optimization - Why would one ever want to compile with -O2 instead of -O3

Question

We usually compile with -O2 because -O3 would "trigger subtle bugs".

For our GCC version -O3 enables more aggressive inlining which would actually reveal bugs otherwise unnoticed (e.g. use of uninitialized values from functions taking them as reference arguments or out-of-bounds access for arrays). It seems to me this aggressive inlining also allows a more expressive way of coding with smaller functions and -funswitch-loops helps keeping variable definitions more local in loops.

Given that bugs in our code are orders of magnitude more likely than compiler bugs and that we use -Wall -Wextra without any issues what kind of bugs should we be looking for?

If it matters we use gcc-4.3.2. Compile time is not a major issue for us.

Answer 1

Size. Of course if size does really matters (sometimes is does, like embedded), one would use -Os. But main difference at O3 is the (from you already mentioned) inlining. This can increase the generated code size (but it is faster). Maybe you want speed, but not at all (space) cost? Otherwise I would see no reason why not to use O3 (except you know of a gcc compiler bug that only occurs in your code at O3, but as long as you dont have an error, you cant reproduce at O2, I would not care).