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From:   Jamie Cunliffe <Jamie.Cunliffe@arm.com>
To:     boqun.feng@gmail.com
Cc:     Jamie.Cunliffe@arm.com, catalin.marinas@arm.com,
        lina@asahilina.net, linux-arm-kernel@lists.infradead.org,
        ojeda@kernel.org, rust-for-linux@vger.kernel.org,
        steve.capper@arm.com, will@kernel.org
Subject: Re: [PATCH v2 1/3] arm64: rust: Enable Rust support for AArch64
Date:   Mon, 12 Jun 2023 16:26:39 +0100
Message-Id: <20230612152639.1446766-1-Jamie.Cunliffe@arm.com>
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> I don't think you need "-fp-armv8" here, because "aarch64-unknown-none"
> target doesn't have the target-feature "fp-arm64":
> 
> 	rustc +1.68.2 --print cfg --target aarch64-unknown-none | grep target_feature
> 
> 	target_feature="neon"
> 
> , and having the "-fp-armv8" gave me a warning at compile time:
> 
> 	  RUSTC L rust/core.o warning: unknown feature specified for `-Ctarget-feature`: `fp-armv8`
> 	  |
> 	  = note: it is still passed through to the codegen backend
> 	  = help: consider filing a feature request

Rust removed the fp feature for arm64 and tied it with Neon. However
this initially had a bug that meant it couldn't be disabled. This has
now been fixed in https://github.com/rust-lang/rust/pull/107294 so
once we can update rustc we can fix that warning by removing the
`-fp-armv8`. Until we do that we still need the `-fp-armv8` otherwise
we will get some floating point usage. This can be seen by doing an
objdump of core.o if you build after removing the `-fp-armv8`.


You can also see that fp-armv8 is enabled in the aarch64-unknown-none target
here: https://raw.githubusercontent.com/rust-lang/rust/master/compiler/rustc_target/src/spec/aarch64_unknown_none.rs


Thanks,
Jamie.