From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wr1-f53.google.com (mail-wr1-f53.google.com [209.85.221.53]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6DAED345CA5 for ; Tue, 20 Jan 2026 21:46:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.53 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768945585; cv=none; b=EC/ujVrkcBITIBglND7DKSHQSJ1hG6+rl8nFIixFZ2ngXGa1rcjfG0WCFE61Z0itNcMy7fT5yfr0hQQZj89H38MNWoNj+wuif6j/G0PekZIFg3z3e3SipCYdZpycEPG+xmW4ESGBFTC/ehoTpGthWiqj59O2NEEs4TAyte/XuHI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768945585; c=relaxed/simple; bh=3khI2KFCyX6nCuLsFR/Ob36rf63ptQZrVn6eGXlqFiU=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=nE8bnPr+SFT/psKYfTkMElPj4540o5yRH0I3fdTZ7kRu9GpfmCAASgXr8IAcjlhMuwdJ6gzkXG501bIiDJjAhFHDAXvHrAPAlSO1c6iz7zf62wLAPtWz8dQEvfK9OH/BOusT/qSTh8Wf1w1GDULA0lMlBnIsVNwaW7JlSdMOC3s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=NUwVouti; arc=none smtp.client-ip=209.85.221.53 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="NUwVouti" Received: by mail-wr1-f53.google.com with SMTP id ffacd0b85a97d-42fb5810d39so4438705f8f.2 for ; Tue, 20 Jan 2026 13:46:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1768945579; x=1769550379; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:subject:cc:to:from:date:from:to:cc:subject:date :message-id:reply-to; bh=ocLXKSDLVOhiyhUWjD1e+8YJMBcInzZT8TNvO6TIBuI=; b=NUwVoutiKFLKUup8AQ2E1RSrn5kQVHncfW2YeAhWmoUfuxRqj67MoIEbinDtG4ECi9 xpulXQ0raIE/gkrBZfehF6fkDxKN7twIiJ5o1lr5sIpNZ9qs4rwEoESoJkdxS+bqolS8 vdR0c6AXogdAarmJLMhXm8MjWbPRMj0ScohBCyW/3CQ4JFifo7qb3Imcoh0TIWNHJVIq ueYpE4JzL0RBy3zEsf53b++2xJDwV415C/y+qXUMO6N/V+jhUPOqAqT0hiQK9+VapNRH NKadVX7UsshLTB810WR37xmGGujiR8VW8rG5aDl6HJYsgtj2ro4PjPLH0TJlEbme8wrW T3XQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1768945579; x=1769550379; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to; bh=ocLXKSDLVOhiyhUWjD1e+8YJMBcInzZT8TNvO6TIBuI=; b=mbsT+U3vG78HeucwruUp16+cRiKEgoIJy7M4Y1DsGSWoh4BfQrHZ2Pf3MQhA/+W7dn uA6BYnR2nczEE2qOe1Wv6xcf1fxw6aQVAqj/ZwAyNdDXb8dxpxQudujc068f4nfPmgs9 /oQGMjcPN5To6OFj8MyL3YVxUQo5agvzbufLF7NDK0ijKz4c2UkmPEW/5IqCYoClq96m XbtnWLX27R/0e6ier3UTuTWnhNs2tVKLHPWCWbI1BT6DSWzXh4SikIwzNIJpz8hrfkum CP6sn2l9WQuw3+Tja+/YTF2PqgrsHEgo8s/FA0kvChYpAkdMznnSTbP5ZTV7DMtZ6ZmA ALUg== X-Forwarded-Encrypted: i=1; AJvYcCVHl4GGCMDGO+yZn2DVyJk9SsTv/zAlLEYmCVNoN0+HWqcMN9SPz4Z2XW9Ves+Tj1xmn/JDOFKOEq5HDyw=@vger.kernel.org X-Gm-Message-State: AOJu0YxyQWKJxnft4wvFglnmgaoBAe1cE6gZQ1b0EgTdy0PtxuZ8akN2 u7B6tG4HiNQ5Wz2qWlvHYI3nOdUJZQcJPDKeue+Ktv7XQv/qt5QDSkAj X-Gm-Gg: AZuq6aKJaJs6Sq97tl69ejgMZei+xM5X7KYrscJx8spoasVzFihrp/sCaoYal2pp0ke SDkq5fiIUOQYbVWoWL5V0jsMMXgHyfSFMAMbxNshwruSuWGAJjeyMnTIYm9LzOTJl4c0E8VT+AL Aii5zi2y9HaOHVKEapUiqcUfcGSqe+RDRiczW+DWV9Oc+hKGi2b7SOVw3YimKg2kzbVrbAa/3YQ Yql+0cNth0qPvLpGk/su5FaNoXznb534WhY8lbZxqI/aaywln1eNQ39gPpKQ4/u6VgZ2GlhAbdX 3z6GH//sz9vwZjMRrLiRRQEy8EWF3L7+7PlXONXusnW1j+yPygGs0uwIfW+zg+hmErFeCdozeuq DlklHlk+QmEJYB9LzpSt3MtjY6ZyxcnA+udwjsI/UyuFpicWMozjs6imJwSD1PlL8Nps5pwBRi9 ceFpSW7zD5p3K5yYUZAz3/fA+gZykPiXyEXfCMzQMtMGz+oCH8XVLA X-Received: by 2002:a05:6000:2f85:b0:432:e00b:8669 with SMTP id ffacd0b85a97d-4356a029a98mr20173594f8f.18.1768945579163; Tue, 20 Jan 2026 13:46:19 -0800 (PST) Received: from pumpkin (82-69-66-36.dsl.in-addr.zen.co.uk. [82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43569927007sm31348953f8f.16.2026.01.20.13.46.