linux-kernel - Re: [PATCH 19/19] arm64: vdso32: Enable Clang Compilation

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Message-ID: <20200217164608.GA2708@willie-the-truck>
Date:   Mon, 17 Feb 2020 16:46:09 +0000
From:   Will Deacon <will@...nel.org>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        linux-arch@...r.kernel.org, ndesaulniers@...gle.com,
        0x7f454c46@...il.com, avagin@...nvz.org, arnd@...db.de,
        sboyd@...nel.org, catalin.marinas@....com, x86@...nel.org,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, clang-built-linux@...glegroups.com,
        paul.burton@...s.com, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, linux@...linux.org.uk, tglx@...utronix.de,
        salyzyn@...roid.com, pcc@...gle.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 19/19] arm64: vdso32: Enable Clang Compilation

On Mon, Feb 17, 2020 at 12:26:16PM +0000, Vincenzo Frascino wrote:
> On 13/02/2020 18:44, Nathan Chancellor wrote:
> > On Thu, Feb 13, 2020 at 04:16:14PM +0000, Vincenzo Frascino wrote:
> >> Enable Clang Compilation for the vdso32 library.
> 
> [...]
> 
> >> +LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
> > 
> > Well this is unfortunate :/
> > 
> > It looks like adding the --target flag to VDSO_LDFLAGS allows
> > clang to link the vDSO just fine although it does warn that -nostdinc
> > is unused:
> > 
> > clang-11: warning: argument unused during compilation: '-nostdinc'
> > [-Wunused-command-line-argument]
> >
> 
> This is why ended up in this "unfortunate" situation :) I wanted to avoid the
> warning.
> 
> > It would be nice if the logic of commit fe00e50b2db8 ("ARM: 8858/1:
> > vdso: use $(LD) instead of $(CC) to link VDSO") could be adopted here
> > but I get that this Makefile is its own beast :) at the very least, I
> > think that the --target flag should be added to VDSO_LDFLAGS so that gcc
> > is not a requirement for this but I am curious if you tried that already
> > and noticed any issues with it.
> > 
> 
> --target is my preferred way as well, I can try to play another little bit with
> the flags and see what I can come up with in the next version.

Yes, please. I'd even prefer the warning rather than silently assuming that
a cross gcc is kicking around on the path.

Will