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

[Jan Beulich]

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.

>  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).

Jan