Re: 4.8.2 -Og vs. -O1

[Joel Sherrill <Joel dot Sherrill at OARcorp dot com>]

Have you compared it to -Os?  That seems to produce assembly closer to what you would likely write by hand. I haven't benchmarked it much but it gives 7-10% smaller code in general. In many cases, fewer instructions is also a performance win.

Gene Smith <gds@chartertn.net> wrote:


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