diff --git a/benchmark/file_basename.yml b/benchmark/file_basename.yml
new file mode 100644
index 00000000000000..fbd78785aa2262
--- /dev/null
+++ b/benchmark/file_basename.yml
@@ -0,0 +1,6 @@
+prelude: |
+  # frozen_string_literal: true
+benchmark:
+  long: File.basename("/Users/george/src/github.com/ruby/ruby/benchmark/file_dirname.yml")
+  long_name: File.basename("Users_george_src_github.com_ruby_ruby_benchmark_file_dirname.yml")
+  withext: File.basename("/Users/george/src/github.com/ruby/ruby/benchmark/file_dirname.yml", ".yml")
diff --git a/file.c b/file.c
index a98bb9728e00db..c23e92d22e029f 100644
--- a/file.c
+++ b/file.c
@@ -3749,7 +3749,7 @@ strrdirsep(const char *path, const char *end, bool mb_enc, rb_encoding *enc)
 }
 
 static char *
-chompdirsep(const char *path, const char *end, rb_encoding *enc)
+chompdirsep(const char *path, const char *end, bool mb_enc, rb_encoding *enc)
 {
     while (path < end) {
         if (isdirsep(*path)) {
@@ -3758,7 +3758,7 @@ chompdirsep(const char *path, const char *end, rb_encoding *enc)
             if (path >= end) return (char *)last;
         }
         else {
-            Inc(path, end, true, enc);
+            Inc(path, end, mb_enc, enc);
         }
     }
     return (char *)path;
@@ -3768,7 +3768,7 @@ char *
 rb_enc_path_end(const char *path, const char *end, rb_encoding *enc)
 {
     if (path < end && isdirsep(*path)) path++;
-    return chompdirsep(path, end, enc);
+    return chompdirsep(path, end, true, enc);
 }
 
 static rb_encoding *
@@ -4088,7 +4088,7 @@ rb_file_expand_path_internal(VALUE fname, VALUE dname, int abs_mode, int long_na
                 rb_enc_associate(result, enc = fs_enc_check(result, fname));
                 p = pend;
             }
-            p = chompdirsep(skiproot(buf, p), p, enc);
+            p = chompdirsep(skiproot(buf, p), p, true, enc);
             s += 2;
         }
     }
@@ -4113,7 +4113,7 @@ rb_file_expand_path_internal(VALUE fname, VALUE dname, int abs_mode, int long_na
         }
         else
 #endif /* defined DOSISH || defined __CYGWIN__ */
-            p = chompdirsep(skiproot(buf, p), p, enc);
+            p = chompdirsep(skiproot(buf, p), p, true, enc);
     }
     else {
         size_t len;
@@ -4656,7 +4656,7 @@ rb_check_realpath_emulate(VALUE basedir, VALUE path, rb_encoding *origenc, enum
   root_found:
     RSTRING_GETMEM(resolved, prefixptr, prefixlen);
     pend = prefixptr + prefixlen;
-    ptr = chompdirsep(prefixptr, pend, enc);
+    ptr = chompdirsep(prefixptr, pend, true, enc);
     if (ptr < pend) {
         prefixlen = ++ptr - prefixptr;
         rb_str_set_len(resolved, prefixlen);
@@ -4910,8 +4910,8 @@ rmext(const char *p, long l0, long l1, const char *e, long l2, rb_encoding *enc)
     return 0;
 }
 
-const char *
-ruby_enc_find_basename(const char *name, long *baselen, long *alllen, rb_encoding *enc)
+static inline const char *
+enc_find_basename(const char *name, long *baselen, long *alllen, bool mb_enc, rb_encoding *enc)
 {
     const char *p, *q, *e, *end;
 #if defined DOSISH_DRIVE_LETTER || defined DOSISH_UNC
@@ -4919,13 +4919,22 @@ ruby_enc_find_basename(const char *name, long *baselen, long *alllen, rb_encodin
 #endif
     long f = 0, n = -1;
 
