Thread: plenty code is confused about function level static

Hi,

We have a fair amount of code that uses non-constant function level static
variables for read-only data. Which makes little sense - it prevents the
compiler from understanding

a) that the data is read only and can thus be put into a segment that's shared
   between all invocations of the program
b) the data will be the same on every invocation, and thus from optimizing
   based on that.

The most common example of this is that all our binaries use
  static struct option long_options[] = { ... };
which prevents long_options from being put into read-only memory.


Is there some reason we went for this pattern in a fair number of places? I
assume it's mostly copy-pasta, but...


In practice it often is useful to use 'static const' instead of just
'const'. At least gcc otherwise soemtimes fills the data on the stack, instead
of having a read-only data member that's already initialized. I'm not sure
why, tbh.


Attached are fixes for struct option and a few more occurrences I've found
with a bit of grepping.


There are lots of places that could benefit from adding 'static
const'.

E.g. most, if not all, HASHCTL's should be that, but that's more verbose to
change, so I didn't do that.

Greetings,

Andres Freund

On 18/04/2024 00:39, Andres Freund wrote:
> Hi,
> 
> We have a fair amount of code that uses non-constant function level static
> variables for read-only data. Which makes little sense - it prevents the
> compiler from understanding
> 
> a) that the data is read only and can thus be put into a segment that's shared
>     between all invocations of the program
> b) the data will be the same on every invocation, and thus from optimizing
>     based on that.
> 
> The most common example of this is that all our binaries use
>    static struct option long_options[] = { ... };
> which prevents long_options from being put into read-only memory.
> 
> 
> Is there some reason we went for this pattern in a fair number of places? I
> assume it's mostly copy-pasta, but...
> 
> 
> In practice it often is useful to use 'static const' instead of just
> 'const'. At least gcc otherwise soemtimes fills the data on the stack, instead
> of having a read-only data member that's already initialized. I'm not sure
> why, tbh.

Weird. I guess it can be faster if you assume the data in the read-only 
section might not be in cache, but the few instructions needed to fill 
the data locally in stack are.

> Attached are fixes for struct option and a few more occurrences I've found
> with a bit of grepping.

+1

-- 
Heikki Linnakangas
Neon (https://neon.tech)

On 17.04.24 23:39, Andres Freund wrote:
> Is there some reason we went for this pattern in a fair number of places? I
> assume it's mostly copy-pasta, but...

Right.  I don't think it is commonly understood that adding const 
qualifiers can help compiler optimization, and it's difficult to 
systematically check for omissions or verify the optimization effects. 
So I think we just have to keep trying to do our best manually for now.

> Attached are fixes for struct option and a few more occurrences I've found
> with a bit of grepping.

These look good to me.

> On 18 Apr 2024, at 02:39, Andres Freund <andres@anarazel.de> wrote:
>
> There are lots of places that could benefit from adding 'static
> const'.

+1 for helping compiler.
GCC has a -Wsuggest-attribute=const, we can count these warnings and threat increase as an error :)


Best regards, Andrey Borodin.

On 18.04.24 10:43, Andrey M. Borodin wrote:
>> On 18 Apr 2024, at 02:39, Andres Freund <andres@anarazel.de> wrote:
>>
>> There are lots of places that could benefit from adding 'static
>> const'.
> 
> +1 for helping compiler.
> GCC has a -Wsuggest-attribute=const, we can count these warnings and threat increase as an error :)

This is different.  It's an attribute, not a qualifier, and it's for 
functions, not variables.  But it could undoubtedly also have a 
performance benefit.

Hi,

On 2024-04-18 10:33:30 +0200, Peter Eisentraut wrote:
> > Attached are fixes for struct option and a few more occurrences I've found
> > with a bit of grepping.
>
> These look good to me.

Thoughts about when to apply these? Arguably they're fixing mildly broken
code, making it appropriate to fix in 17, but it's also something that we
could end up fixing for a while...


There are some variations of this that are a bit harder to fix, btw. We have

objdump -j .data -t src/backend/postgres|sort -k5
...
0000000001474d00 g     O .data  00000000000015f0              ConfigureNamesReal
0000000001479a80 g     O .data  0000000000001fb0              ConfigureNamesEnum
0000000001476300 g     O .data  0000000000003778              ConfigureNamesString
...
00000000014682e0 g     O .data  0000000000005848              ConfigureNamesBool
000000000146db40 g     O .data  00000000000071c0              ConfigureNamesInt

Not that thta's all *that* much these days, but it's still pretty silly to use
~80kB of memory in every postgres instance just because we didn't set
  conf->gen.vartype = PGC_BOOL;
etc at compile time.

Large modifiable arrays with callbacks are also quite useful for exploitation,
as one doesn't need to figure out precise addresses.

Greetings,

Andres Freund

Hi,

On 2024-04-18 09:07:43 -0300, Ranier Vilela wrote:
>  On 18/04/2024 00:39, Andres Freund wrote:
> >There are lots of places that could benefit from adding 'static
> >const'.
> 
> I found a few more places.

Good catches.


> Patch 004
> 
> The opposite would also help, adding static.
> In these places, I believe it is safe to add static,
> allowing the compiler to transform into read-only, definitively.

I don't think this would even compile? E.g. LockTagTypeNames, pg_wchar_table
are declared in a header and used across translation units.

Greetings,

Andres Freund

On 18.04.24 19:11, Andres Freund wrote:
> Thoughts about when to apply these? Arguably they're fixing mildly broken
> code, making it appropriate to fix in 17, but it's also something that we
> could end up fixing for a while...

Yeah, let's keep these for later.  They are not regressions, and there 
is no end in sight yet.  I have some other related stuff queued up, so 
if we're going to start adjusting these kinds of things now, it would 
open a can of worms.

On Thu, Apr 18, 2024 at 07:56:35PM +0200, Peter Eisentraut wrote:
> On 18.04.24 19:11, Andres Freund wrote:
>> Thoughts about when to apply these? Arguably they're fixing mildly broken
>> code, making it appropriate to fix in 17, but it's also something that we
>> could end up fixing for a while...
>
> Yeah, let's keep these for later.  They are not regressions, and there is no
> end in sight yet.  I have some other related stuff queued up, so if we're
> going to start adjusting these kinds of things now, it would open a can of
> worms.

This is a set of optimizations for stuff that has accumulated across
the years in various code paths, so I'd vote on the side of caution
and wait until v18 opens for business.
--
Michael