llvm dev - Jun 2011 - [LLVMdev] What is the optimization level means?

I tried to understand what the difference in optimization level and I got
confuse...
When I use *llc -O1, -O2, or -O3*, I always got the same assembly code in my
test program.
Can anybody teach me that what's the actual effect for using different
optimization level ?

Best,
Foster
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110607/3ebcde12/attachment.html>

On Mon, Jun 6, 2011 at 9:49 AM, don <dondum113 at gmail.com>
wrote:
> I tried to understand what the difference in optimization level and I got
> confuse...
> When I use llc -O1, -O2, or -O3, I always got the same assembly code in my
> test program.
> Can anybody teach me that what's the actual effect for using different
> optimization level ?
There really isn't a big difference between llc at -O1, -O2, and -O3;
IIRC, some optimizations are slightly more aggressive, but you
normally won't notice much difference.  Most of the differences are in
the IR optimization passes (which can be run with "opt -O3" etc.).

-Eli