From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out162-62-57-49.mail.qq.com (out162-62-57-49.mail.qq.com [162.62.57.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 486CE200B99; Fri, 21 Feb 2025 08:54:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=162.62.57.49 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740128074; cv=none; b=hNS2Rl+58Vlj3wGBX/BH0cdqAIFIyFtSujhjqeM/zvuo3lPoIM8M4xrtz3fTcVAu/6ceEyAAaI/IfzAwWUKdjiW36kF5KGAIVeZrtcx151m4LVUVIiFbh0HBlgZMlQ0envsOmK4RoRuXLDPbGSlp4yzxpdbpbflcaDXIWQoDWW8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740128074; c=relaxed/simple; bh=sOpoaaYi70tIuun+k4bCpzt72Cgn3AbfOmLUzsM8dlo=; h=Message-ID:From:To:Cc:Subject:Date:In-Reply-To:References: MIME-Version; b=A2kD1/jw/0haU+aBjoFCRZvSjoF76UW62GKVwtIINX87X7nH+p888CcasQ7c4BdLNumnKHRPE49pLyX/Nph5qMKpAL7Sp6m+LeZ8LOkElcyk0Y0d3La6BERybknDUvebOsPy5lZMKn4H1bfJ/bc99/aE66cmXoeCcQCv4mV/Tso= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=foxmail.com; spf=pass smtp.mailfrom=foxmail.com; dkim=pass (1024-bit key) header.d=foxmail.com header.i=@foxmail.com header.b=E8+H5pAu; arc=none smtp.client-ip=162.62.57.49 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=foxmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=foxmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=foxmail.com header.i=@foxmail.com header.b="E8+H5pAu" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=foxmail.com; s=s201512; t=1740128058; bh=sOpoaaYi70tIuun+k4bCpzt72Cgn3AbfOmLUzsM8dlo=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=E8+H5pAuiy2qfrB4e4BISZp/rtcEF7Q7lg2V5kHxwBuKek0x0J5+uCZopzUA82Pme bS26LK0wOly3w0JbeAYENaZSGsoIM/48y+BckS/lAbQ/CwfFzJpnkpjN83TK0j0uX4 ykWudvPj72Zf/YBDQAQdbdfUqNb3EKIrz/bxDhuw= Received: from localhost.localdomain ([112.2.252.119]) by newxmesmtplogicsvrszb16-1.qq.com (NewEsmtp) with SMTP id 7DB05E70; Fri, 21 Feb 2025 16:31:27 +0800 X-QQ-mid: xmsmtpt1740126687tmzom3y51 Message-ID: X-QQ-XMAILINFO: MqswyhUqVe0Clb7vetySGil+QBrDiZ9UDLfyhWTRdv38s6p/9DZL03cObi+ps0 TDR4+jSOSddH6EeX/N6vw14iDreotcGMJncR7grnxz/hhvQOKO6DGoisUr99ENwKtkMq6bVEwO+9 0uY/E7wKnrK32VyqrtrPxok7fYlBE/+BxKTGssd9dAi3uAIWJcctxirSauwGkB3rdbCGu0gNMQiv 7Na68RqwSaq6hsa7jrt3KroV8gl5Q6CX904QZ6axfUOQ5YJ0Hg2UcB153paY0LLRkk75Cc4EEQJ/ Qx5Ro0hM9z8Fj76T8v5mf1GSYBRxAfTDat5+PRjstW/fljah10yRD/Lb4phn1CfnnggFKlRs7lzg Nf471kOdeMeUgSTFc/6X4/+Hun9lWe76DaBX4XL1UMqT+z6MHQyCowz2WZSv6IZ63FITCVZtgqRG wsu7JtBfN4/PxcEwPk2xlSzMqZ90sHgEFbaWA6lJ2Hd8/CqB6rVv7MgpoCwG3uewLaRmkINK6Oj0 4Ytr4N9ss3UKUB4kABWjtsvYGc0mdME+rbqJcOvOfKXsgaTI02zWImp5f6r/hy4XzyjBHYXgZtSi 31HhRDDE+4rRDpYhEMO45eQ7tj/bkZPtOIhtWcOdrc7uV0kbJIgQLyJ21/+hME7fwEvG7YAOXa9V bxjyioJGGeum0EURXYldvF6t867LkxzMXU1LD6j620wJOpgsj14vQb6a7ysTWlNJukFSq+YJuOni nrLo8Hj46sBXMsQLzkkVF3eYE/T/Qbee1T44fsoe/zPdHp2w0LYBuQM+eAgKJmGimUdGoBEYlb07 cDnmadBraNUxaS/wmmb2Kkd1Y2HUGn6VwJcMRqqZoRJzSaiwbL2hig0j4ooj4HALPbl5YyOxHCUv /RWgWJyU3JRbMdqc0Vg6q6LKRqs2VbNDgyTagdwCFfY1Gp68VqmJZaVcieaUQKHk7rwNWrll/tmN l/yKSDWoA16sbnGqiRglgBnS067o4gG6AT7YKM2DOmfff1sibmYw== X-QQ-XMRINFO: NI4Ajvh11aEj8Xl/2s1/T8w= From: HUANG Zhaobin To: ej@inai.de Cc: airlied@gmail.com, boqun.feng@gmail.com, gregkh@linuxfoundation.org, hch@infradead.org, hpa@zytor.com, ksummit@lists.linux.dev, linux-kernel@vger.kernel.org, miguel.ojeda.sandonis@gmail.com, rust-for-linux@vger.kernel.org, torvalds@linux-foundation.org Subject: Re: C aggregate passing (Rust kernel policy) Date: Fri, 21 Feb 2025 16:31:27 +0800 X-OQ-MSGID: <20250221083127.54494-1-huang_zhaobin@foxmail.com> X-Mailer: git-send-email 2.48.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Thu, 20 Feb 2025 16:17:07 +0100 (CET), Jan Engelhardt wrote: > > Returning aggregates in C++ is often implemented with a secret extra > pointer argument passed to the function. The C backend does not > perform that kind of transformation automatically. I surmise ABI reasons. No, in both C and C++, fff accepts a secret extra pointer argument. https://godbolt.org/z/13K9aEffe For gcc, the difference is that `sb` is allocated then copied back in C, while in C++ NRVO is applied so there is no extra allocation and copy. Clang does NRVO for both C and C++ in this case, thus generating exactly the same codes for them. I have no idea why gcc doesn't do the same.