Re: plenty code is confused about function level static

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)