Ifort activated optimization flags with the -O1 option

Hello,

I am facing a strange behavior in terms of cumulated CPU time for a F90 code, which is radically changing when I am compiling it with O0 or O1 options.

After reading the doc, I did not have seen the main flags that are activated by the O1 options. It could be very useful to understand the code behavior.  Someone has any clues to know what are those flags?

Thanks in advance