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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-487306acb47sm3065875e9.15.2026.03.27.12.57.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 27 Mar 2026 12:57:43 -0700 (PDT) From: david.laight.linux@gmail.com To: Andrew Morton , Kees Cook , Andy Shevchenko , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, Linus Torvalds Cc: David Laight Subject: [PATCH next] string: Optimise strlen() Date: Fri, 27 Mar 2026 19:57:37 +0000 Message-Id: <20260327195737.89537-1-david.laight.linux@gmail.com> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: David Laight Unrolling the loop once significantly improves performance on some CPU. Userspace testing on a Zen-5 shows it runs at two bytes/clock rather than one byte/clock with only a marginal additional overhead. Using 'byte masking' is faster for longer strings - the break-even point is around 56 bytes on the same Zen-5 (there is much larger overhead, then it runs at 16 bytes in 3 clocks). But the majority of kernel calls won't be near that length. There will also be extra overhead for big-endian systems and those without a fast ffs(). Signed-off-by: David Laight --- For reference 'rep scasb' comes in at 150 + 3 per byte on Zen-5. I've not tested any Intel CPU, I don't think they can run a '1 clock loop' but the change might improve performance from 2 clocks/byte to 1 clock/byte. I can test Intel up to i7-7xxx but don't have any older AMD CPU or any other architecutes (apart from a pi-5). Other architectures may well see an improvement. If only because of a reduced number of taken branches. I did notice that arm64 uses a very large asm block that is clearly optimised for very long strings - I suspect the C version will be faster in the kernel. lib/string.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/lib/string.c b/lib/string.c index b632c71df1a5..31de9aa86409 100644 --- a/lib/string.c +++ b/lib/string.c @@ -415,11 +415,13 @@ EXPORT_SYMBOL(strnchr); #ifndef __HAVE_ARCH_STRLEN size_t strlen(const char *s) { - const char *sc; + size_t len; - for (sc = s; *sc != '\0'; ++sc) - /* nothing */; - return sc - s; + for (len = 0; likely(s[len]); len += 2) { + if (!s[len + 1]) + return len + 1; + } + return len; } EXPORT_SYMBOL(strlen); #endif -- 2.39.5