Re: [BUGS] BUG #14897: Segfault on statitics SQL request

Michael Paquier <michael(dot)paquier(at)gmail(dot)com> writes:
> On Tue, Nov 14, 2017 at 12:31 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> As I said before, I don't like moving the int128 typedefs into a section
>> where they don't belong, but that's just cosmetic --- this is good enough
>> for testing.

> Section 3 could be moved after the section 4 listing the alignment
> macros. It seems that it won't hurt to back-patch the refactoring as
> well.

After some further consideration of what's where in c.h, I propose that
we redefine section 1 (hacks to cope with non-ANSI C compilers) as
"compiler characteristics", and then move all of these items into it:

"#ifdef PG_FORCE_DISABLE_INLINE" stanza

all the pg_attribute_xxx() macros, also pg_noinline

PG_USED_FOR_ASSERTS_ONLY

maybe pg_unreachable(), although it's not insane to keep it in section 7

ditto for likely()/unlikely()

If anyone's really hot to keep the "non-ANSI hacks" segregated from those,
we could invent a "section 1.1" for that ... but I'd say that at least one
of the stanzas that are in there now is unrelated to ANSI compatibility
already.

Having gotten pg_attribute_aligned in front of the int128 stanza,
my vision for a full fix for the bug is:

* Explicitly document somewhere that MAXALIGN ignores int128.

* Have configure test for and define ALIGNOF_PG_INT128_TYPE
if it defines PG_INT128_TYPE.

* Set up the int128 stanza like this:

#if defined(PG_INT128_TYPE)
#if defined(pg_attribute_aligned) || ALIGNOF_PG_INT128_TYPE <= MAXIMUM_ALIGNOF
#define HAVE_INT128 1
typedef PG_INT128_TYPE int128
#if defined(pg_attribute_aligned)
pg_attribute_aligned(MAXIMUM_ALIGNOF)
#endif
;
typedef unsigned PG_INT128_TYPE uint128
#if defined(pg_attribute_aligned)
pg_attribute_aligned(MAXIMUM_ALIGNOF)
#endif
;
#endif
#endif

That is, even if configure finds an int128 type, we'll only use it if
it fits or can be made to fit into our system-wide MAXALIGN assumption.

* It'd be kind of nice to insert a
StaticAssertStmt(alignof(int128) <= MAXIMUM_ALIGNOF)
someplace to cross-check this, but as far as I can find we aren't
relying on alignof() to exist, so I don't see a good way to do it.

BTW, looking at the existing uses of pg_attribute_aligned
along with this example, I can't help but think that
this decision:

/*
* NB: aligned and packed are not given default definitions because they
* affect code functionality; they *must* be implemented by the compiler
* if they are to be used.
*/

was just useless pedantry. If we're going to ifdef around it everywhere
the macro is of use, why not just #define it to empty? That would
certainly make the above a lot easier to read. We could add a
HAVE_PG_ATTRIBUTE_ALIGNED symbol for use in #if tests.

Barring objections, I'll get on with making this happen.

regards, tom lane