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

[Cary Coutant]

> Let's take GNU_PROPERTY_X86_FEATURE_2_USED.  Here are
> what x86-64 psABI says:

OK. Pay particular attention to the parts I've highlighted...

> GNU_PROPERTY_X86_FEATURE_2_USED GNU_PROPERTY_X86_UINT32_OR_AND_LO + 1
>
> GNU_PROPERTY_X86_FEATURE_2_USED The x86 processor features
> indicated by the corresponding bits are used in program.
> *** THEIR SUPPORT IN THE HARDWARE IS OPTIONAL. ***
> Its absence in an x86 ELF binary implies that any x86 processor
> features may be used. GNU_PROPERTY_X86_FEATURE_2_USED can be used
> to check for features used in the program.

Earlier in this thread [1], you said:

> 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. ***

You say USED bits correspond to features that are optional, but you
say OR_AND is for features that are required. You say OR may be
generated automatically by the assembler, but you have the assembler
generating the USED bits automatically. You say OR_AND must be
originated by the compiler or programmer -- those are clearly for the
NEEDED features, since the assembler cannot automatically determine
whether the use of an instruction is guarded by a CPUID check.

-cary

[1] https://sourceware.org/ml/binutils/2018-11/msg00238.html