[ruby-core:124631] [Ruby Bug#21851] Performance difference between / operator and fdiv

27 Jan 2026

      Issue #21851 has been reported by mbrown1intel (Michael Brown).

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Bug #21851: Performance difference between / operator and fdiv
https://bugs.ruby-lang.org/issues/21851

* Author: mbrown1intel (Michael Brown)
* Status: Open
* ruby -v: ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x64-mingw-ucrt]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
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Benchmarking fdiv against the / operator, it seems like fdiv is significantly slower. As far as I'm aware there's no functional difference between these two methods.

I first ran this on 2.6, and then checked against 4.0 to make sure it hadn't been fixed in the meantime, and saw the same results.

``` ruby
require 'benchmark'

n = 500_000_000
c = nil

Benchmark.bm do |benchmark|
  benchmark.report("to_f ") do
    n.times do |i|
      c = n / i.to_f
    end
  end

  benchmark.report("fdiv ") do
    n.times do |i|
      c = n.fdiv(i)
    end
  end
end
```

```
           user     system      total        real
to_f  37.672000   0.110000  37.782000 ( 38.714876)
fdiv 113.016000   0.265000 113.281000 (116.428442)
```

-- 
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