From: Eric Wong <normalperson@...>
Date: 2015-08-04T03:09:14+00:00
Subject: [ruby-core:70234] Re: [Ruby trunk - Feature #11375] Decreased Object Allocation in Pathname.rb

Eric Wong <normalperson@yhbt.net> wrote:
> richard.schneeman@gmail.com wrote:
> > You've mentioned the case statement optimization previously in a patch I sent to Rack. I agree that the difference is pretty negligible. Although I'm able to consistently see one running `benchmark/ips`
> 
> Thanks.  We need to see if we can optimize case/when for smaller
> statements.  It looks to be a problem with st_lookup (from
> opt_case_dispatch) being pointless for small case/when statements.
> 
> Perhaps try something like the following patch to compile.c

NAK on my own patch to compile.c

st_lookup is still faster when there is no match in the case/when,
or when there's multiple "when" statements.

So I think the only place where a literal freeze for a "when" is faster
is when there's a guaranteed hit, and only one element to match against
(as in [ruby-core:70103]).

In every other case, emitting the opt_case_dispatch insn seems faster.

Benchmarks I used for testing this:

diff --git a/benchmark/bm_vm2_case_opt1.rb b/benchmark/bm_vm2_case_opt1.rb
new file mode 100644
index 0000000..e804d4b
--- /dev/null
+++ b/benchmark/bm_vm2_case_opt1.rb
@@ -0,0 +1,9 @@
+# [ruby-core:70103]
+i = 0
+string = 'foo'
+while i<6_000_000 # while loop 2
+  case string
+  when 'foo'
+    i += 1
+  end
+end
diff --git a/benchmark/bm_vm2_case_opt1miss.rb b/benchmark/bm_vm2_case_opt1miss.rb
new file mode 100644
index 0000000..800f8f1
--- /dev/null
+++ b/benchmark/bm_vm2_case_opt1miss.rb
@@ -0,0 +1,10 @@
+# [ruby-core:70103]
+i = 0
+string = 'foo'
+while i<6_000_000 # while loop 2
+  case string
+  when 'FOO'
+    i += 1
+  end
+  i += 1
+end
diff --git a/benchmark/bm_vm2_case_opt2.rb b/benchmark/bm_vm2_case_opt2.rb
new file mode 100644
index 0000000..774892e
--- /dev/null
+++ b/benchmark/bm_vm2_case_opt2.rb
@@ -0,0 +1,11 @@
+# [ruby-core:70103]
+i = 0
+string = 'foo'
+while i<6_000_000 # while loop 2
+  case string
+  when 'bar'
+    i -= 1
+  when 'foo'
+    i += 1
+  end
+end
diff --git a/benchmark/bm_vm2_case_opt2miss.rb b/benchmark/bm_vm2_case_opt2miss.rb
new file mode 100644
index 0000000..de820ae
--- /dev/null
+++ b/benchmark/bm_vm2_case_opt2miss.rb
@@ -0,0 +1,12 @@
+# [ruby-core:70103]
+i = 0
+string = 'foo'
+while i<6_000_000 # while loop 2
+  case string
+  when 'bar'
+    i -= 1
+  when 'FOO'
+    i += 1
+  end
+  i += 1
+end