Improve compiler code layout in elog/ereport ERROR calls

Here we use a bit of preprocessor trickery to coax supporting compilers
into laying out their generated code so that the code that's in the same
branch as elog(ERROR)/ereport(ERROR) calls is moved away from the hot
path.  Effectively, this reduces the size of the hot code meaning that it
can sit on fewer cache lines.

Performance improvements of between 10-15% have been seen on highly CPU
bound workloads using pgbench's TPC-b benchmark.

What's achieved here is very similar to putting the error condition inside
an unlikely() macro. For example;

if (unlikely(x < 0))
    elog(ERROR, "invalid x value");

now there's no need to make use of unlikely() here as the common macro
used by elog and ereport will now see that elevel is >= ERROR and make use
of a pg_attribute_cold marked version of errstart().

When elevel < ERROR or if it cannot be determined to be constant, the
original behavior is maintained.

Author: David Rowley
Reviewed-by: Andres Freund, Peter Eisentraut
Discussion: https://postgr.es/m/CAApHDvrVpasrEzLL2er7p9iwZFZ%3DJj6WisePcFeunwfrV0js_A%40mail.gmail.com

src/backend/utils/error/elog.c

@@ -219,6 +219,19 @@ err_gettext(const char *str)
 #endif
 }
 
+/*
+ * errstart_cold
+ *		A simple wrapper around errstart, but hinted to be "cold".  Supporting
+ *		compilers are more likely to move code for branches containing this
+ *		function into an area away from the calling function's code.  This can
+ *		result in more commonly executed code being more compact and fitting
+ *		on fewer cache lines.
+ */
+pg_attribute_cold bool
+errstart_cold(int elevel, const char *domain)
+{
+	return errstart(elevel, domain);
+}
 
 /*
  * errstart --- begin an error-reporting cycle

src/include/utils/elog.h

@@ -111,6 +111,15 @@
  * ereport_domain() directly, or preferably they can override the TEXTDOMAIN
  * macro.
  *
+ * When __builtin_constant_p is available and elevel >= ERROR we make a call
+ * to errstart_cold() instead of errstart().  This version of the function is
+ * marked with pg_attribute_cold which will coax supporting compilers into
+ * generating code which is more optimized towards non-ERROR cases.  Because
+ * we use __builtin_constant_p() in the condition, when elevel is not a
+ * compile-time constant, or if it is, but it's < ERROR, the compiler has no
+ * need to generate any code for this branch.  It can simply call errstart()
+ * unconditionally.
+ *
  * 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
@@ -124,7 +133,9 @@
 #define ereport_domain(elevel, domain, ...)	\
 	do { \
 		pg_prevent_errno_in_scope(); \
-		if (errstart(elevel, domain)) \
+		if (__builtin_constant_p(elevel) && (elevel) >= ERROR ? \
+			errstart_cold(elevel, domain) : \
+			errstart(elevel, domain)) \
 			__VA_ARGS__, errfinish(__FILE__, __LINE__, PG_FUNCNAME_MACRO); \
 		if (__builtin_constant_p(elevel) && (elevel) >= ERROR) \
 			pg_unreachable(); \
@@ -146,6 +157,7 @@
 
 #define TEXTDOMAIN NULL
 
+extern bool pg_attribute_cold errstart_cold(int elevel, const char *domain);
 extern bool errstart(int elevel, const char *domain);
 extern void errfinish(const char *filename, int lineno, const char *funcname);