-    end = name + (alllen ? (size_t)*alllen : strlen(name));
-    name = skipprefix(name, end, true, enc);
+    long len = (alllen ? (size_t)*alllen : strlen(name));
+
+    if (len <= 0) {
+        return name;
+    }
+
+    end = name + len;
+    name = skipprefix(name, end, mb_enc, enc);
 #if defined DOSISH_DRIVE_LETTER || defined DOSISH_UNC
     root = name;
 #endif
-    while (isdirsep(*name))
+
+    while (isdirsep(*name)) {
         name++;
+    }
+
     if (!*name) {
         p = name - 1;
         f = 1;
@@ -4947,32 +4956,47 @@ ruby_enc_find_basename(const char *name, long *baselen, long *alllen, rb_encodin
 #endif /* defined DOSISH_DRIVE_LETTER || defined DOSISH_UNC */
     }
     else {
-        if (!(p = strrdirsep(name, end, true, enc))) {
+        p = strrdirsep(name, end, mb_enc, enc);
+        if (!p) {
             p = name;
         }
         else {
-            while (isdirsep(*p)) p++; /* skip last / */
+            while (isdirsep(*p)) {
+                p++; /* skip last / */
+            }
         }
 #if USE_NTFS
         n = ntfs_tail(p, end, enc) - p;
 #else
-        n = chompdirsep(p, end, enc) - p;
+        n = chompdirsep(p, end, mb_enc, enc) - p;
 #endif
         for (q = p; q - p < n && *q == '.'; q++);
-        for (e = 0; q - p < n; Inc(q, end, true, enc)) {
+        for (e = 0; q - p < n; Inc(q, end, mb_enc, enc)) {
             if (*q == '.') e = q;
         }
-        if (e) f = e - p;
-        else f = n;
+        if (e) {
+            f = e - p;
+        }
+        else {
+            f = n;
+        }
     }
 
-    if (baselen)
+    if (baselen) {
         *baselen = f;
-    if (alllen)
+    }
+    if (alllen) {
         *alllen = n;
+    }
     return p;
 }
 
+const char *
+ruby_enc_find_basename(const char *name, long *baselen, long *alllen, rb_encoding *enc)
+{
+    return enc_find_basename(name, baselen, alllen, true, enc);
+}
+
 /*
  *  call-seq:
  *     File.basename(file_name [, suffix] )  ->  base_name
@@ -4993,7 +5017,7 @@ ruby_enc_find_basename(const char *name, long *baselen, long *alllen, rb_encodin
 static VALUE
 rb_file_s_basename(int argc, VALUE *argv, VALUE _)
 {
-    VALUE fname, fext, basename;
+    VALUE fname, fext;
     const char *name, *p;
     long f, n;
     rb_encoding *enc;
@@ -5006,15 +5030,19 @@ rb_file_s_basename(int argc, VALUE *argv, VALUE _)
         enc = rb_str_enc_get(fext);
     }
     fname = argv[0];
-    FilePathStringValue(fname);
+    CheckPath(fname, name);
     if (NIL_P(fext) || !(enc = rb_enc_compatible(fname, fext))) {
-        enc = rb_enc_get(fname);
+        enc = rb_str_enc_get(fname);
         fext = Qnil;
     }
-    if ((n = RSTRING_LEN(fname)) == 0 || !*(name = RSTRING_PTR(fname)))
-        return rb_str_new_shared(fname);
 
