optimize repeat(string, n) for repeating single ASCII chars (#22462)

(cherry picked from commit b1c2d73)

Author: stevengj

base/strings/string.jl

@@ -427,8 +427,12 @@ function repeat(s::String, r::Integer)
     r < 0 && throw(ArgumentError("can't repeat a string $r times"))
     n = s.len
     out = _string_n(n*r)
-    for i=1:r
-        unsafe_copy!(pointer(out, 1+(i-1)*n), pointer(s), n)
+    if n == 1 # common case: repeating a single ASCII char
+        ccall(:memset, Ptr{Void}, (Ptr{UInt8}, Cint, Csize_t), out, unsafe_load(pointer(s)), r)
+    else
+        for i=1:r
+            unsafe_copy!(pointer(out, 1+(i-1)*n), pointer(s), n)
+        end
     end
     return out
 end

test/strings/basic.jl

@@ -461,6 +461,10 @@ Base.endof(x::CharStr) = endof(x.chars)
 @test cmp("\U1f596\U1f596", CharStr("\U1f596")) == 1   # Gives BoundsError with bug
 @test cmp(CharStr("\U1f596"), "\U1f596\U1f596") == -1
 
+# repeat function
+@test repeat("xx",3) == repeat("x",6) == "xxxxxx"
+@test repeat("αα",3) == repeat("α",6) == "αααααα"
+
 # issue #12495: check that logical indexing attempt raises ArgumentError
 @test_throws ArgumentError "abc"[[true, false, true]]
 @test_throws ArgumentError "abc"[BitArray([true, false, true])]