What's the difference between a compiler's `-O0` option and `-Og` option?

Question

When I want to do debugging of C or C++ programs, I've been taught to use -O0 to turn optimization OFF, and -ggdb to insert symbols into the executable which are optimized for using the GNU gdb debugger, which I use (or, you can use -glldb for LLVM/clang's lldb debugger, or just -g for general debugging symbols, but that won't be as good as -ggdb apparently...). However, I recently stumbled upon someone saying to use -Og (instead of -O0 ), and it caught me off-guard. Sure enough though, it's in man gcc ::

-Og Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.

So, what's the difference? Here's the -O0 description from man gcc :

-O0 Reduce compilation time and make debugging produce the expected results. This is the default.

man gcc clearly says -Og "should be the optimization level of choice for the standard edit-compile-debug cycle", though.

This makes it sound like -O0 is truly "no optimizations", whereas -Og is "some optimizations on, but only those which don't interfere with debugging." Is this correct? So, which should I use, and why?

Related:

1. related, but NOT a duplicate, (read it closely: it's not at all a duplicate): What is the difference between -O0,-O1 and -g
2. my answer on debugging --copt= settings to use with Bazel: gdb: No symbol "i" in current context

Answer 1

@kaylum just provided some great insight in their comment under my question: And the key part I really care about the most is this:

[ -Og ] is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0.

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Optimize-Options

So, from now on I'm using -Og (NOT -O0 ) in addition to -ggdb .

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UDPATE 13 Aug. 2020:

Heck with this. Nevermind. I'm sticking with -O0 .

With -Og I get <optimized out> and Can't take address of "var" which isn't an lvalue. errors all over the place: I can't print my variables or examine their memory anymore! Ex:

(gdb) print &angle
Can't take address of "angle" which isn't an lvalue.
(gdb) print angle_fixed_p
$6 = <optimized out>

With -O0 , however, everything works fine!

(gdb) print angle
$7 = -1.34869879e+20
(gdb) print &angle
$8 = (float *) 0x7ffffffefbbc
(gdb) x angle
0x8000000000000000:     Cannot access memory at address 0x8000000000000000
(gdb) x &angle
0x7ffffffefbbc: 0xe0e9f642

So, back to using -O0 instead of -Og it is!

Related:

1. [they also recommend -O0 , and I concur] What does <value optimized out> mean in gdb?

Answer 2

Due to the editing, the addition of -ggdb isn't entirely clear from Gabriel's excellent answer above. I have found that if GDB is your preferred debugger, then along with -O0 it is definitely advantageous to always also use -ggdb3 (the 3 is important, better than just ggdb ).