Implement Hash.new(capacity:)

[Feature #19236]

When building a large hash, pre-allocating it with enough
capacity can save many re-hashes and significantly improve
performance.

```
/opt/rubies/3.3.0/bin/ruby --disable=gems -rrubygems -I./benchmark/lib ./benchmark/benchmark-driver/exe/benchmark-driver \
	            --executables="compare-ruby::../miniruby-master -I.ext/common --disable-gem" \
	            --executables="built-ruby::./miniruby --disable-gem" \
	            --output=markdown --output-compare -v $(find ./benchmark -maxdepth 1 -name 'hash_new' -o -name '*hash_new*.yml' -o -name '*hash_new*.rb' | sort)
compare-ruby: ruby 3.4.0dev (2024-03-25T11:48:11Z master f53209f) +YJIT dev [arm64-darwin23]
last_commit=[ruby/irb] Cache RDoc::RI::Driver.new (ruby/irb#911)
built-ruby: ruby 3.4.0dev (2024-03-25T15:29:40Z hash-new-rb 77652b08a2) +YJIT dev [arm64-darwin23]
warming up...

|                    |compare-ruby|built-ruby|
|:-------------------|-----------:|---------:|
|new                 |      7.614M|    5.976M|
|                    |       1.27x|         -|
|new_with_capa_1k    |     13.931k|   15.698k|
|                    |           -|     1.13x|
|new_with_capa_100k  |     124.746|   148.283|
|                    |           -|     1.19x|
```

benchmark/hash_new.yml

@@ -0,0 +1,16 @@
+prelude: |
+  has_hash_with_capa = Hash.instance_method(:initialize).parameters.include?([:key, :capacity])
+  strings_1k = 1_000.times.map { |i| i.to_s.freeze }
+  strings_100k = 100_000.times.map { |i| i.to_s.freeze }
+benchmark:
+  new: Hash.new
+  new_with_capa_1k: |
+    h = has_hash_with_capa ? Hash.new(capacity: strings_1k.size) : {}
+    strings_1k.each do |x|
+      h[x] = true
+    end
+  new_with_capa_100k: |
+    h = has_hash_with_capa ? Hash.new(capacity: strings_100k.size) : {}
+    strings_100k.each do |x|
+      h[x] = true
+    end

common.mk

@@ -1200,6 +1200,7 @@ BUILTIN_RB_SRCS = \
 		$(srcdir)/trace_point.rb \
 		$(srcdir)/warning.rb \
 		$(srcdir)/array.rb \
+		$(srcdir)/hash.rb \
 		$(srcdir)/kernel.rb \
 		$(srcdir)/ractor.rb \
 		$(srcdir)/symbol.rb \
@@ -7775,12 +7776,14 @@ hash.$(OBJEXT): {$(VPATH)}backward/2/limits.h
 hash.$(OBJEXT): {$(VPATH)}backward/2/long_long.h
 hash.$(OBJEXT): {$(VPATH)}backward/2/stdalign.h
 hash.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
+hash.$(OBJEXT): {$(VPATH)}builtin.h
 hash.$(OBJEXT): {$(VPATH)}config.h
 hash.$(OBJEXT): {$(VPATH)}constant.h
 hash.$(OBJEXT): {$(VPATH)}debug_counter.h
 hash.$(OBJEXT): {$(VPATH)}defines.h
 hash.$(OBJEXT): {$(VPATH)}encoding.h
 hash.$(OBJEXT): {$(VPATH)}hash.c
+hash.$(OBJEXT): {$(VPATH)}hash.rbinc
 hash.$(OBJEXT): {$(VPATH)}id.h
 hash.$(OBJEXT): {$(VPATH)}id_table.h
 hash.$(OBJEXT): {$(VPATH)}intern.h

hash.c

@@ -48,6 +48,7 @@
 #include "ruby/thread_native.h"
 #include "ruby/ractor.h"
 #include "vm_sync.h"
+#include "builtin.h"
 
 /* Flags of RHash
  *
@@ -1762,59 +1763,30 @@ set_proc_default(VALUE hash, VALUE proc)
     RHASH_SET_IFNONE(hash, proc);
 }
 
-/*
- *  call-seq:
- *     Hash.new(default_value = nil) -> new_hash
- *     Hash.new {|hash, key| ... } -> new_hash
- *
- *  Returns a new empty +Hash+ object.
- *
- *  The initial default value and initial default proc for the new hash
- *  depend on which form above was used. See {Default Values}[rdoc-ref:Hash@Default+Values].
- *
- *  If neither an argument nor a block given,
- *  initializes both the default value and the default proc to <tt>nil</tt>:
- *    h = Hash.new
- *    h.default # => nil
- *    h.default_proc # => nil
- *
- *  If argument <tt>default_value</tt> given but no block given,
- *  initializes the default value to the given <tt>default_value</tt>
- *  and the default proc to <tt>nil</tt>:
- *    h = Hash.new(false)
- *    h.default # => false
- *    h.default_proc # => nil
- *
- *  If a block given but no argument, stores the block as the default proc
- *  and sets the default value to <tt>nil</tt>:
- *    h = Hash.new {|hash, key| "Default value for #{key}" }
- *    h.default # => nil
- *    h.default_proc.class # => Proc
- *    h[:nosuch] # => "Default value for nosuch"
- */
-
 static VALUE
-rb_hash_initialize(int argc, VALUE *argv, VALUE hash)
+rb_hash_init(VALUE hash, long capa, VALUE ifnone_unset, VALUE ifnone, VALUE block)
 {
-    rb_hash_modify(hash);
+    RUBY_ASSERT(RHASH_SIZE(hash) == 0);
+    RUBY_ASSERT(RHASH_AR_TABLE_P(hash));
 
