llvm dev - Mar 2016 - difference between --target, -mcpu, -march

On Fri, Mar 18, 2016 at 12:49 PM, Eric Christopher <echristo at gmail.com>
wrote:
>
>
> On Wed, Mar 16, 2016 at 2:53 PM Rail Shafigulin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I'm confused about the clang --target, -mcpu, -march
>>
>> Can someone give a clear explanation on what is the difference between
>> them?
>>
>>
> I can try :)
>
>
>> Originally I thought need to specify -mcpu (which I assume means CPU)
and
>> -march but then I can't figure out how --target fits into the
picture.
>> Sometimes it tells me that -march or -mcpu options is not used. I would
>> really appreciate any help on this.
>>
>>
> Let's say you're on an x86_64-linux-gnu machine (basically any
linux
> machine) and you want to compile some code for a haswell specific binary,
> you'd use:
>
> clang -march=haswell foo.c
>
> Which tells clang to use the current host as the "OS" for your
compile and
> to tell the backend to generate haswell specific code.
>
> Let's say you instead want to compile for arm-linux-gnu (just your
basic
> arm linux machine), you'd use:
>
> clang -target arm-linux-gnu foo.c <some other options>
>
> I'm glossing over this a bit because it's not relevant to what you
asked,
> but the other options there are going to be things like a sysroot where you
> can get the headers and libraries for an arm-linux-gnu machine, because
> they're likely not installed on your x86_64-linux-gnu machine.
>
> Now, let's say you want to target a specific arm processor as part of
your
> cross compile, you'd do this:
>
> clang -target arm-linux-gnu foo.c -mcpu=armv7
>
> or you could do this:
>
> clang -target armv7-linux-gnu foo.c
>
> this is partially because of legacy reasons, and partially because arm is
> weird here. The -mcpu and -march options are very similar in clang. In
> general, they're setting the processor you're compiling your code
to run
> on. (Arm is even weirder here so I'm glossing over a lot of details).
>
> The reasons behind this are largely historical and option compatibility
> with gcc. If you've done cross compilation with gcc all that the
--target
> option does in clang is select at runtime the same thing that you'd use
> --target for on the configure line.
>
> Make sense?
>
> -eric
>
It somewhat makes sense. Basically one has to know very well what options
to use for their specific target. It looks like at the moment there is no
consistency.

-- 
Rail Shafigulin
Software Engineer
Esencia Technologies
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160318/ee7fd643/attachment.html>

On Fri, Mar 18, 2016 at 1:44 PM Rail Shafigulin <rail at esenciatech.com>
wrote:
>
> On Fri, Mar 18, 2016 at 12:49 PM, Eric Christopher <echristo at
gmail.com>
> wrote:
>
>>
>>
>> On Wed, Mar 16, 2016 at 2:53 PM Rail Shafigulin via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I'm confused about the clang --target, -mcpu, -march
>>>
>>> Can someone give a clear explanation on what is the difference
between
>>> them?
>>>
>>>
>> I can try :)
>>
>>
>>> Originally I thought need to specify -mcpu (which I assume means
CPU)
>>> and -march but then I can't figure out how --target fits into
the picture.
>>> Sometimes it tells me that -march or -mcpu options is not used. I
would
>>> really appreciate any help on this.
>>>
>>>
>> Let's say you're on an x86_64-linux-gnu machine (basically any
linux
>> machine) and you want to compile some code for a haswell specific
binary,
>> you'd use:
>>
>> clang -march=haswell foo.c
>>
>> Which tells clang to use the current host as the "OS" for
your compile
>> and to tell the backend to generate haswell specific code.
>>
>> Let's say you instead want to compile for arm-linux-gnu (just your
basic
>> arm linux machine), you'd use:
>>
>> clang -target arm-linux-gnu foo.c <some other options>
>>
>> I'm glossing over this a bit because it's not relevant to what
you asked,
>> but the other options there are going to be things like a sysroot where
you
>> can get the headers and libraries for an arm-linux-gnu machine, because
>> they're likely not installed on your x86_64-linux-gnu machine.
>>
>> Now, let's say you want to target a specific arm processor as part
of
>> your cross compile, you'd do this:
>>
>> clang -target arm-linux-gnu foo.c -mcpu=armv7
>>
>> or you could do this:
>>
>> clang -target armv7-linux-gnu foo.c
>>
>> this is partially because of legacy reasons, and partially because arm
is
>> weird here. The -mcpu and -march options are very similar in clang. In
>> general, they're setting the processor you're compiling your
code to run
>> on. (Arm is even weirder here so I'm glossing over a lot of
details).
>>
>> The reasons behind this are largely historical and option compatibility
>> with gcc. If you've done cross compilation with gcc all that the
--target
>> option does in clang is select at runtime the same thing that you'd
use
>> --target for on the configure line.
>>
>> Make sense?
>>
>> -eric
>>
>
> It somewhat makes sense. Basically one has to know very well what options
> to use for their specific target. It looks like at the moment there is no
> consistency.
>
-Most- targets use -march.

-eric

>
>
> --
> Rail Shafigulin
> Software Engineer
> Esencia Technologies
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160318/003ac68c/attachment.html>

>
> -Most- targets use -march.
>
> -eric
>
Do you meat that in most cases -target is used along with -march instead of
-target and -mcpu?

-- 
Rail Shafigulin
Software Engineer
Esencia Technologies
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160318/9efe3d1b/attachment.html>