-    p = ruby_enc_find_basename(name, &f, &n, enc);
+    n = RSTRING_LEN(fname);
+    if (n == 0 || !*name) {
+        rb_enc_str_new(0, 0, enc);
+    }
+
+    bool mb_enc = !rb_str_encindex_fastpath(rb_enc_to_index(enc));
+    p = enc_find_basename(name, &f, &n, mb_enc, enc);
     if (n >= 0) {
         if (NIL_P(fext)) {
             f = n;
@@ -5027,12 +5055,12 @@ rb_file_s_basename(int argc, VALUE *argv, VALUE _)
             }
             RB_GC_GUARD(fext);
         }
-        if (f == RSTRING_LEN(fname)) return rb_str_new_shared(fname);
+        if (f == RSTRING_LEN(fname)) {
+            return rb_str_new_shared(fname);
+        }
     }
 
-    basename = rb_str_new(p, f);
-    rb_enc_copy(basename, fname);
-    return basename;
+    return rb_enc_str_new(p, f, enc);
 }
 
 static VALUE rb_file_dirname_n(VALUE fname, int n);
@@ -5350,7 +5378,7 @@ rb_file_join_ary(VALUE ary)
             rb_enc_copy(result, tmp);
         }
         else {
-            tail = chompdirsep(name, name + len, rb_enc_get(result));
+            tail = chompdirsep(name, name + len, true, rb_enc_get(result));
             if (RSTRING_PTR(tmp) && isdirsep(RSTRING_PTR(tmp)[0])) {
                 rb_str_set_len(result, tail - name);
             }
diff --git a/internal/string.h b/internal/string.h
index dd5e20c0c68ed1..9212ce898653be 100644
--- a/internal/string.h
+++ b/internal/string.h
@@ -33,7 +33,13 @@ enum ruby_rstring_private_flags {
 static inline bool
 rb_str_encindex_fastpath(int encindex)
 {
-    // The overwhelming majority of strings are in one of these 3 encodings.
+    // The overwhelming majority of strings are in one of these 3 encodings,
+    // which are all either ASCII or perfect ASCII supersets.
+    // Hence you can use fast, single byte algorithms on them, such as `memchr` etc,
+    // without all the overhead of fetching the rb_encoding and using functions such as
+    // rb_enc_mbminlen etc.
+    // Many other encodings could qualify, but they are expected to be rare occurences,
+    // so it's better to keep that list small.
     switch (encindex) {
       case ENCINDEX_ASCII_8BIT:
       case ENCINDEX_UTF_8:
diff --git a/lib/rubygems/ext/builder.rb b/lib/rubygems/ext/builder.rb
index 350daf1e16d7b7..62d36bcf48d304 100644
--- a/lib/rubygems/ext/builder.rb
+++ b/lib/rubygems/ext/builder.rb
@@ -163,8 +163,6 @@ def initialize(spec, build_args = spec.build_args, target_rbconfig = Gem.target_
     @gem_dir    = spec.full_gem_path
     @target_rbconfig = target_rbconfig
     @build_jobs = build_jobs
-
-    @ran_rake = false
   end
 
   ##
@@ -177,7 +175,6 @@ def builder_for(extension) # :nodoc:
     when /configure/ then
       Gem::Ext::ConfigureBuilder
     when /rakefile/i, /mkrf_conf/i then
-      @ran_rake = true
       Gem::Ext::RakeBuilder
     when /CMakeLists.txt/ then
       Gem::Ext::CmakeBuilder.new
@@ -250,8 +247,6 @@ def build_extensions
     FileUtils.rm_f @spec.gem_build_complete_path
 
     @spec.extensions.each do |extension|
-      break if @ran_rake
-
       build_extension extension, dest_path
     end
 
diff --git a/spec/ruby/core/file/basename_spec.rb b/spec/ruby/core/file/basename_spec.rb
index 989409d76b91de..87695ab97be3ca 100644
--- a/spec/ruby/core/file/basename_spec.rb
+++ b/spec/ruby/core/file/basename_spec.rb
@@ -151,8 +151,34 @@
       File.basename("c:\\bar.txt", ".*").should == "bar"
       File.basename("c:\\bar.txt.exe", ".*").should == "bar.txt"
     end
+
+    it "handles Shift JIS 0x5C (\\) as second byte of a multi-byte sequence" do
+      # dir\fileソname.txt
+      path = "dir\\file\x83\x5cname.txt".b.force_encoding(Encoding::SHIFT_JIS)
+      path.valid_encoding?.should be_true
+      File.basename(path).should == "file\x83\x5cname.txt".b.force_encoding(Encoding::SHIFT_JIS)
+    end
   end
 
