From: Joe Perches <joe@perches.com> Date: 2021-03-03 18:41:30
On Wed, 2021-03-03 at 10:41 +0100, Rasmus Villemoes wrote:
On 02/03/2021 18.42, Joe Perches wrote:
quoted
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.
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From: Julia Lawall <julia.lawall@inria.fr> Date: 2021-03-07 19:15:01
On Wed, 3 Mar 2021, Joe Perches wrote:
On Wed, 2021-03-03 at 10:41 +0100, Rasmus Villemoes wrote:
quoted
On 02/03/2021 18.42, Joe Perches wrote:
quoted
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
From: Joe Perches <joe@perches.com> Date: 2021-03-08 05:40:25
On Sun, 2021-03-07 at 20:14 +0100, Julia Lawall wrote:
On Wed, 3 Mar 2021, Joe Perches wrote:
quoted
On Wed, 2021-03-03 at 10:41 +0100, Rasmus Villemoes wrote:
quoted
On 02/03/2021 18.42, Joe Perches wrote:
quoted
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.
You really need consider using defconfig so whatever object code
does not have tracing/debugging support.
For instance, this code with defconfig and analog joystick:
Original:
$ size drivers/input/joystick/analog.o
text data bss dec hex filename
8115 261 224 8600 2198 drivers/input/joystick/analog.o
with const:
$ size drivers/input/joystick/analog.o
text data bss dec hex filename
8179 201 224 8604 219c drivers/input/joystick/analog.o
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From: Julia Lawall <julia.lawall@inria.fr> Date: 2021-03-08 06:55:57
On Sun, 7 Mar 2021, Joe Perches wrote:
On Sun, 2021-03-07 at 20:14 +0100, Julia Lawall wrote:
quoted
On Wed, 3 Mar 2021, Joe Perches wrote:
quoted
On Wed, 2021-03-03 at 10:41 +0100, Rasmus Villemoes wrote:
quoted
On 02/03/2021 18.42, Joe Perches wrote:
quoted
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.
You really need consider using defconfig so whatever object code
does not have tracing/debugging support.
For instance, this code with defconfig and analog joystick:
Original:
$ size drivers/input/joystick/analog.o
text data bss dec hex filename
8115 261 224 8600 2198 drivers/input/joystick/analog.o
with const:
$ size drivers/input/joystick/analog.o
text data bss dec hex filename
8179 201 224 8604 219c drivers/input/joystick/analog.o
Thanks for the suggestion. It occurred to me that in one case my
transformation was wrong, because the array was multi-level, and a sub
array was being stored in a structure field that was not declared as
const. So the argument that the compiler would figure out to put the
array in .rodata didn't make sense. But I still go the same sizes for
that file. So I'll try the whole thing again.
thanks,
julia