From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by sourceware.org (Postfix) with ESMTPS id 606EE3888C59 for ; Mon, 3 Jun 2024 15:51:27 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 606EE3888C59 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=redhat.com ARC-Filter: OpenARC Filter v1.0.0 sourceware.org 606EE3888C59 Authentication-Results: server2.sourceware.org; arc=none smtp.remote-ip=170.10.133.124 ARC-Seal: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1717429888; cv=none; b=hGE1oGEJe3uTNK+QTse07j0iVE70LOfObc4dR7gq1HRNixhbwY0PCZHNCPoQjKHdQwAjdW9YeXLiifAHVwbAdhqwN1s2edIV0vFaWPVRfMNT63KZiYybxC36yMM7Ot74sJTyDZQAvbZqFaWcCWa4BGBNMpYMnWlXr9XV9r8Dthc= ARC-Message-Signature: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1717429888; c=relaxed/simple; bh=ptA+t30C5SwItSE8BjdVu6kugKm3qeqsEtIP7/2hnck=; h=DKIM-Signature:MIME-Version:From:Date:Message-ID:Subject:To; b=J7o6a9pdDsy3F1sFbxWL6EruCbRec4HXujqWUfyLJXOTJ1HrEERdnGcUuUFOHEX6dHj9hJzsyEAex1iB9gvVWXe4TPe/7Zbm2x9Cgxbh4qA22MK0U+xNFkJeU3eAMCoi17qj6uKkuKtu1kWf2s23jTJxxt+ckyxi0K15w+EeKLY= ARC-Authentication-Results: i=1; server2.sourceware.org DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1717429887; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=pQYUVwaUXQSSsz98FxIPCSwAdGYQFUWI/iDtlCXcO1c=; b=XwwXuaYyx8ViEJmM27Zbq2rDMLWhdqu9hZTii3sZLMoTN1yXtALijpXYCxI1+yxwC8WLoZ rc0ERRsVPedGAJSBlVOlLs+4gNRk9Xlti5Jp6B+Cj02QoE0tOulGXCpBrMkIUMEk1mjCtF yC8ZqSc2i5vRZk1QbWGgyBlCDkHfZ8M= Received: from mail-yw1-f200.google.com (mail-yw1-f200.google.com [209.85.128.200]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-306-xqpW143qM9asRm-g4eZmGg-1; Mon, 03 Jun 2024 11:51:25 -0400 X-MC-Unique: xqpW143qM9asRm-g4eZmGg-1 Received: by mail-yw1-f200.google.com with SMTP id 00721157ae682-629fe12b380so1223307b3.1 for ; Mon, 03 Jun 2024 08:51:25 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1717429885; x=1718034685; h=to:subject:message-id:date:from:in-reply-to:references:mime-version :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=pQYUVwaUXQSSsz98FxIPCSwAdGYQFUWI/iDtlCXcO1c=; b=OHTPyKda8CG++NcQstCh5xCkH5AwtoN7UiYaeiXZENMw15VfwbLU5Qm9z7AKY4OTka xtzJMC0qnt3zY05K1/8VttrDJ9nREv0XrfKUpMueGoE0ePfX+EYpLbfg4tgGBYRiXy89 KJ9mHYHa3MDJb4RpcUmhurv5PILd7WeAOdWbCT7gmbDgr/t0P0K3usFZXpRSkB48rwMT JbinlepsXeaAAsllUd+gKkDUd+p+zP5q66JoYEsO3KTlORAh5pQ+KERxfgDRtX/w33HB qG2KelQDSaejWmi/yAYDP7XFBAzaPh/YJOBP7L5JqC619sy+ISoe1F+OluZ5k5mvef7G lbvQ== X-Forwarded-Encrypted: i=1; AJvYcCXlFS5KdcZb5RZ9FnMFVFHz2e18cqDXMjZ+d9u64pUfazCFABVPLXFOlyF+4HOR0Z4wmkmnszdhyCNBnn7isF02gupJtuG4lg== X-Gm-Message-State: AOJu0YybTjqPi3S5mDP9L6W4SZqzOYcTf8N116JB8gKY8rO+lkUIVIkE DHzdZBraJrRHzwE61qRLiwQK08KkVNNEypemp/ATbfYLQz4+d9WetI7mYC+RD0PTAMHb3HBgqaQ jyU3S2av3aoAbdj6TrrLrNgW//VHFkyq27uzQM+Y52FWV8b70B/RdRZPb8Un1i4sUKXX6qjawxl zV7Cjdx9F3kz4kRs2VMJEDyMkZs7MDtA== X-Received: by 2002:a25:ae26:0:b0:de5:4a6d:96f9 with SMTP id 3f1490d57ef6-dfa73d9ebc1mr9476517276.51.1717429885091; Mon, 03 Jun 2024 08:51:25 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFl8eamIBHQGLYReCa7UlKxeqHRH0WuzbpD0nyrIdZpgKxILKqDf25ZfeI5zziaRKf0YQ8j6Hw+Mgh+udRtNqo= X-Received: by 2002:a25:ae26:0:b0:de5:4a6d:96f9 with SMTP id 3f1490d57ef6-dfa73d9ebc1mr9476491276.51.1717429884672; Mon, 03 Jun 2024 08:51:24 -0700 (PDT) MIME-Version: 1.0 References: <20240601102508.878398-1-jwakely@redhat.com> In-Reply-To: <20240601102508.878398-1-jwakely@redhat.com> From: Jonathan Wakely Date: Mon, 3 Jun 2024 16:51:08 +0100 Message-ID: Subject: Re: [PATCH] libstdc++: Optimize std::basic_string_view::starts_with To: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-13.5 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: Pushed to trunk. On Sat, 1 Jun 2024 at 11:26, Jonathan Wakely wrote: > > We get smaller code at all optimization levels by not creating a > temporary object, just comparing lengths first and then using > traits_type::compare. This does less work than calling substr then > operator==. > > libstdc++-v3/ChangeLog: > > * include/std/string_view (starts_with(basic_string_view)): > Compare lengths first and then call traits_type::compare > directly. > --- > libstdc++-v3/include/std/string_view | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/libstdc++-v3/include/std/string_view b/libstdc++-v3/include/std/string_view > index a7c5a126461..740aa9344f0 100644 > --- a/libstdc++-v3/include/std/string_view > +++ b/libstdc++-v3/include/std/string_view > @@ -385,7 +385,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > [[nodiscard]] > constexpr bool > starts_with(basic_string_view __x) const noexcept > - { return this->substr(0, __x.size()) == __x; } > + { > + return _M_len >= __x._M_len > + && traits_type::compare(_M_str, __x._M_str, __x._M_len) == 0; > + } > > [[nodiscard]] > constexpr bool > -- > 2.45.1 >