linux-kernel - Re: [PATCH] scripts/Makefile.clang: default to LLVM

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Date:   Thu, 5 Aug 2021 11:40:15 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Khem Raj <raj.khem@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SIFIVE DRIVERS" <linux-riscv@...ts.infradead.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] scripts/Makefile.clang: default to LLVM_IAS=1

On Thu, Aug 5, 2021 at 8:16 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Tue, Aug 3, 2021 at 8:43 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> >
> > diff --git a/Makefile b/Makefile
> > index 444558e62cbc..b24b48c9ebb7 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -845,7 +845,7 @@ else
> >  DEBUG_CFLAGS   += -g
> >  endif
> >
> > -ifneq ($(LLVM_IAS),1)
> > +ifeq ($(LLVM_IAS),0)
> >  KBUILD_AFLAGS  += -Wa,-gdwarf-2
> >  endif
> >
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index bc74afdbf31e..807f7c94bc6f 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -41,7 +41,7 @@ endif
> >  ifeq ($(CONFIG_LD_IS_LLD),y)
> >         KBUILD_CFLAGS += -mno-relax
> >         KBUILD_AFLAGS += -mno-relax
> > -ifneq ($(LLVM_IAS),1)
> > +ifeq ($(LLVM_IAS),0)
> >         KBUILD_CFLAGS += -Wa,-mno-relax
> >         KBUILD_AFLAGS += -Wa,-mno-relax
> >  endif
>
>
>
> Please drop these two hunks.
>
> I will apply my patch instead.
> https://lore.kernel.org/patchwork/patch/1472580/

Sure.  Will send a v2 with Matthew's suggestion as well.

> When we negate a flag that is enabled by default,
> which is a common way?
>  - set it to '0'
>  - set is to empty
>
>
> So, I was wondering if we should support
> not only LLVM_IAS=0 but also LLVM_IAS=.
>
> What do you think?

LLVM_IAS= looks weird (so I agree with Khem's response), but if it's
common/expected then maybe if there's a way to write a concise check
for either =<blank> or =0?  I don't feel strongly about how it's
specified to disable the integrated assembler, but let's sort that out
_before_ I send a v2.

How can you tell the difference between `make CC=clang` and `make
CC=clang LLVM_IAS=`? (maybe that doesn't matter here, as either imply
"don't use clang as the assembler when compiling with clang")
-- 
Thanks,
~Nick Desaulniers