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

[Cary Coutant]

Thanks, this is much clearer:

> GNU_PROPERTY_X86_UINT32_AND_LO..GNU_PROPERTY_X86_UINT32_AND_HI
> A bit in the output pr_data field is set only if it is set in all
> relocatable input pr_data
> fields. If all bits in the the output pr_data field are zero, this
> property should be
> removed from output.
>
> GNU_PROPERTY_X86_UINT32_OR_LO..GNU_PROPERTY_X86_UINT32_OR_HI
> A bit in the output pr_data field is set if it is set in any
> relocatable input pr_data
> fields. If all bits in the the output pr_data field are zero, this
> property should be
> removed from output.
>
> GNU_PROPERTY_X86_UINT32_OR_AND_LO..GNU_PROPERTY_X86_UINT32_OR_AND_HI
> A bit in the output pr_data field is set if it is set in any
> relocatable input pr_data
> fields and this property is present in all relocatable input files. A
> missing property
> implies that its bits may have any values. When all bits in the the
> output pr_data
> field are zero, this property should not be removed from output.

So the "linker must not remove properties with all zeroes" rule is
only for the OR_AND properties, right? That is, ISA_1_USED and
FEATURE_2_USED. And the rationale for this rule is that any object
that doesn't contain the property is presumed old and therefore uses
an unknown set of ISAs and FEATUREs. But an object that does contain a
property with all zeroes uses only the base ISA or no extra FEATUREs.

What's the point of the rule that ISA_1_USED is "valid only if
GNU_PROPERTY_X86_FEATURE_2_USED type is also present"?

It might have been simpler to define a bit in FEATURE_1_AND to
indicate that the object is not old and that its ISA and FEATURE bits
can be trusted (even if missing and assumed zero). The OR_AND
properties could then just be simple OR properties, and a missing
property would be synonymous with all zero bits. If that new bit is
off in the output file, the loader would know that the ISA and FEATURE
bits may not completely describe the object.

A version number might be even more useful, since we've already seen
two or three revisions since this scheme was first deployed. Define a
new property with an integer value and a "MIN" combining rule that
says to output the minimum of all values seen in the input objects
(where an object with a missing property is taken as version 0). As an
example, this would cover you if you decide at some point in the
future to define a new ISA or FEATURE bit to represent some feature
that may already be in use in some existing objects.

-cary