[PATCH] conditionalget: avoid const lookup in case/when

[Eric Wong <e@80x24.org>]

case/when dispatches already optimize away allocation of constant
string literals in all C Ruby 1.9.x/2.x releases
(ref: opt_case_dispatch in Ruby insns.def)

Other Ruby implementations presumably have similar optimizations
to encourage prettier code.

The following code snippet does not cause GC.count to increase
during the two loops, regardless of what `nr' is.
Tested on Ruby 1.9.3, 2.1.3, and trunk r47786:

    GET = "GET"
    HEAD = "HEAD"
    REQUEST_METHOD = "REQUEST_METHOD" # unnecessary in 2.2.0+
    env = { REQUEST_METHOD => "GET" }

    nr = 10000000
    nr.times do |i|
      case env[REQUEST_METHOD]
      when GET, HEAD
        :foo
      else
        :bar
      end
    end
    a = GC.count

    nr.times do |i|
      case env[REQUEST_METHOD]
      when "GET", "HEAD"
        :foo
      else
        :bar
      end
    end
    b = GC.count
    p [ a, b ]
---
  Eric Wong <e@80x24.org> wrote:
  > I don't disagree with this commit at this time

  OK, I disagree with one part :)

  I think the following allocations may be elided in future C Ruby, too:
    a) foo == "lit" (and "lit" == foo)
    b) str << "lit"

  If you prefer `git pull" for this:

  The following changes since commit 022b0076b0eacad03eac48060198f05aa776a866:

    Merge pull request #739 from schneems/schneems/fix-respond_to (2014-10-02 21:29:17 +0200)

  are available in the git repository at:

    git://bogomips.org/rack.git when-lit

  for you to fetch changes up to 68e086d2b42ee2c9fc8017264aa353a0d543fd13:

    conditionalget: avoid const lookup in case/when (2014-10-05 08:09:50 +0000)

  ----------------------------------------------------------------
  Eric Wong (1):
        conditionalget: avoid const lookup in case/when

 lib/rack/conditionalget.rb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/rack/conditionalget.rb b/lib/rack/conditionalget.rb
index 3d4c78a..441dd38 100644
--- a/lib/rack/conditionalget.rb
+++ b/lib/rack/conditionalget.rb
@@ -21,7 +21,7 @@ module Rack
 
     def call(env)
       case env[REQUEST_METHOD]
-      when GET, HEAD
+      when "GET", "HEAD"
         status, headers, body = @app.call(env)
         headers = Utils::HeaderHash.new(headers)
         if status == 200 && fresh?(env, headers)
-- 
EW

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