linux-kernel - Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility

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Date:   Fri, 5 May 2017 19:50:39 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mka@...omium.org, md@...gle.com, tglx@...utronix.de, hpa@...or.com,
        torvalds@...ux-foundation.org, grundler@...omium.org,
        linux-kernel@...r.kernel.org, ghackmann@...gle.com,
        keescook@...omium.org, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for
 multiplication to work around Clang incompatibility


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, May 05, 2017 at 01:11:47AM -0700, tip-bot for Matthias Kaehlcke wrote:
> > Commit-ID:  121843eb02a6e2fa30aefab64bfe183c97230c75
> > Gitweb:     http://git.kernel.org/tip/121843eb02a6e2fa30aefab64bfe183c97230c75
> > Author:     Matthias Kaehlcke <mka@...omium.org>
> > AuthorDate: Mon, 1 May 2017 15:47:41 -0700
> > Committer:  Ingo Molnar <mingo@...nel.org>
> > CommitDate: Fri, 5 May 2017 08:31:05 +0200
> > 
> > x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility
> > 
> > The constraint "rm" allows the compiler to put mix_const into memory.
> > When the input operand is a memory location then MUL needs an operand
> > size suffix, since Clang can't infer the multiplication width from the
> > operand.
> 
> *sigh*, this is another shining example of how LLVM is a better, faster
> moving compiler?

Well, I don't like it - but we already have similar patterns to cover some asm 
complications so I didn't mind. Apparently Clang is very close to being able to 
build a working Linux kernel, right?

In that sense it would be unfair to expect it to not have various legacies, 
missing features and quirks - just like the kernel has dozens of GCC related 
workarounds.

Thanks,

	Ingo