4.8.2 -Og vs. -O1

[Gene Smith]

I tried -Og optimization on a recent svn snapshot of 4.8 and don't see 
much difference in the code compared to -O1. If anything, at least for 
one case, -Og is actually less debuggable than -O1, e.g., for a simple 
buffer selection like this:

uint8_t* buffer;

if (condx == true)
	buffer = buf1;  // buf1 is a static external buffer
else
	buffer = buf2;  // buf2 is a static external buffer
uint8_t foo = buffer[1];

With -O1 there is assembly code associated with each buffer assignment 
statement. But with -Og there is no code under the first buffer = buf1 
with it all under the 2nd buffer = buf2.

So, with -Og, when stepping through the code with condx true, it appears 
that the wrong line is executing since the first buffer = buf1 has no 
code and never occurs. Of course, the result is still correct and is 
actually maybe more efficient or at least equal to the -O1 code, but 
there is no improved debug experience in this case.

In this case, the debug experience with -O1 is closer to -O0 than -Og is.

Also with -Og, some variables are still optimized away like -O1 and 
higher, but unlike -O0 where all variables are, of course, visible with 
the debugger (gdb).

-gene