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Improve handling of ereport(ERROR) and elog(ERROR).

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In commit 71450d7fd6, we added code to inform
suitably-intelligent compilers that ereport() doesn't return if the elevel
is ERROR or higher.  This patch extends that to elog(), and also fixes a
double-evaluation hazard that the previous commit created in ereport(),
as well as reducing the emitted code size.

The elog() improvement requires the compiler to support __VA_ARGS__, which
should be available in just about anything nowadays since it's required by
C99.  But our minimum language baseline is still C89, so add a configure
test for that.

The previous commit assumed that ereport's elevel could be evaluated twice,
which isn't terribly safe --- there are already counterexamples in xlog.c.
On compilers that have __builtin_constant_p, we can use that to protect the
second test, since there's no possible optimization gain if the compiler
doesn't know the value of elevel.  Otherwise, use a local variable inside
the macros to prevent double evaluation.  The local-variable solution is
inferior because (a) it leads to useless code being emitted when elevel
isn't constant, and (b) it increases the optimization level needed for the
compiler to recognize that subsequent code is unreachable.  But it seems
better than not teaching non-gcc compilers about unreachability at all.

Lastly, if the compiler has __builtin_unreachable(), we can use that
instead of abort(), resulting in a noticeable code savings since no
function call is actually emitted.  However, it seems wise to do this only
in non-assert builds.  In an assert build, continue to use abort(), so that
the behavior will be predictable and debuggable if the "impossible"
happens.

These changes involve making the ereport and elog macros emit do-while
statement blocks not just expressions, which forces small changes in
a few call sites.

Andres Freund, Tom Lane, Heikki Linnakangas
This commit is contained in:
Tom Lane
2013-01-13 18:39:20 -05:00
parent 4ae5ee6c9b
commit b853eb9718
12 changed files with 361 additions and 15 deletions
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66
src/include/utils/elog.h
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@ -101,15 +101,33 @@
* ereport_domain() directly, or preferably they can override the TEXTDOMAIN
* macro.
*
* When elevel >= ERROR, we add an abort() call to give the compiler a hint
* that the ereport() expansion will not return, but the abort() isn't actually
* reached because the longjmp happens in errfinish().
* If elevel >= ERROR, the call will not return; we try to inform the compiler
* of that via pg_unreachable(). However, no useful optimization effect is
* obtained unless the compiler sees elevel as a compile-time constant, else
* we're just adding code bloat. So, if __builtin_constant_p is available,
* use that to cause the second if() to vanish completely for non-constant
* cases. We avoid using a local variable because it's not necessary and
* prevents gcc from making the unreachability deduction at optlevel -O0.
*----------
*/
#ifdef HAVE__BUILTIN_CONSTANT_P
#define ereport_domain(elevel, domain, rest) \
(errstart(elevel, __FILE__, __LINE__, PG_FUNCNAME_MACRO, domain) ? \
(errfinish rest) : (void) 0), \
((elevel) >= ERROR ? abort() : (void) 0)
do { \
if (errstart(elevel, __FILE__, __LINE__, PG_FUNCNAME_MACRO, domain)) \
errfinish rest; \
if (__builtin_constant_p(elevel) && (elevel) >= ERROR) \
pg_unreachable(); \
} while(0)
#else /* !HAVE__BUILTIN_CONSTANT_P */
#define ereport_domain(elevel, domain, rest) \
do { \
const int elevel_ = (elevel); \
if (errstart(elevel_, __FILE__, __LINE__, PG_FUNCNAME_MACRO, domain)) \
errfinish rest; \
if (elevel_ >= ERROR) \
pg_unreachable(); \
} while(0)
#endif /* HAVE__BUILTIN_CONSTANT_P */
#define ereport(elevel, rest) \
ereport_domain(elevel, TEXTDOMAIN, rest)
@ -212,7 +230,37 @@ extern int getinternalerrposition(void);
* elog(ERROR, "portal \"%s\" not found", stmt->portalname);
*----------
*/
#define elog elog_start(__FILE__, __LINE__, PG_FUNCNAME_MACRO), elog_finish
#ifdef HAVE__VA_ARGS
/*
* If we have variadic macros, we can give the compiler a hint about the
* call not returning when elevel >= ERROR. See comments for ereport().
* Note that historically elog() has called elog_start (which saves errno)
* before evaluating "elevel", so we preserve that behavior here.
*/
#ifdef HAVE__BUILTIN_CONSTANT_P
#define elog(elevel, ...) \
do { \
elog_start(__FILE__, __LINE__, PG_FUNCNAME_MACRO); \
elog_finish(elevel, __VA_ARGS__); \
if (__builtin_constant_p(elevel) && (elevel) >= ERROR) \
pg_unreachable(); \
} while(0)
#else /* !HAVE__BUILTIN_CONSTANT_P */
#define elog(elevel, ...) \
do { \
int elevel_; \
elog_start(__FILE__, __LINE__, PG_FUNCNAME_MACRO); \
elevel_ = (elevel); \
elog_finish(elevel_, __VA_ARGS__); \
if (elevel_ >= ERROR) \
pg_unreachable(); \
} while(0)
#endif /* HAVE__BUILTIN_CONSTANT_P */
#else /* !HAVE__VA_ARGS */
#define elog \
elog_start(__FILE__, __LINE__, PG_FUNCNAME_MACRO), \
elog_finish
#endif /* HAVE__VA_ARGS */
extern void elog_start(const char *filename, int lineno, const char *funcname);
extern void
@ -299,14 +347,14 @@ extern PGDLLIMPORT ErrorContextCallback *error_context_stack;
/*
* gcc understands __attribute__((noreturn)); for other compilers, insert
* a useless exit() call so that the compiler gets the point.
* pg_unreachable() so that the compiler gets the point.
*/
#ifdef __GNUC__
#define PG_RE_THROW() \
pg_re_throw()
#else
#define PG_RE_THROW() \
(pg_re_throw(), exit(1))
(pg_re_throw(), pg_unreachable())
#endif
extern PGDLLIMPORT sigjmp_buf *PG_exception_stack;