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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43eda4662a0sm101920725e9.36.2025.04.07.13.23.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 07 Apr 2025 13:23:11 -0700 (PDT) Date: Mon, 7 Apr 2025 21:23:10 +0100 From: David Laight To: Linus Torvalds Cc: Kees Cook , linux-kernel@vger.kernel.org, Andrey Konovalov , Andy Shevchenko , Catalin Marinas , Nathan Chancellor , Peter Collingbourne , Vincenzo Frascino , Will Deacon Subject: Re: [GIT PULL] string fixes for v6.15-rc1 Message-ID: <20250407212310.0a934bad@pumpkin> In-Reply-To: References: <202504061053.F27227CA@keescook> 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 Sun, 6 Apr 2025 19:04:29 -0700 Linus Torvalds wrote: ... > For something like wcslen() the answer is "DON'T DO THIS". Because > there is absolutely zero upside to trying to recognize this pattern, > and there is real downside. gcc also has a nasty habit of converting: for (i = 0; i < len; i++) dst[i] = src[i]; into a call to memcpy(). If I wanted a memcpy() call I'd write one - so will most people. But if 'len' is very small (may even known to be less than, say, 4) you really want the loop - which is why it was written. I've even seen (not gcc) it converted to a 'rep movsw' 'rep movsb' pair at a time when a P4 might have been a likely target cpu. The 0 to 3 byte 'rep movsb' had a setup cost of IIRC 150 clocks. David