What is the difference between the /Ox and /O2 compiler options?

Question

Microsoft\'s C++ compiler (cl.exe, as included with Visual Studio) offers several optimization switches. The difference between most of them seems self-explanat

Answer 1

Asha's answer cites a blog post about Visual Studio 2005, and is rather out of date.

The latest version of the documentation is available here:

• /Ox: https://msdn.microsoft.com/en-us/library/59a3b321.aspx
• /O2: https://msdn.microsoft.com/en-us/library/8f8h5cxt.aspx

According to those:

• /Ox → /Og /Oi /Ot /Oy /Ob2

• /O2 → the same, but further adds /Gs /GF /Gy

  • /GF eliminates duplicate strings
  • /Gy does function level linking

You may additionally be interested in /GS- which turns off security checks around the stack, which can be a significant performance hit (see the MS docs for /GS).

You should benchmark your specific application, as ever.

Answer 2

I found it here:

Ox and O2 are almost identical. They differ only in the fact that O2 also throws GF and Gy. There is almost no reason to avoid throwing these two switches.