linux-kernel - Re: [PATCH 2/3] powerpc: fix build errors

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Message-Id: <1645748553.sa2ewgy7dr.astroid@bobo.none>
Date:   Fri, 25 Feb 2022 10:23:07 +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 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.

Thanks,
Nick