+  it "rejects strings encoded with non ASCII-compatible encodings" do
+    Encoding.list.reject(&:ascii_compatible?).reject(&:dummy?).each do |enc|
+      begin
+        path = "/foo/bar".encode(enc)
+      rescue Encoding::ConverterNotFoundError
+        next
+      end
+
+      -> {
+        File.basename(path)
+      }.should raise_error(Encoding::CompatibilityError)
+    end
+  end
+
+  it "works with all ASCII-compatible encodings" do
+    Encoding.list.select(&:ascii_compatible?).each do |enc|
+      File.basename("/foo/bar".encode(enc)).should == "bar".encode(enc)
+    end
+  end
 
   it "returns the extension for a multibyte filename" do
     File.basename('/path/Офис.m4a').should == "Офис.m4a"
diff --git a/test/rubygems/test_gem_ext_builder.rb b/test/rubygems/test_gem_ext_builder.rb
index 34f85e6b756983..5fcbc3e2acfdec 100644
--- a/test/rubygems/test_gem_ext_builder.rb
+++ b/test/rubygems/test_gem_ext_builder.rb
@@ -18,7 +18,7 @@ def setup
 
     @spec = util_spec "a"
 
-    @builder = Gem::Ext::Builder.new @spec, ""
+    @builder = Gem::Ext::Builder.new @spec
   end
 
   def teardown
@@ -201,6 +201,57 @@ def test_build_extensions_install_ext_only
     Gem.configuration.install_extension_in_lib = @orig_install_extension_in_lib
   end
 
+  def test_build_multiple_extensions
+    pend if RUBY_ENGINE == "truffleruby"
+    pend "terminates on ruby/ruby" if ruby_repo?
+
+    extension_in_lib do
+      @spec.extensions << "ext/Rakefile"
+      @spec.extensions << "ext/extconf.rb"
+
+      ext_dir = File.join @spec.gem_dir, "ext"
+
+      FileUtils.mkdir_p ext_dir
+
+      extconf_rb = File.join ext_dir, "extconf.rb"
+      rakefile = File.join ext_dir, "Rakefile"
+
+      File.open extconf_rb, "w" do |f|
+        f.write <<-'RUBY'
+          require 'mkmf'
+
+          create_makefile 'a'
+        RUBY
+      end
+
+      File.open rakefile, "w" do |f|
+        f.write <<-RUBY
+          task :default do
+            FileUtils.touch File.join "#{ext_dir}", 'foo'
+          end
+        RUBY
+      end
+
+      ext_lib_dir = File.join ext_dir, "lib"
+      FileUtils.mkdir ext_lib_dir
+      FileUtils.touch File.join ext_lib_dir, "a.rb"
+      FileUtils.mkdir File.join ext_lib_dir, "a"
+      FileUtils.touch File.join ext_lib_dir, "a", "b.rb"
+
+      use_ui @ui do
+        @builder.build_extensions
+      end
+
+      assert_path_exist @spec.extension_dir
+      assert_path_exist @spec.gem_build_complete_path
+      assert_path_exist File.join @spec.gem_dir, "ext", "foo"
+      assert_path_exist File.join @spec.extension_dir, "gem_make.out"
+      assert_path_exist File.join @spec.extension_dir, "a.rb"
+      assert_path_exist File.join @spec.gem_dir, "lib", "a.rb"
+      assert_path_exist File.join @spec.gem_dir, "lib", "a", "b.rb"
+    end
+  end
+
   def test_build_extensions_none
     use_ui @ui do
       @builder.build_extensions