[cfe-dev] [llvm-dev] difference between --target, -mcpu, -march

[Rail Shafigulin via cfe-dev]

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
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