-    if (rb_block_given_p()) {
-        rb_check_arity(argc, 0, 0);
-        SET_PROC_DEFAULT(hash, rb_block_proc());
+    if (capa > 0) {
+        hash_st_table_init(hash, &objhash, capa);
     }
-    else {
-        rb_check_arity(argc, 0, 1);
 
-        VALUE options, ifnone;
-        rb_scan_args(argc, argv, "01:", &ifnone, &options);
-        if (NIL_P(ifnone) && !NIL_P(options)) {
-            ifnone = options;
-            rb_warn_deprecated_to_remove("3.4", "Calling Hash.new with keyword arguments", "Hash.new({ key: value })");
+    if (!NIL_P(block)) {
+        if (ifnone_unset != Qtrue) {
+            rb_check_arity(1, 0, 0);
         }
+        else {
+            SET_PROC_DEFAULT(hash, block);
+        }
+    }
+    else if (ifnone_unset != Qtrue) {
         RHASH_SET_IFNONE(hash, ifnone);
     }
 
-    return hash;
+    hash_verify(hash);
+    return Qnil;
 }
 
 static VALUE rb_hash_to_a(VALUE hash);
@@ -7150,7 +7122,6 @@ Init_Hash(void)
     rb_define_alloc_func(rb_cHash, empty_hash_alloc);
     rb_define_singleton_method(rb_cHash, "[]", rb_hash_s_create, -1);
     rb_define_singleton_method(rb_cHash, "try_convert", rb_hash_s_try_convert, 1);
-    rb_define_method(rb_cHash, "initialize", rb_hash_initialize, -1);
     rb_define_method(rb_cHash, "initialize_copy", rb_hash_replace, 1);
     rb_define_method(rb_cHash, "rehash", rb_hash_rehash, 0);
 
@@ -7477,3 +7448,5 @@ Init_Hash(void)
 
     HASH_ASSERT(sizeof(ar_hint_t) * RHASH_AR_TABLE_MAX_SIZE == sizeof(VALUE));
 }
+
+#include "hash.rbinc"

hash.rb

@@ -0,0 +1,43 @@
+class Hash
+  # call-seq:
+  #    Hash.new(default_value = nil) -> new_hash
+  #    Hash.new(default_value = nil, capacity: size) -> new_hash
+  #    Hash.new {|hash, key| ... } -> new_hash
+  #    Hash.new(capacity: size) {|hash, key| ... } -> new_hash
+  #
+  # Returns a new empty +Hash+ object.
+  #
+  # The initial default value and initial default proc for the new hash
+  # depend on which form above was used. See {Default Values}[rdoc-ref:Hash@Default+Values].
+  #
+  # If neither an argument nor a block is given,
+  # initializes both the default value and the default proc to <tt>nil</tt>:
+  #   h = Hash.new
+  #   h.default # => nil
+  #   h.default_proc # => nil
+  #
+  # If argument <tt>default_value</tt> is given but no block is given,
+  # initializes the default value to the given <tt>default_value</tt>
+  # and the default proc to <tt>nil</tt>:
+  #   h = Hash.new(false)
+  #   h.default # => false
+  #   h.default_proc # => nil
+  #
+  # If a block is given but no <tt>default_value</tt>, stores the block as the default proc
+  # and sets the default value to <tt>nil</tt>:
+  #   h = Hash.new {|hash, key| "Default value for #{key}" }
+  #   h.default # => nil
+  #   h.default_proc.class # => Proc
+  #   h[:nosuch] # => "Default value for nosuch"
+  #
+  # If both a block and a <tt>default_value</tt> are given, raises an +ArgumentError+
+  #
+  # If the optional keyword argument +capacity+ is given, the hash will be allocated
+  # with enough capacity to accomodate this many keys without having to be resized.
+  def initialize(ifnone = (ifnone_unset = true), capacity: 0, &block)
+    b = block # TODO: builtin bug
+    Primitive.cexpr!(%{
+      rb_hash_init(self, NUM2LONG(capacity), ifnone_unset, ifnone, b)
+    })
+  end
+end

inits.c

@@ -95,6 +95,7 @@ rb_call_builtin_inits(void)
     BUILTIN(pack);
     BUILTIN(warning);
     BUILTIN(array);
+    BUILTIN(hash);
     BUILTIN(kernel);
     BUILTIN(symbol);
     BUILTIN(timev);

spec/ruby/core/hash/new_spec.rb

@@ -34,7 +34,7 @@
     -> { Hash.new(nil) { 0 } }.should raise_error(ArgumentError)
   end
 
-  ruby_version_is "3.3" do
+  ruby_version_is "3.3"..."3.4" do
     it "emits a deprecation warning if keyword arguments are passed" do
       -> { Hash.new(unknown: true) }.should complain(
         Regexp.new(Regexp.escape("Calling Hash.new with keyword arguments is deprecated and will be removed in Ruby 3.4; use Hash.new({ key: value }) instead"))
@@ -46,4 +46,18 @@
       Hash.new({ unknown: true }).default.should == { unknown: true }
     end
   end
+
+  ruby_version_is "3.4" do
+    it "accepts a capacity: argument" do
+      Hash.new(5, capacity: 42).default.should == 5
+      Hash.new(capacity: 42).default.should == nil
+      (Hash.new(capacity: 42) { 1 }).default_proc.should_not == nil
+    end
+
+    it "raises an error if unknown keyword arguments are passed" do
+      -> { Hash.new(unknown: true) }.should raise_error(ArgumentError)
+      -> { Hash.new(1, unknown: true) }.should raise_error(ArgumentError)
+      -> { Hash.new(unknown: true) { 0 } }.should raise_error(ArgumentError)
+    end
+  end
 end