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Date: Fri, 10 May 2024 14:43:10 -0700
From: Charlie Jenkins <charlie@rivosinc.com>
To: Conor Dooley <conor@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Conor Dooley <conor.dooley@microchip.com>,
	Song Liu <song@kernel.org>, Xi Wang <xi.wang@gmail.com>,
	=?iso-8859-1?Q?Bj=F6rn_T=F6pel?= <bjorn@rivosinc.com>,
	=?iso-8859-1?Q?Cl=E9ment_L=E9ger?= <cleger@rivosinc.com>,
	Jessica Clarke <jrtc27@jrtc27.com>,
	Andy Chiu <andy.chiu@sifive.com>, linux-riscv@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/8] riscv: Add PLATFORM_MAY_SUPPORT_RISCV_ISA_V
 Kconfig option
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References: <20240507-compile_kernel_with_extensions-v2-0-722c21c328c6@rivosinc.com>
 <20240507-compile_kernel_with_extensions-v2-2-722c21c328c6@rivosinc.com>
 <20240510-earthly-regress-7a8c8dba55db@spud>
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On Fri, May 10, 2024 at 09:43:33PM +0100, Conor Dooley wrote:
> Hey Charlie,
> 
> On Tue, May 07, 2024 at 06:36:28PM -0700, Charlie Jenkins wrote:
> > Current versions of the kernel add "v" to the march and then immeidately
> > filter it out such that "v" is not passed to CFLAGS.  Instead of doing
> > this filtering, code blocks in the kernel that want to use vector
> > assembly have been changed to locally enable vector (using ".option
> > arch, +v").
> 
> Other content in the series aside, since this is a change that could be
> made independently of the main series objectives, I figured it was worth
> pointing out that this is not a change without downsides: I think that
> it would drop support for vector with most versions of LLVM as
> .option arch support there is much more recent thing than it is for gcc.
> Off the top of my head I don't know exactly the versions involved, but
> it is something like LLVM-14 supports vector but only LLVM-17 and later
> supports .option arch.

Toolchain incompatibilities are always fun. It does look like .option
arch was introduced in LLVM-17. That would be a regression. We do use
option arch for every other extension, but vector was treated special
when it was introduced unfortunately so maybe we have to live with the
weird march parsing hack.

- Charlie

> 
> Thanks,
> Conor.