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[90.40.114.60]) by smtp.gmail.com with ESMTPSA id g19sm1867689wme.48.2021.05.05.23.42.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 05 May 2021 23:42:24 -0700 (PDT) Subject: Re: [PATCH 07/23] hw/block/nvme: Use definition to avoid dynamic stack allocation To: Keith Busch , Eric Blake References: <20210505211047.1496765-1-philmd@redhat.com> <20210505211047.1496765-8-philmd@redhat.com> <20210505212238.GA1186879@dhcp-10-100-145-180.wdc.com> <285e17d3-93fb-7317-8aba-689fda772f84@redhat.com> <20210506021556.GA1187168@dhcp-10-100-145-180.wdc.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <43f60568-6fa2-0fd4-5672-48e5b767de80@redhat.com> Date: Thu, 6 May 2021 08:42:22 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210506021556.GA1187168@dhcp-10-100-145-180.wdc.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.693, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , qemu-block@nongnu.org, Richard Henderson , qemu-devel@nongnu.org, Max Reitz , qemu-ppc@nongnu.org, Gerd Hoffmann , Paolo Bonzini , Klaus Jensen , =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 5/6/21 4:15 AM, Keith Busch wrote: > On Wed, May 05, 2021 at 06:09:10PM -0500, Eric Blake wrote: >> On 5/5/21 5:07 PM, Philippe Mathieu-Daudé wrote: >>> +Eric >>> >>> On 5/5/21 11:22 PM, Keith Busch wrote: >>>> On Wed, May 05, 2021 at 11:10:31PM +0200, Philippe Mathieu-Daudé wrote: >>>>> The compiler isn't clever enough to figure 'SEG_CHUNK_SIZE' is >>>>> a constant! Help it by using a definitions instead. >>>> >>>> I don't understand. >>> >>> Neither do I TBH... >>> >>>> It's labeled 'const', so any reasonable compiler >>>> will place it in the 'text' segment of the executable rather than on the >>>> stack. While that's compiler specific, is there really a compiler doing >>>> something bad with this? If not, I do prefer the 'const' here if only >>>> because it limits the symbol scope ('static const' is also preferred if >>>> that helps). >>> >>> Using: gcc version 10.2.1 20201125 (Red Hat 10.2.1-9) (GCC) >>> >>> Both static+const / const trigger: >>> >>> hw/block/nvme.c: In function ‘nvme_map_sgl’: >>> hw/block/nvme.c:818:5: error: ISO C90 forbids variable length array >>> ‘segment’ [-Werror=vla] >>> 818 | NvmeSglDescriptor segment[SEG_CHUNK_SIZE], *sgld, *last_sgld; >>> | ^~~~~~~~~~~~~~~~~ >>> cc1: all warnings being treated as errors >> >> C99 6.7.5.2 paragraph 4: >> "If the size is an integer constant expression and the element type has >> a known constant size, the array type is not a variable length array >> type; otherwise, the array type is a variable length array type." >> >> 6.6 paragraph 6: >> "An integer constant expression shall have integer type and shall only >> have operands that are integer constants, enumeration constants, >> character constants, sizeof expressions whose results are integer >> constants, and floating constants that are the immediate operands of >> casts. Cast operators in an integer constant expression shall only >> convert arithmetic types to integer types, except as part of an operand >> to the sizeof operator." >> >> Notably absent from that list are 'const int' variables, which even >> though they act as constants (in that the name always represents the >> same value), do not actually qualify as such under C99 rules. Yes, it's >> a pain. What's more, 6.6 paragraph 10: >> >> "An implementation may accept other forms of constant expressions." >> >> which means it _should_ be possible for the compiler to do what we want. >> But just because it is permitted does not make it actually work. :( > > Thank you, that makes sense. In this case, we are talking about an > integer constant expression for the value of a 'const' symbol. That > seems like perfect fodder for a compiler to optimize. I understand it's > not obligated to do that, but I assumed it would. > > Anyway, Philippe, I have no objection if you want to push forward with > this series. Thanks both. I'll amend Eric explanation in the commit description. Regards, Phil.