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Make assorted performance improvements in snprintf.c.

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In combination, these changes make our version of snprintf as fast
or faster than most platforms' native snprintf, except for cases
involving floating-point conversion (which we still delegate to
the native sprintf).  The speed penalty for a float conversion
is down to around 10% though, much better than before.

Notable changes:

* Rather than always parsing the format twice to see if it contains
instances of %n$, do the extra scan only if we actually find a $.
This obviously wins for non-localized formats, and even when there
is use of %n$, we can avoid scanning text before the first % twice.

* Use strchrnul() if available to find the next %, and emit the
literal text between % escapes as strings rather than char-by-char.

* Create a bespoke function (dopr_outchmulti) for the common case
of emitting N copies of the same character, in place of writing
loops around dopr_outch.

* Simplify construction of the format string for invocations of sprintf
for floats.

* Const-ify some internal functions, and avoid unnecessary use of
pass-by-reference arguments.

Patch by me, reviewed by Andres Freund

Discussion: https://postgr.es/m/11787.1534530779@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2018-10-03 10:18:15 -04:00
parent 9bc9f72b28
commit abd9ca377d
5 changed files with 432 additions and 273 deletions
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2
configure vendored
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@ -15100,7 +15100,7 @@ fi
LIBS_including_readline="$LIBS"
LIBS=`echo "$LIBS" | sed -e 's/-ledit//g' -e 's/-lreadline//g'`
for ac_func in cbrt clock_gettime fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll posix_fallocate ppoll pstat pthread_is_threaded_np readlink setproctitle setproctitle_fast setsid shm_open symlink sync_file_range utime utimes wcstombs_l
for ac_func in cbrt clock_gettime fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll posix_fallocate ppoll pstat pthread_is_threaded_np readlink setproctitle setproctitle_fast setsid shm_open strchrnul symlink sync_file_range utime utimes wcstombs_l
do :
as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"