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Wed, 03 Mar 2021 11:08:34 +0100 (CET) X-Greylist: Sender DNS name whitelisted, not delayed by milter-greylist-4.4.3 (isis.lip6.fr [132.227.60.2]); Wed, 03 Mar 2021 10:42:01 +0100 (CET) X-Scanned-By: MIMEDefang 2.78 on 132.227.60.2 X-Scanned-By: MIMEDefang 2.78 on 132.227.60.2 X-Mailman-Approved-At: Wed, 03 Mar 2021 11:08:32 +0100 Cc: LKML Subject: Re: [Cocci] linux-kernel janitorial RFP: Mark static arrays as const X-BeenThere: cocci@systeme.lip6.fr X-Mailman-Version: 2.1.13 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: cocci-bounces@systeme.lip6.fr Errors-To: cocci-bounces@systeme.lip6.fr On 02/03/2021 18.42, Joe Perches wrote: > Here is a possible opportunity to reduce data usage in the kernel. > > $ git grep -P -n '^static\s+(?!const|struct)(?:\w+\s+){1,3}\w+\s*\[\s*\]' drivers/ | \ > grep -v __initdata | \ > wc -l > 3250 > > Meaning there are ~3000 declarations of arrays with what appears to be > file static const content that are not marked const. > > So there are many static arrays that could be marked const to move the > compiled object code from data to text minimizing the total amount of > exposed r/w data. You can add const if you like, but it will rarely change the generated code. gcc is already smart enough to take a static array whose contents are provably never modified within the TU and put it in .rodata: static int x = 7; static int y[2] = {13, 19}; static int p(int a, const int *foo) { return a + *foo; } int q(int a) { int b = p(a, &x); return p(b, &y[b & 1]); } $ nm c.o 0000000000000000 T q 0000000000000000 r y $ size c.o text data bss dec hex filename 111 0 0 111 6f c.o So x gets optimized away completely, but y isn't so easy to get rid of - nevertheless, it's never modified and the address doesn't escape the TU, so gcc treats is as if it was declared const. I think you'll see the same if you try adding the const on a few of your real-life examples. (Of course, the control flow may be so convoluted that gcc isn't able to infer the constness, so I'm not saying it will never make a difference - only that you shouldn't expect too much.) Rasmus _______________________________________________ Cocci mailing list Cocci@systeme.lip6.fr https://systeme.lip6.fr/mailman/listinfo/cocci