[PATCH] x86: Don't remove empty x86 properties

[H.J. Lu]

On Tue, Nov 27, 2018 at 11:45 PM Cary Coutant <ccoutant@gmail.com> wrote:
>
> > 3 kinds of properties provide answers for different questions:
> >
> > 1. AND:  Do all input relocatables have the feature?
> > 2. OR:  Does any input relocatables have the feature?
> > 3. OR_AND:  Do all input relocatables have the property and
> > some inputs have the feature? In turn, it can tell if none of input
> > relocatables have the feature.
> >
> > It works better to keep both OR and OR_AND.
>
> You can tell if none of the input relocatables have the feature only
> if *all* of the input relocatables have the property. If even one of
> the input relocatables is missing the property, you lose all the
> feature bits -- you're discarding potentially useful information.
> Remember, all the 1 bits are trustworthy even if a relocatable is
> missing the property; it's the 0 bits that might not be accurate
> (i.e., some of them might have been 1, depending on features used by
> the objects missing the property info).

1 bits are accurate for AND, OR and OR_AND.  0 bits are accurate only
for OR_AND.

> So now what do you expect the loader to do with the information in the
> property note? If you have feature bits, presumably it will compare
> the 1 bits with the available features on the hardware, and refuse to
> run the program if its demands exceed the available features. And it
> could give a meaningful error message explaining exactly which
> features are required but not available.

Correct.

> But what if you've removed the property entirely? Should the loader
> refuse to run the program? No, you're going to want it to run anyway,
> right? And it'll fail at run time if it tries to use an unavailable
> feature. But if you hadn't thrown away those 1 bits, you could still
> get a meaningful error message for the features we do know about. A
> single new AND bit (or a version number) will tell you the same thing
> that a missing property tells you, without throwing away all those
> useful bits of information.
>

AND and OR_AND are used for different purposes:

1.  AND:  It is OK if run-time doesn't have the feature the program supports,
like CET.
2. OR_AND: It is not OK if run-time doesn't have the feature the program
requires, like needed AVX512

OR is informational, like used AVX512:

1 bit: Yes.
0 bit: Unknown.

AND and OR_AND are useful for loaders and must be originated by
compiler or programmer.   OR may be generated automatically by
assembler.

-- 
H.J.