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Commit913ec71

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Improve compiler code layout in elog/ereport ERROR calls
Here we use a bit of preprocessor trickery to coax supporting compilersinto laying out their generated code so that the code that's in the samebranch as elog(ERROR)/ereport(ERROR) calls is moved away from the hotpath. Effectively, this reduces the size of the hot code meaning that itcan sit on fewer cache lines.Performance improvements of between 10-15% have been seen on highly CPUbound workloads using pgbench's TPC-b benchmark.What's achieved here is very similar to putting the error condition insidean 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 macroused by elog and ereport will now see that elevel is >= ERROR and make useof a pg_attribute_cold marked version of errstart().When elevel < ERROR or if it cannot be determined to be constant, theoriginal behavior is maintained.Author: David RowleyReviewed-by: Andres Freund, Peter EisentrautDiscussion:https://postgr.es/m/CAApHDvrVpasrEzLL2er7p9iwZFZ%3DJj6WisePcFeunwfrV0js_A%40mail.gmail.com
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‎src/backend/utils/error/elog.c‎

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

‎src/include/utils/elog.h‎

Lines changed: 13 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -111,6 +111,15 @@
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* ereport_domain() directly, or preferably they can override the TEXTDOMAIN
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* macro.
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*
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* When __builtin_constant_p is available and elevel >= ERROR we make a call
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* to errstart_cold() instead of errstart(). This version of the function is
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* marked with pg_attribute_cold which will coax supporting compilers into
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* generating code which is more optimized towards non-ERROR cases. Because
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* we use __builtin_constant_p() in the condition, when elevel is not a
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* compile-time constant, or if it is, but it's < ERROR, the compiler has no
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* need to generate any code for this branch. It can simply call errstart()
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* unconditionally.
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*
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* If elevel >= ERROR, the call will not return; we try to inform the compiler
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* of that via pg_unreachable(). However, no useful optimization effect is
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* obtained unless the compiler sees elevel as a compile-time constant, else
@@ -124,7 +133,9 @@
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#defineereport_domain(elevel,domain, ...)\
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do { \
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pg_prevent_errno_in_scope(); \
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if (errstart(elevel, domain)) \
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if (__builtin_constant_p(elevel) && (elevel) >= ERROR ? \
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errstart_cold(elevel, domain) : \
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errstart(elevel, domain)) \
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__VA_ARGS__, errfinish(__FILE__, __LINE__, PG_FUNCNAME_MACRO); \
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if (__builtin_constant_p(elevel) && (elevel) >= ERROR) \
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pg_unreachable(); \
@@ -146,6 +157,7 @@
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#defineTEXTDOMAIN NULL
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externboolpg_attribute_colderrstart_cold(intelevel,constchar*domain);
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externboolerrstart(intelevel,constchar*domain);
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externvoiderrfinish(constchar*filename,intlineno,constchar*funcname);
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