linux-kernel - Re: Clang and X86-EFlags (was Re: [PATCH] usbhid: Fix lockdep unannotated irqs-off warning)

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Date:   Tue, 24 Apr 2018 17:46:55 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        David Laight <David.Laight@...lab.com>,
        Jiri Kosina <jikos@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Michael Hordijk <hoffbrinkle@...mail.com>,
        Chandler Carruth <chandlerc@...il.com>
Subject: Re: Clang and X86-EFlags (was Re: [PATCH] usbhid: Fix lockdep
 unannotated irqs-off warning)

On Tue, Apr 24, 2018 at 5:22 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Apr 24, 2018 at 6:28 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>>
>> $ objdump -S clang-eflag.o
>>
>> Does this now look good?
>
> Looks fine to me. The instruction choice is still pretty odd:
>
>   1a:   f6 c3 ff                test   $0xff,%bl
>   1d:   74 02                   je     21 <bar+0x21>
>
> that "test   $0xff,%bl" is a rather odd way of testing the byte for zero, since
>
>   1a:   84 db                test   %bl,%bl
>   1c:   74 02                   je     20 <bar+0x20>
>
> would have been a byte shorter and is the canonical way on x86 to test
> a register against zero.
>
> But that's just a "looks odd to somebody who is used to x86 asm", not
> a bug or anything really noticeable.
>
>                   Linus

Thanks for the quick reply and the confirmation.

Personally, I do not speak x86/asm.
I hope CCed LLVM folks know on how to improve things.

- Sedat -