On 07/01/2013 02:05 AM, Joel Sherrill wrote:
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.
Hi Joel,
Looking for every var and c/c++ instruction line visible and step-able
in debugger with fastest possible code. A difficult trade-off no doubt.
Currently this only happens at -O0 but speed is really bad.
I haven't seen any other feedback on this list about the new -Og option.
-gene
Gene Smith<g...@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
