[PATCH] Linux/x86: Configure gas with --enable-x86-used-note by default

[Jan Beulich]

On 09.07.2020 17:44, H.J. Lu wrote:
> On Thu, Jul 9, 2020 at 8:36 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 09.07.2020 17:22, H.J. Lu wrote:
>>> On Thu, Jul 9, 2020 at 8:21 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>
>>>> On 09.07.2020 17:02, H.J. Lu via Binutils wrote:
>>>>>       * configure.ac: Configure with --enable-x86-used-note by default
>>>>>       for Linux/x86.
>>>>
>>>> I'm quite unconvinced this is a good idea as long as the contents of
>>>> these notes are neither reliable nor properly settled on what they
>>>> actually mean. This should be considered an experimental feature for
>>>> now, and hence would better not be enabled by default to avoid
>>>> future backwards compatibility issues.
>>>>
>>>
>>> It has been an experimental feature for 2 years.
>>
>> And when I asked questions about the underlying spec I did get at
>> best fuzzy answers from you. I still have on my todo list to get
>> this in shape with a pretty recent reply of yours, which was
>> supporting my view on how things should be (and why things are
>> broken right now), and hence somewhat contrary to earlier replies
>> of yours.
>>
>> To just name the most unclear (to me) aspect of what's there
>> currently: What's the distinction between xmm, ymm, and zmm, when
>> zmm is a superset of ymm (and ymm one of xmm), yet at the same
>> time AVX512 writes to xmm mean writes to ymm and zmm (zeroing
>> upper parts) as well. From my observations e.g. an AVX512 insn
>> accessing just xmm registers will record just an xmm dependency,
>> which then is simply wrong - the resulting binary in fact depends
>> on all of xmm, ymm, zmm, and the mask registers. IIRC I proposed
>> back then to tie what an insn records to the XCR0 bits it
>> requires to be set in order for it to not fault.
> 
> Should we add a field to i386 opcode table for this?

I was wanting to try without, to see how much logic would be needed.
If it gets too much, doing so may be the better choice.

>>>  Let's see what fallouts will be.
>>
>> Fallout may become noticeable only when the behavior of the notes
>> changes: People may start noticing that new (correct) dependencies
>> prevent things from working (assuming there's any consumer of
>> these notes).
> 
> We have a couple months to address this before 2.36 is released.

Under the assumption that people test non-released code ...

Jan