Book: "Pentaho Kettle Solutions", Matt Casters, Roland Bouman, & Jos van Dongen, Wiley 2010 Book: "Pentaho Solutions", Roland Bouman & Jos van Dongen, Wiley 2009

Monday, January 22, 2007

GCC: -march and -mtune

Some might've noticed that the check-cpu script in the MySQL BUILD directory does not always succeed in detecting the right processor:

roland@roland-laptop:/opt/mysql/5.1/bitkeeper/BUILD$ ./check-cpu
BUILD/check-cpu: Oops, could not find out what kind of cpu this machine is using.

When you dig a little deeper into that script, you will notice that the name of you particular processor (in my case: Intel(R) Core(TM)2 CPU T5600 @ 1.83GHz) is indeed not there and thus not checked. Finally you may find that the model and name of your processor are passed to the gcc compiler.

As far as I cans see the information is used to set the values of the --mtune and --march compiler options. What those are? Hehe! Ok, here's a partial result from man gcc:

-march=cpu-type: Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mtune. Moreover, specifying -march=cpu-type implies -mtune=cpu-type.

Mmm, not sure why -mtune=cpu-type is there in the first place then...

Anybody?

(PS: use /proc/cpuinfo to find out what kind of processor you really have....)

6 comments:

Anonymous said...

gcc 4.3 adds "core2" as arch. I've resisted the temptation to add that quite yet as pretty much nobody has 4.3 yet.
-- Azundris

Anonymous said...

I'm not 100% sure, but I think -mcpu doesn't break compatability with other CPUs but -march (-mtune) does...

Anonymous said...

The manual (info gcc) is your friend:

-mtune (not --mtune): Tune to CPU-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions.

-march: Generate instructions for the machine type CPU-TYPE. Specifying -march=CPU-TYPE implies -mtune=CPU-TYPE.

-mcpu: A deprecated synonym for -mtune.

Unfortunately the manual for GCC 4.1 will not tell you what CPU-TYPE to use for core2. Dunno about 4.2, though.

//Carl T

Anonymous said...

-mtune is used predominantly for instruction scheduling, gcc uses it when it's reordering instructions to favour a specific micro-architecture over others. Code which is tuned for one microarchitecture will run on another chip of the same architecture, but the reordering may hurt performance on these chips.

-mcpu tells it to generate code SPECIFICALLY for a certain chip, which obviously includes instruction reordering (thus why -mcpu implies -mtune), but will also cause the compiler to emit instructions specific to the target processor. This does not guarantee binary compatibility with other chips based on a different microarchitecture.

pixelbeat said...

Here is a script that finds the
optimum gcc cpu settings for your
cpu in conjuntion with your currently
installed gcc.

Anonymous said...

This is why I hate blogs / wiki answers. People post answers when they don't really know all the facts, even *after* the issue has been correctly addressed by a previous comment! I'm concluding that these type of people just like to here themselves type. :|

-march={cpu} : Generate a binary *only* for the specific cpu type (and later, compatible models [most later cpu's support earlier optimizations, or gracefully bypass them]). This flag should generate the faster code on x86 architectures. Fall back to the most recent (mainstream) model in your vendors line if yours is not yet available.

-mtune={cpu} : Tunes the binary for given cpu on x86 architecture. This means that the binary will still execute on a 386 (if it did in the first place), while also including optimization for the cpu supplied to the '-mtune=' switch.

-mcpu={cpu} : Deprecated (phased out) for the x86 architecture.