Xinliang David Li wrote:
Thanks for the comments. FDO will probably improve SPEC2000 score.
Although it is not obvious for some tests because the train data sets for
them are different from the reference data sets and it might actually
mislead the compiler.
FDO is important for optimizations where all possible data sets do not
change branch probability distribution much. IMHO therefore FDO is not
widely used by most of developers (although I am sure that for Google
applications it is extremely important) and therefore I don't measure it and
it is not so interesting for me. Although bigger reason not use FDO is
inconvenience to use it for regular compiler user.
As for vortex FDO improvement, vortex contains a moderate size loop in which
most of time is spent. The loop has if-then-else on the top loop level. On
all SPEC2000 data sets, one if-branch is taken practically always (like 1
to 1,000,000). So it is not amazing for me that FDO gives such
improvement for vortex.
Actually what I was trying to say is that LTO will be more powerful
when combined with FDO. In other words, I expect LTO + FDO improves
over plain FDO more than 1.86%.
It would be great if there is number collected comparing LTO + FDO vs
plain FDO in the same setup.
Usually after such posting the comparisons, I am getting a lot of requests.
I'd like to do all of them but unfortunately running and the result
preparation takes a lot of my time. May be I'll do such comparison next
year.
Ok. Another comment is that using SPEC2000 for performance testing
won't be indicative of today's real world program size. Even
SPEC2006's largest C++ programs are not that big.
It is very subjective what is today's real world program size. Usually
it reflects what you are working on. I understand that Google
applications are huge and their speed is important for saving money (or
energy) for their employees. Firefox is big enough but for regular
desktop user 1% improvement may be invisible or not important if it is
already fast enough.
A math-physics program can be small but its speed may be really
important because it takes hours or days on fast machine. Even big and
intensively used applications like some logistic system can have small
program parts (e.g. ILP solver or compression algorithms like gzip for
speeding Internet communication up) whose optimization are the most
important for the application and SPEC contains such
calculation-intensive code (a lot of NP-complete task solvers and math
physics programs). So I would not say using SPEC for performance
testing is not important for improving today's real world size
program. Of course it is not so important than testing the program you
are working on. In order words, this program is most important
benchmark for you but probably not for others.
As for me, GCC itself is very important program and SPEC contains it
(2000 old one version and 2006 more recent one). So SPEC is pretty
important and good for me (not perfect of course at least because it is
not free) although it is not the single one which I care of.
