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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43d43f33242sm119320825e9.5.2025.03.22.14.53.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 22 Mar 2025 14:53:15 -0700 (PDT) Date: Sat, 22 Mar 2025 21:53:10 +0000 From: David Laight To: "Arnd Bergmann" Cc: "Tor Vic" , x86@kernel.org, linux-kernel@vger.kernel.org, "Borislav Petkov" , "Thomas Gleixner" , "Ingo Molnar" , "H. Peter Anvin" Subject: Re: [PATCH] arch/x86: Add an option to build the kernel with '-march=native' on x86-64 Message-ID: <20250322215310.7b69b8a1@pumpkin> In-Reply-To: References: <20250321142859.13889-1-torvic9@mailbox.org> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Sat, 22 Mar 2025 12:40:08 +0100 "Arnd Bergmann" wrote: ... > I assume that the difference here is that -march=native on > your machine gets turned into -march=skylake, which then turns > on both additional instructions and a different instruction > scheduler. > > Are you able to quickly run the same tests again using > just one of the two? > > a) -march=x86-64 -mtune=skylake > b) -march=skylake -mtune=generic I've wondered what -mtune=generic is actually optimised for? I've seen gcc convert 32 bit add into an lea and then have to use another instruction to clear the high bits. I've not fiddled with the options to see why it does that. My only guess is it is avoiding false dependencies against the flags register - but the flags are split on non-archaic cpu so that doesn't matter. David