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Jan 2026 13:46:18 -0800 (PST) Date: Tue, 20 Jan 2026 21:46:12 +0000 From: David Laight To: David Desobry Cc: tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] x86/lib: Optimize num_digits() and fix INT_MIN overflow Message-ID: <20260120214612.73b83cbe@pumpkin> In-Reply-To: <20260120174748.302078-1-david.desobry@formalgen.com> References: <20260120174748.302078-1-david.desobry@formalgen.com> 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 Tue, 20 Jan 2026 18:47:48 +0100 David Desobry wrote: > The current implementation of num_digits() uses a loop with repeated > multiplication, which is inefficient. Furthermore, the negation of > the input value "val = -val" causes undefined behavior when val is > INT_MIN, as its absolute value cannot be represented as a 32-bit integer. > > Replace the loop with a switch statement using GCC case ranges. This > allows the compiler to generate a jump table or a series of optimized > comparisons, providing O(1) performance. By using an unsigned int to > handle the magnitude, we safely handle the INT_MIN case without > relying on 64-bit types or undefined signed overflow. > > Removed the outdated comment. > > Signed-off-by: David Desobry > --- > v2: > - Replaced loop with switch statement and GCC case ranges. > - Fixed INT_MIN overflow using unsigned int cast instead of s64/long long. > - Removed outdated comment regarding mobile submission. > arch/x86/lib/misc.c | 35 +++++++++++++++++++++++++---------- > 1 file changed, 25 insertions(+), 10 deletions(-) > > diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c > index 40b81c338ae5..03ba028d5326 100644 > --- a/arch/x86/lib/misc.c > +++ b/arch/x86/lib/misc.c > @@ -3,22 +3,37 @@ > > /* > * Count the digits of @val including a possible sign. > - * > - * (Typed on and submitted from hpa's mobile phone.) > */ > int num_digits(int val) > { > - long long m = 10; > - int d = 1; > + unsigned int v = val; > + int d = 0; > > if (val < 0) { > - d++; > - val = -val; > + d = 1; > + v = -v; > } Maybe better to write as: if (val < 0) { d = 1; v = -val; } else { d = 0; v = val; } The compiler will only generate one jump. > > - while (val >= m) { > - m *= 10; > - d++; > + switch (v) { > + case 0 ... 9: > + return d + 1; > + case 10 ... 99: > + return d + 2; > + case 100 ... 999: > + return d + 3; > + case 1000 ... 9999: > + return d + 4; > + case 10000 ... 99999: > + return d + 5; > + case 100000 ... 999999: > + return d + 6; > + case 1000000 ... 9999999: > + return d + 7; > + case 10000000 ... 99999999: > + return d + 8; > + case 100000000 ... 999999999: > + return d + 9; > + default: > + return d + 10; clang generates something really horrid for that. Either: if (v <= 9) return d + 1; if (v <= 99) return d + 2; if (v <= 999) return d + 3; if (v <= 9999) return d + 4; if (v <= 99999) return d + 5; if (v <= 999999) return d + 6; if (v <= 9999999) return d + 7; if (v <= 99999999) return d + 8; if (v <= 999999999) return d + 9; return d + 10; or: if ((++d && v > 9) && (++d && v > 99) && (++d && v > 999) && (++d && v > 9999) && (++d && v > 99999) && (++d && v > 999999) && (++d && v > 9999999) && (++d && v > 99999999) && (++d && v > 999999999)) d++; return d; generate better code. In particular it is almost certainly best to only have one taken branch. Dumping in a load of unlikely() might help that as well. (Neither compiler does the ++d inline, the add is done before the return.) David > } > - return d; > }