Re: [PATCH v3 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for CC=clang LLVM_IAS=1

From: Masahiro Yamada <masahiroy@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>,
	Miguel Ojeda <ojeda@kernel.org>,
	Fangrui Song <maskray@google.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Arnd Bergmann <arnd@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Christoph Hellwig <hch@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v3 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for CC=clang LLVM_IAS=1
Date: Sat, 31 Jul 2021 00:15:22 +0900	[thread overview]
Message-ID: <CAK7LNATbbHosb7uB53HPo3BpdFH_g5YAKkn1ouL=ymHb5nkEXw@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdm0xs4ikb0K0_b8Az0T=Kxu_-6AHjWHOhjsKZb3hTrH2A@mail.gmail.com>

On Fri, Jul 30, 2021 at 9:19 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Jul 29, 2021 at 2:00 PM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > While I understand that the LLVM=1 LLVM_IAS=1 case works perfectly fine
> > with this series, I am of the belief that making it work for CC=clang
> > LLVM_IAS=1 is a mistake because there is no way for that configuration
> > to work for cross compiling without CROSS_COMPILE.
>
> So with v3 of this change, rather than:
>
> $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang -j72
>
> If you wanted to omit CROSS_COMPILE, you'd need:
>
> $ ARCH=arm64 make CC=clang LLVM_IAS=1 LD=ld.lld OBJCOPY=llvm-objcopy
> STRIP=llvm-strip
>
> or
>
> $ ARCH=arm64 make CC=clang LLVM_IAS=1 LD=aarch64-linux-gnu-ld
> OBJCOPY=aarch64-linux-gnu-objcopy STRIP=aarch64-linux-gnu-strip

or

  $ ARCH=arm64 make LLVM=1 LLVM_IAS=1

still works.

> That's straight up worse IMO and defeats the purpose of "shortening
> the command line," which should be the goal.  Not "making CC=clang
> maximally flexible."  We don't want folks generally using CC=clang;
> preferably they'd use LLVM=1.  I need to rewrite our docs to make that
> more explicit and straightforward.  And if folks would prefer to use
> CC=clang for whatever reason, let them explicitly state CROSS_COMPILE
> then.
>
> So I agree with Nathan, and hope Masahiro will reconsider that perhaps
> the v2 variant that required LLVM=1 maybe makes more sense.

We can always infer the target triple from ARCH unless CROSS_COMPILE is given.

Doing this all the time makes nothing wrong.

"Whether CROSS_COMPILE is unneeded" is a different thing.

> Either way, I need to fix the comment in the new script, commit
> message, and docs, so v4 is necessary.
>
> I'm tempted to add a rewrite of our docs to say "just use LLVM=1"
> front and center, then get into finer grain details below, moving this
> second patch to be the third in a series.  Let's see what Masahiro's
> thoughts are though first. (I do appreciate them, even when I
> disagree).

I am not sure about that.

LLVM=1 is a very strong all-or-nothing flag,
but technically there is no reason to force it.

(At least, target-tools and host-tools look
independent to each other to me)

When you send v4, one more request:
Please drop LLVM_IAS=1 check as well.

-- 
Best Regards
Masahiro Yamada