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

[James Tucker <jftucker@gmail.com>]

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Thanks Eric!

On Sun, Oct 5, 2014 at 1:15 AM, Eric Wong <e@80x24.org> wrote:

> 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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