4 Jan
2006
4 Jan
'06
1:18 a.m.
Alex Ionescu wrote:
- -O2 makes the compiler do additional checks. For
example, gcc will NOT
detect uninitialized variables unless -O2 is being used, even though
they are a very important programming bug. Apart from finding more bugs,
it also makes trunk compilable. Right now, I see at least two commits by
Thomas or others being made every week in order to fix some code which
used unitinialized variables (I myself have been guilty of this). This
means that some of us, like Thomas, have to constantly fix other
people's mistakes.
- -O2 means less last-minute blockers. Because we release in -O2 but
almost never build it like that, this creates a big problem for people
like Andrew or Brandon, which handle the release process and do testing.
Because the -O2 build gets less testing coverage, it is very possible
for a critical bug to be in ROS for a month before anyone notices it at
release time, in which case we will all have to scramble to find a fix
for it.
- -O2 will not undefine DBG or change anything else in the code. All the
advatanges, extra error checking and assertions of the DBG =1 build
would remain.
- -O2 builds are much faster, greatly helping testing speed.
- -O2 builds are much more likely to bring up race conditions and other
important timing bugs we need to watch out for.
- -O2 means easier debugging. This point is really important because
until I realized how true it was, I didn't want to bring this up. Here
is a pseudo(but real) disassembly of something I've seen in my dbg = 1
kernel binary while debugging:
I've been building with -O3 exclusively for most of these reasons for a
long time. I've had to fix countless bugs/warnings in the past that were
only exposed with optimizations enabled. So I really do support a switch
to -O2/3 although people with slower development machines might not like
this. Depending on the hardware and OS the build time might increase
dramatically. However, at least GCC 4.0.x still has the sibling call
optimization bug, so -fno-optimize-sibling-calls should be used at least
for now. I'm not sure if it's been fixed in the 4.1 branch.
- Thomas