[cfe-dev] When to use '-mcpu' versus '-march'

[Richard Hadsell via cfe-dev]

Does clang differ from g++ in this respect?

This is from the man page for g++ 4.8.5 for Intel x86 and x86-64 processors:

        -march=cpu-type
            Generate instructions for the machine type cpu-type.  In contrast to -mtune=cpu-type, which merely tunes the generated code
            for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the
            one indicated.  Specifying -march=cpu-type implies -mtune=cpu-type.
...
        -mtune=cpu-type
            Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions.
            While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any
            code that cannot run on the default machine type unless you use a -march=cpu-type option.  ...
...
        -mcpu=cpu-type
            A deprecated synonym for -mtune.



On 03/20/2018 12:40 AM, Eric Christopher via cfe-dev wrote:
> Hi Martin,
>
> Sorry about the delay here, but I've got some advice even though a lot of things aren't solid anywhere.
>
> a) Does this match an out of tree target for gcc? If so, I'd match that.
> b) Typically -march/-mtune (though the latter isn't supported in llvm at the moment, see an earlier post from me to llvm-dev on how to support that) are what I'd use for new ports. We used it in a number of new/out of tree ports in gcc as well.
> c) -mcpu is occastionally used for "-march+-mtune" when -march by itself is generally "this architecture, but generic tuning".
>

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