Issue #12647 has been reported by Chia-sheng Chen.

----------------------------------------
Feature #12647: Make tanh faster when it lacks of HAVE_TANH
https://bugs.ruby-lang.org/issues/12647

* Author: Chia-sheng Chen
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
I found tanh(x) in math.c return sinh(x) / cosh(x), which function call is a waist of time.

So I changed to (exp(2*x) - 1) / (exp(2*x) + 1), the extract form of tanh().

But if HAVE_SINH and HAVE_COSH are defined, only HAVE_TANH undefined (a weird library but possible)
Then the origin formulae are faster.

Eventually the solution is to determined whether HAVE_SINH/COSH exists, and set the formulae accordingly.

Testing Code:
~~~
require 'benchmark/ips'

Benchmark.ips do |x|
  x.config(:times => 1000, :warmup => 5)
  x.report(RUBY_DESCRIPTION) { Math.tanh(12.345) } 
end
~~~
Outcome:
~~~
# Origin with HAVE_SINH, HAVE_COSH, HAVE_TANH all undef
Warming up --------------------------------------
ruby 2.4.0dev (2016-08-02 trunk 55797) [x86_64-linux]
                       122.226k i/100ms
Calculating -------------------------------------
ruby 2.4.0dev (2016-08-02 trunk 55797) [x86_64-linux]
                          1.856M (± 4.9%) i/s -      9.289M in   5.016718s

# All tag undifined, modified tanh()
Warming up --------------------------------------
ruby 2.4.0dev (2016-08-02 trunk 55797) [x86_64-linux]
                       165.404k i/100ms
Calculating -------------------------------------
ruby 2.4.0dev (2016-08-02 trunk 55797) [x86_64-linux]
                          2.857M (± 2.5%) i/s -     14.390M in   5.039981s

# Only HAVE_TANH undifined
Warming up --------------------------------------
ruby 2.4.0dev (2016-08-02 trunk 55797) [x86_64-linux]
                       205.575k i/100ms
Calculating -------------------------------------
ruby 2.4.0dev (2016-08-02 trunk 55797) [x86_64-linux]
                          3.732M (± 2.2%) i/s -     18.707M in   5.015756s
~~~
About 50% improvement when all three tags are undefined.
Doesn't drop when only HAVE_TANH is undefined.

BTW, I want to ask how to send a patch.
Adding a feature here and attaching my patch is enough?

---Files--------------------------------
Update-tanh-function-to-be-faster.patch (598 Bytes)


-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>