Horton

Re: Large writable variables

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@enterprisedb.com>
Date: 2018-10-17T20:02:00Z
Lists: pgsql-hackers 
On 2018-10-17 21:06:13 +0200, Peter Eisentraut wrote:
> On 16/10/2018 08:30, Andres Freund 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?
> 
> I've had the attached patch lying around.

Heh.


> As you can see there, there are a few places where it could be useful,
> but not too many.

Yea.

> The name CONST_CAST() is adapted from C++.
> 
> Your version with __builtin_types_compatible_p() doesn't work for
> casting between char * and const char *, so it wouldn't be very useful.

Huh, why wouldn't it work for char *? Seems to work fine for me?


> +/*
> + * Safer casting -- use this for casting away const or volatile.  It ensures
> + * that the source and target types are otherwise compatible.
> + */
> +#define CONST_CAST(t, x) ((void)((t)0 == (x)), (t)(x))
> +

Has the advantage that it probably works for globals, but OTOH, it only
works correctly for pointers, and it doesn't reliably trigger warnigns /
errors.

Greetings,

Andres Freund

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.