From nobody Wed Nov 13 00:10:29 2024 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4CB14C05027 for ; Tue, 14 Feb 2023 17:16:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232633AbjBNRQI (ORCPT ); Tue, 14 Feb 2023 12:16:08 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41600 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232123AbjBNRPQ (ORCPT ); Tue, 14 Feb 2023 12:15:16 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9D35DCC08 for ; Tue, 14 Feb 2023 09:14:28 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676394867; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9PNg3WPaFWzxoLjSof2qU8psox3R+qpqOMDOMyjiLi8=; b=OqGm8++iNY9gpd7HVvl4dXkXgTPrOeS+T2u6jJH/TeoTy74o4FxCaR/ZhTDwoWhmHjVkkZ tMDI5U3SxsAHrD9Sr8A7T9bexi1F9uJ51+0UF7M/u47Vk5IvUxEqYBZftd8mWMMQgsVHtj hQ+7PfjlecsLUff+fb1FzDQx9wwgk6c= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-88-Gp-NasbONamSrWK0LXoPrQ-1; Tue, 14 Feb 2023 12:14:22 -0500 X-MC-Unique: Gp-NasbONamSrWK0LXoPrQ-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8698B1C08798; Tue, 14 Feb 2023 17:14:21 +0000 (UTC) Received: from warthog.procyon.org.uk (unknown [10.33.36.24]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9817DC15BAD; Tue, 14 Feb 2023 17:14:19 +0000 (UTC) From: David Howells To: Jens Axboe , Al Viro , Christoph Hellwig Cc: David Howells , Matthew Wilcox , Jan Kara , Jeff Layton , David Hildenbrand , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Hellwig , John Hubbard Subject: [PATCH v14 13/17] block: Fix bio_flagged() so that gcc can better optimise it Date: Tue, 14 Feb 2023 17:13:26 +0000 Message-Id: <20230214171330.2722188-14-dhowells@redhat.com> In-Reply-To: <20230214171330.2722188-1-dhowells@redhat.com> References: <20230214171330.2722188-1-dhowells@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Fix bio_flagged() so that multiple instances of it, such as: if (bio_flagged(bio, BIO_PAGE_REFFED) || bio_flagged(bio, BIO_PAGE_PINNED)) can be combined by the gcc optimiser into a single test in assembly (arguably, this is a compiler optimisation issue[1]). The missed optimisation stems from bio_flagged() comparing the result of the bitwise-AND to zero. This results in an out-of-line bio_release_page() being compiled to something like: <+0>: mov 0x14(%rdi),%eax <+3>: test $0x1,%al <+5>: jne 0xffffffff816dac53 <+7>: test $0x2,%al <+9>: je 0xffffffff816dac5c <+11>: movzbl %sil,%esi <+15>: jmp 0xffffffff816daba1 <__bio_release_pages> <+20>: jmp 0xffffffff81d0b800 <__x86_return_thunk> However, the test is superfluous as the return type is bool. Removing it results in: <+0>: testb $0x3,0x14(%rdi) <+4>: je 0xffffffff816e4af4 <+6>: movzbl %sil,%esi <+10>: jmp 0xffffffff816dab7c <__bio_release_pages> <+15>: jmp 0xffffffff81d0b7c0 <__x86_return_thunk> instead. Also, the MOVZBL instruction looks unnecessary[2] - I think it's just 're-booling' the mark_dirty parameter. Signed-off-by: David Howells Reviewed-by: Christoph Hellwig Reviewed-by: John Hubbard cc: Jens Axboe cc: linux-block@vger.kernel.org Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108370 [1] Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108371 [2] Link: https://lore.kernel.org/r/167391056756.2311931.356007731815807265.stg= it@warthog.procyon.org.uk/ # v6 --- include/linux/bio.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/bio.h b/include/linux/bio.h index c1da63f6c808..10366b8bdb13 100644 --- a/include/linux/bio.h +++ b/include/linux/bio.h @@ -227,7 +227,7 @@ static inline void bio_cnt_set(struct bio *bio, unsigne= d int count) =20 static inline bool bio_flagged(struct bio *bio, unsigned int bit) { - return (bio->bi_flags & (1U << bit)) !=3D 0; + return bio->bi_flags & (1U << bit); } =20 static inline void bio_set_flag(struct bio *bio, unsigned int bit)