linux-kernel - Re: [Cocci] linux-kernel janitorial RFP: Mark static arrays as const

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Date:   Tue, 02 Mar 2021 18:47:51 -0800
From:   Joe Perches <joe@...ches.com>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     kernelnewbies <kernelnewbies@...nelnewbies.org>,
        kernel-janitors <kernel-janitors@...r.kernel.org>,
        cocci <cocci@...teme.lip6.fr>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Cocci] linux-kernel janitorial RFP: Mark static arrays as const

On Tue, 2021-03-02 at 22:41 +0100, Julia Lawall wrote:
> 
> On Tue, 2 Mar 2021, Joe Perches wrote:
> 
> > Here is a possible opportunity to reduce data usage in the kernel.
> 
> Does it actually reduce data usage?

Yes, at least for gcc.  For instance:

$ gcc --version
gcc (Ubuntu 10.2.0-13ubuntu1) 10.2.0

And with the diff below (x86-64 defconfig with hwmon/pmbus and max20730)

$ diff --git a/drivers/hwmon/pmbus/max20730.c b/drivers/hwmon/pmbus/max20730.c
index 9dd3dd79bc18..b824eab95456 100644
--- a/drivers/hwmon/pmbus/max20730.c
+++ b/drivers/hwmon/pmbus/max20730.c
@@ -434,7 +434,7 @@ static long direct_to_val(u16 w, enum pmbus_sensor_classes >
        return d;
 }
 
-static u32 max_current[][5] = {
+static const u32 max_current[][5] = {
        [max20710] = { 6200, 8000, 9700, 11600 },
        [max20730] = { 13000, 16600, 20100, 23600 },
        [max20734] = { 21000, 27000, 32000, 38000 },

$ size drivers/hwmon/pmbus/max20730.o*
   text	   data	    bss	    dec	    hex	filename
   9245	    256	      0	   9501	   251d	drivers/hwmon/pmbus/max20730.o.gcc.new
   9149	    352	      0	   9501	   251d	drivers/hwmon/pmbus/max20730.o.gcc.old

with clang-11 it doesn't seem to make a difference:

$ /usr/bin/clang --version
Ubuntu clang version 11.0.0-2

$ size drivers/hwmon/pmbus/max20730.o*
   text	   data	    bss	    dec	    hex	filename
   9166	    256	      1	   9423	   24cf	drivers/hwmon/pmbus/max20730.o.clang-11.new
   9166	    256	      1	   9423	   24cf	drivers/hwmon/pmbus/max20730.o.clang-11.old