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From: Richard Sandiford <richard.sandiford@arm.com>
To: Christer Solskogen via Gcc-help <gcc-help@gcc.gnu.org>
Mail-Followup-To: Christer Solskogen via Gcc-help <gcc-help@gcc.gnu.org>,
 Christer Solskogen <christer.solskogen@gmail.com>, richard.sandiford@arm.com
Cc: Christer Solskogen <christer.solskogen@gmail.com>
Subject: Re: aarch64 feature modifiers
References: <sunu47$1l4$1@ciao.gmane.io>
Date: Mon, 21 Feb 2022 11:35:44 +0000
In-Reply-To: <sunu47$1l4$1@ciao.gmane.io> (Christer Solskogen via Gcc-help's
 message of "Fri, 18 Feb 2022 11:58:14 +0100")
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Christer Solskogen via Gcc-help <gcc-help@gcc.gnu.org> writes:
> On https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html there's a 
> list of feature modifiers to -march and -mcpu. Is there a way to check 
> or detect what features are available on a particular CPU?

If you want to compile for a specific CPU then it's better to use
-mcpu=<that-cpu> with no feature modifiers, and leave out -march.
GCC will then make full use of the available features.
E.g. -mcpu=neoverse-n1 (on its own) will enable all features
available on Neoverse N1 and optimise code for Neoverse N1.

If you're just curious which features a given CPU implements,
you can find that out by compiling a dummy file with -mcpu.  E.g.:

  echo | gcc -mcpu=neoverse-n1 -xc - -o - -S | grep '\.arch'

will print out:

        .arch armv8.2-a+crc+fp16+rcpc+dotprod+profile

But it should never be necessary to convert that back into an
-march option.  Using -mcpu with no feature modifiers is better.

-march is really for the case in which you don't know which CPU
the code will run on, but you instead have a baseline set of
requirements.

Thanks,
Richard