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

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.

Asha's answer cites a blog post about Visual Studio 2005, and is rather out of date.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