Re: Proposal: Add more compile-time asserts to expose inconsistencies.

From: Andres Freund <andres(at)anarazel(dot)de>
To: "Smith, Peter" <peters(at)fast(dot)au(dot)fujitsu(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>
Cc: "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(at)paquier(dot)xyz>, Dagfinn Ilmari Mannsåker <ilmari(at)ilmari(dot)org>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposal: Add more compile-time asserts to expose inconsistencies.
Date: 2019-11-12 19:00:46
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Hi Peter, Peter, :)

On 2019-10-28 00:30:11 +0000, Smith, Peter wrote:
> From: Andres Freund <andres(at)anarazel(dot)de> Sent: Sunday, 27 October 2019 12:03 PM
> > My proposal for this really was just to use this as a fallback for when static assert isn't available. Which in turn I was just suggesting because Tom wanted a fallback.
> The patch is updated to use "extern" technique only when "_Static_assert" is unavailable.


> /*
> * forkname_to_number - look up fork number by name
> diff --git a/src/include/c.h b/src/include/c.h
> index d752cc0..3e24ff4 100644
> --- a/src/include/c.h
> +++ b/src/include/c.h
> @@ -838,11 +838,16 @@ extern void ExceptionalCondition(const char *conditionName,
> do { _Static_assert(condition, errmessage); } while(0)
> #define StaticAssertExpr(condition, errmessage) \
> ((void) ({ StaticAssertStmt(condition, errmessage); true; }))
> +/* StaticAssertDecl is suitable for use at file scope. */
> +#define StaticAssertDecl(condition, errmessage) \
> + _Static_assert(condition, errmessage)
> #else /* !HAVE__STATIC_ASSERT */
> #define StaticAssertStmt(condition, errmessage) \
> ((void) sizeof(struct { int static_assert_failure : (condition) ? 1 : -1; }))
> #define StaticAssertExpr(condition, errmessage) \
> StaticAssertStmt(condition, errmessage)
> +#define StaticAssertDecl(condition, errmessage) \
> + extern void static_assert_func(int static_assert_failure[(condition) ? 1 : -1])
> #endif /* HAVE__STATIC_ASSERT */
> #else /* C++ */
> #if defined(__cpp_static_assert) && __cpp_static_assert >= 200410
> @@ -858,7 +863,6 @@ extern void ExceptionalCondition(const char *conditionName,
> #endif
> #endif /* C++
> */

Peter Smith:

Is there a reason to not just make StaticAssertExpr and StaticAssertStmt
be the same? I don't want to proliferate variants that users have to
understand if there's no compelling need. Nor do I think do we really
need two different fallback implementation for static asserts.

As far as I can tell we should be able to use the prototype based
approach in all the cases where we currently use the "negative bit-field
width" approach?

Should then also update
* Otherwise we fall back on a kluge that assumes the compiler will complain
* about a negative width for a struct bit-field. This will not include a
* helpful error message, but it beats not getting an error at all.

Peter Eisentraut:

Looking at the cplusplus variant, I'm somewhat surprised to see that you
made both fallback and plain version unconditionally use GCC style
compound expressions:

commit a2c8e5cfdb9d82ae6d4bb8f37a4dc7cbeca63ec1
Author: Peter Eisentraut <peter_e(at)gmx(dot)net>
Date: 2016-08-30 12:00:00 -0400

Add support for static assertions in C++


+#if defined(__cpp_static_assert) && __cpp_static_assert >= 200410
+#define StaticAssertStmt(condition, errmessage) \
+ static_assert(condition, errmessage)
+#define StaticAssertExpr(condition, errmessage) \
+ StaticAssertStmt(condition, errmessage)
+#define StaticAssertStmt(condition, errmessage) \
+ do { struct static_assert_struct { int static_assert_failure : (condition) ? 1 : -1; }; } while(0)
+#define StaticAssertExpr(condition, errmessage) \
+ ({ StaticAssertStmt(condition, errmessage); })

Was that intentional? The C version intentionally uses compound
expressions only for the _Static_assert case, where configure tests for
the compound expression support? As far as I can tell this'll not allow
using our headers e.g. with msvc in C++ mode if somebody introduce a
static assertion in a header - which seems like a likely and good
outcome with the changes proposed here?

Btw, it looks to me like msvc supports using the C++ static_assert()
even in C mode:


Andres Freund

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