Horton

Re: Large writable variables

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@enterprisedb.com>
Date: 2018-10-16T15:22:31Z
Lists: pgsql-hackers 
On Tue, Oct 16, 2018 at 2:30 AM Andres Freund <andres@anarazel.de> wrote:
> This just reminded me that a couple times I wanted a cast that casts
> away const, but otherwise makes sure the type stays the same. I don't
> think there's a way to do that in C, but we can write one that verifies
> the cast doesn't do something bad if gcc is used:
>
> #if defined(HAVE__BUILTIN_TYPES_COMPATIBLE_P)
> #define unconstify(cst, var) StaticAssertExpr(__builtin_types_compatible_p(__typeof(var), const cst), "wrong cast"), (cst) (var)
> #else
> #define unconstify(cst, var) ((cst) (var))
> #endif
>
> Does anybody besides me see value in adding a cleaned up version of
> that?

Under what circumstances would we consider this to be a legitimate thing to use?

I think if we add something this, we'd better accompany it with some
detailed and very clearly-written statements about when you're allowed
to use it.  Otherwise, I predict that people will use it in cases
where it's not actually safe, and we'll end up with low-grade bugs.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Apply unconstify() in more places

  2. Improve unconstify() documentation

  3. Drop const cast from dlsym() calls

  4. Const-ify a few more large static tables.

  5. Improve tzparse's handling of TZDEFRULES ("posixrules") zone data.

  6. Avoid statically allocating statement cache in ecpglib/prepare.c.

  7. Reorder FmgrBuiltin members, saving 25% in size.

  8. Add macro to cast away const without allowing changes to underlying type.

  9. Mark constantly allocated dest receiver as const.

  10. Avoid statically allocating formatting.c's format string caches.

  11. Correct constness of system attributes in heap.c & prerequisites.

  12. Avoid statically allocating gmtsub()'s timezone workspace.

  13. Correct constness of a few variables.

  14. Move the replication lag tracker into heap memory.