Message-Id: <1645833637.za1t01a9md.astroid@bobo.none>
Date:   Sat, 26 Feb 2022 10:07:38 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Anders Roxell <anders.roxell@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH 2/3] powerpc: fix build errors

Excerpts from Segher Boessenkool's message of February 26, 2022 8:28 am:
> On Fri, Feb 25, 2022 at 10:23:07AM +1000, Nicholas Piggin wrote:
>> Excerpts from Segher Boessenkool's message of February 25, 2022 3:29 am:
>> > On Thu, Feb 24, 2022 at 09:13:25PM +1000, Nicholas Piggin wrote:
>> >> Excerpts from Arnd Bergmann's message of February 24, 2022 8:20 pm:
>> >> > Again, there should be a minimum number of those .machine directives
>> >> > in inline asm as well, which tends to work out fine as long as the
>> >> > entire kernel is built with the correct -march= option for the minimum
>> >> > supported CPU, and stays away from inline asm that requires a higher
>> >> > CPU level.
>> >> 
>> >> There's really no advantage to them, and they're ugly and annoying
>> >> and if we applied the concept consistently for all asm they would grow 
>> >> to a very large number.
>> > 
>> > The advantage is that you get machine code that *works*.  There are
>> > quite a few mnemonics that translate to different instructions with
>> > different machine options!  We like to get the intended instructions
>> > instead of something that depends on what assembler options the user
>> > has passed behind our backs.
>> > 
>> >> The idea they'll give you good static checking just doesn't really
>> >> pan out.
>> > 
>> > That never was a goal of this at all.
>> > 
>> > -many was very problematical for GCC itself.  We no longer use it.
>> 
>> You have the wrong context. We're not talking about -many vs .machine
>> here.
> 
> Okay, so you have no idea what you are talking about?  Wow.

Wrong context. It's not about -many. We're past that everyone agrees 
it's wrong.

> The reason GCC uses .machine *itself* is because assembler -mmachine
> options *cannot work*, for many reasons.  We hit problems often enough
> that years ago we started moving away from it already.  The biggest
> problems are that on one hand there are mnemonics that encode to
> different instructions depending on target arch or cpu selected (like
> mftb, lxvx, wait, etc.), and on the other hand GCC needs to switch that
> target halfway through compilation (attribute((target(...)))).
> 
> Often these problems were hidden most of the time by us passing -many.
> But not all of the time, and over time, problems became more frequent
> and nasty.
> 
> Passing assembler -m options is nasty when you have to mix it with
> .machine statements (and we need the latter no matter what), and it

No it's not nasty, read the gas manual. -m specifies the machine and
so does .machine. It's simple.

> becomes completely unpredictable if the user passes other -m options
> manually.
> Inline assembler is inserted textually in the generated assembler code.
> This is a big part of the strength of inline assembler.  It does mean
> that if you need a different target selected for your assembler code
> then you need to arrange for that in your assembler code.
> 
> So yes, this very much is about -many, other -m options, and .machine .
> I discourage the kernel (as well as any other project) from using -m
> options, especially -many, but that is your own choice of course.  I
> get sick and tired from you calling a deliberate design decision we
> arrived at after years of work and weighing alternatives a "bug" though.

Alan posted a good summary here

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102485#c10

Thanks,
Nick

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