > diff --git a/include/qemu/cutils.h b/include/qemu/cutils.h > index 62b153e603..7a2145ffef 100644 > --- a/include/qemu/cutils.h > +++ b/include/qemu/cutils.h > @@ -209,11 +209,12 @@ static inline bool buffer_is_zero(const void *vbuf, size_t len) > return true; > } > > +#if defined(CONFIG_AVX2_OPT) || defined(__SSE2__) > if (len >= 256) { > return buffer_is_zero_len_256_plus(vbuf, len); > - } else { > - return buffer_is_zero_len_4_plus(vbuf, len); > } > +#endif > + return buffer_is_zero_len_4_plus(vbuf, len); Plausible. Also, now that we're down to two variants instead of four, perhaps a statically predicted direct branch or two might be better than an indirect branch? E.g. bool buffer_is_zero_len_256_plus(buf, len) { #ifdef CONFIG_AVX2_OPT if (select_accel & CPUINFO_AVX2) { return buffer_zero_avx2(buf, len); } #endif #ifdef __SSE2__ if (select_accel & CPUINFO_SSE2) { return buffer_zero_sse2(buf, len); } #endif return buffer_is_zero_len_4_plus(buf, len); } where select_accel would be set by test_buffer_is_zero_next_accel() etc. r~