improve performance for string(...) (#28876)

* improve performance for various string methods

(cherry picked from commit 7314249)

base/strings/io.jl

@@ -103,31 +103,43 @@ function sprint(f::Function, args...; context=nothing, sizehint::Integer=0)
     String(resize!(s.data, s.size))
 end
 
-tostr_sizehint(x) = 0
+tostr_sizehint(x) = 8
 tostr_sizehint(x::AbstractString) = lastindex(x)
 tostr_sizehint(x::Float64) = 20
 tostr_sizehint(x::Float32) = 12
 
-function print_to_string(xs...; env=nothing)
+function print_to_string(xs...)
     if isempty(xs)
         return ""
     end
+    siz = 0
+    for x in xs
+        siz += tostr_sizehint(x)
+    end
     # specialized for performance reasons
-    s = IOBuffer(sizehint=tostr_sizehint(xs[1]))
-    if env !== nothing
-        env_io = IOContext(s, env)
-        for x in xs
-            print(env_io, x)
-        end
-    else
-        for x in xs
-            print(s, x)
-        end
+    s = IOBuffer(sizehint=siz)
+    for x in xs
+        print(s, x)
     end
     String(resize!(s.data, s.size))
 end
 
-string_with_env(env, xs...) = print_to_string(xs...; env=env)
+function string_with_env(env, xs...)
+    if isempty(xs)
+        return ""
+    end
+    siz = 0
+    for x in xs
+        siz += tostr_sizehint(x)
+    end
+    # specialized for performance reasons
+    s = IOBuffer(sizehint=siz)
+    env_io = IOContext(s, env)
+    for x in xs
+        print(env_io, x)
+    end
+    String(resize!(s.data, s.size))
+end
 
 """
     string(xs...)

base/strings/substring.jl

@@ -142,19 +142,32 @@ function reverse(s::Union{String,SubString{String}})::String
     end
 end
 
-function string(a::Union{String, SubString{String}}...)
-    if length(a) == 1
-        return String(a[1])
-    end
+string(a::String)            = String(a)
+string(a::SubString{String}) = String(a)
+
+function string(a::Union{Char, String, SubString{String}}...)
     n = 0
-    for str in a
-        n += sizeof(str)
+    for v in a
+        if v isa Char
+            n += codelen(v)
+        else
+            n += sizeof(v)
+        end
     end
     out = _string_n(n)
     offs = 1
-    for str in a
-        unsafe_copyto!(pointer(out,offs), pointer(str), sizeof(str))
-        offs += sizeof(str)
+    for v in a
+        if v isa Char
+           x = bswap(reinterpret(UInt32, v))
+           for j in 1:codelen(v)
+               unsafe_store!(pointer(out, offs), x % UInt8)
+               offs += 1
+               x >>= 8
+           end
+        else
+            unsafe_copyto!(pointer(out,offs), pointer(v), sizeof(v))
+            offs += sizeof(v)
+        end
     end
     return out
 end

test/strings/basic.jl

@@ -34,6 +34,7 @@ using Random
     @test string("∀∃", "1∀∃") === "∀∃1∀∃"
     @test string(SubString("∀∃"), SubString("1∀∃", 2)) === "∀∃∀∃"
     @test string(s"123") === s"123"
+    @test string("123", 'α', SubString("1∀∃", 2), 'a', "foo") === "123α∀∃afoo"
     codegen_egal_of_strings(x, y) = (x===y, x!==y)
     @test codegen_egal_of_strings(string("ab", 'c'), "abc") === (true, false)
     let strs = ["", "a", "a b c", "до свидания"]