Merge pull request #43647 from JuliaLang/sv-faster-dateformat

Lift expensive Regex construction from DateFormat method body.

Author: Sacha0

stdlib/Dates/src/io.jl

@@ -332,6 +332,23 @@ const CONVERSION_TRANSLATIONS = IdDict{Type, Any}(
     Time => (Hour, Minute, Second, Millisecond, Microsecond, Nanosecond, AMPM),
 )
 
+# The `DateFormat(format, locale)` method just below consumes the following Regex.
+# Constructing this Regex is fairly expensive; doing so in the method itself can
+# consume half or better of `DateFormat(format, locale)`'s runtime. So instead we
+# construct and cache it outside the method body. Note, however, that when
+# `keys(CONVERSION_SPECIFIERS)` changes, the cached Regex must be updated accordingly;
+# hence the mutability (Ref-ness) of the cache, the helper method with which to populate
+# the cache, the cache of the hash of `keys(CONVERSION_SPECIFIERS)` (to facilitate checking
+# for changes), and the lock (to maintain consistency of these objects across threads when
+# threads simultaneously modify `CONVERSION_SPECIFIERS` and construct `DateFormat`s).
+function compute_dateformat_regex(conversion_specifiers)
+    letters = String(collect(keys(conversion_specifiers)))
+    return Regex("(?<!\\\\)([\\Q$letters\\E])\\1*")
+end
+const DATEFORMAT_REGEX_LOCK = ReentrantLock()
+const DATEFORMAT_REGEX_HASH = Ref(hash(keys(CONVERSION_SPECIFIERS)))
+const DATEFORMAT_REGEX_CACHE = Ref(compute_dateformat_regex(CONVERSION_SPECIFIERS))
+
 """
     DateFormat(format::AbstractString, locale="english") -> DateFormat
 
@@ -379,8 +396,20 @@ function DateFormat(f::AbstractString, locale::DateLocale=ENGLISH)
     prev = ()
     prev_offset = 1
 
-    letters = String(collect(keys(CONVERSION_SPECIFIERS)))
-    for m in eachmatch(Regex("(?<!\\\\)([\\Q$letters\\E])\\1*"), f)
+    # To understand this block, please see the comments attached to the definitions of
+    # DATEFORMAT_REGEX_LOCK, DATEFORMAT_REGEX_HASH, and DATEFORMAT_REGEX_CACHE.
+    lock(DATEFORMAT_REGEX_LOCK)
+    try
+        dateformat_regex_hash = hash(keys(CONVERSION_SPECIFIERS))
+        if dateformat_regex_hash != DATEFORMAT_REGEX_HASH[]
+            DATEFORMAT_REGEX_HASH[] = dateformat_regex_hash
+            DATEFORMAT_REGEX_CACHE[] = compute_dateformat_regex(CONVERSION_SPECIFIERS)
+        end
+    finally
+        unlock(DATEFORMAT_REGEX_LOCK)
+    end
+
+    for m in eachmatch(DATEFORMAT_REGEX_CACHE[], f)
         tran = replace(f[prev_offset:prevind(f, m.offset)], r"\\(.)" => s"\1")
 
         if !isempty(prev)