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 08:22:35 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sedat Dilek <sedat.dilek@...il.com>
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 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