lkml.org 
[lkml]   [2021]   [Mar]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: linux-kernel janitorial RFP: Mark static arrays as const


    On Wed, 3 Mar 2021, Joe Perches wrote:

    > On Wed, 2021-03-03 at 10:41 +0100, Rasmus Villemoes wrote:
    > > 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:
    >
    > At least some or perhaps even most of the time, true, but the gcc compiler
    > from v5 through at least v10 seems inconsistent about when it does the
    > appropriate conversion.
    >
    > See the example I posted:
    > https://lore.kernel.org/lkml/6b8b250a06a98ce42120a14824531a8641f5e8aa.camel@perches.com/
    >
    > It was a randomly chosen source file conversion btw, I had no prior
    > knowledge of whether the text/data use would change.
    >
    > I'm unsure about clang consistently moving static but provably const arrays
    > from data to text. I rarely use clang. At least for v11 it seems to be
    > better though. I didn't try 10.1.

    I tried the relevnt drivers in drivers/input/joystick. I got only one
    driver that changed with gcc 9.3, which was
    drivers/input/joystick/analog.c. It actually got larger:

    original:

    text data bss dec hex filename
    22607 10560 320 33487 82cf drivers/input/joystick/analog.o

    after adding const:

    text data bss dec hex filename
    22728 10816 320 33864 8448 drivers/input/joystick/analog.o

    This was the only case where bss was not 0, but I don't know if there is a
    connection.

    julia
    \
     
     \ /
      Last update: 2021-03-07 20:15    [W:2.267 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site