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

[H.J. Lu]

On Wed, Dec 5, 2018 at 10:17 AM Cary Coutant <ccoutant@gmail.com> wrote:
>
> > 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. If the bit is 1, some input relocatables have the
> > feature. If
> > the bit is 0 or the property is missing, it is unknown whether any
> > input relocatables
> > have the feature.
> >
> > 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 have unknown values. When all bits in the the
> > output pr_data
> > field are zero, this property should not be removed from output to
> > indicate it has
> > zero in all bits. If the property is in output, all input relocatables
> > have the property.
> > If the bit is 1, some input relocatables have the feature. If the bit
> > is 0, none of input
> > relocatables have the feature.
> >
> > How do they differ from bfd linker?
>
> I didn't say they did. Indeed, it seems that bfd linker implements
> these ranges as documented. But see below.
>
> > An x86 program main () may contain AVX512 instructions, whose usage is
> > guarded by CPUID.  NEEDED describes features that main() uses without
> > CPUID check.  In this case, AVX512 is USED, not NEEDED.
>
> Fine. That's what I at first presumed, until you said that bit about
> "NEEDED is for main() and USED is for functions that main() may call".
> That was very misleading.
>
> > How about this?
> >
> > 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.
> >
> > GNU_PROPERTY_X86_FEATURE_2_NEEDED The x86 processor features
> > indicated by the corresponding bits are used in program and they must be
> > supported by the hardware. Loader may refuse to load the program whose
> > GNU_PROPERTY_X86_FEATURE_2_NEEDED features aren’t supported by the
> > hardware.
> >
> > GNU_PROPERTY_X86_ISA_1_NEEDED The x86 instruction sets indicated by the
> > corresponding bits are used in program and they must be supported by
> > the hardware.
> > Loader may refuse to load the program whose GNU_PROPERTY_X86_ISA_1_NEEDED
> > ISAs aren’t supported by the hardware.
> >
> > GNU_PROPERTY_X86_ISA_1_USED The x86 instruction sets indicated by the
> > corresponding bits are used in program. Their support in the hardware
> > is optional.
> > GNU_PROPERTY_X86_ISA_1_USED can be used to check for ISAs used in the
> > program.
>
> This is also fine. But it's not what you've implemented!
>
> You've implemented these with the USED properties in the OR_AND range,
> and the NEEDED properties in the OR range:
>
>    Table 5.3: Program Property Types
>    Name                               Value
>    GNU_PROPERTY_X86_FEATURE_1_AND     GNU_PROPERTY_X86_UINT32_AND_LO + 0
>    GNU_PROPERTY_X86_FEATURE_2_USED    GNU_PROPERTY_X86_UINT32_OR_AND_LO + 1
>    GNU_PROPERTY_X86_FEATURE_2_NEEDED  GNU_PROPERTY_X86_UINT32_OR_LO + 1
>    GNU_PROPERTY_X86_ISA_1_USED        GNU_PROPERTY_X86_UINT32_OR_AND_LO + 0
>    GNU_PROPERTY_X86_ISA_1_NEEDED      GNU_PROPERTY_X86_UINT32_OR_LO + 0
>
> The way you've defined the values, ISA_1_USED uses the OR_AND rule,
> which implements the behavior you want for ISA_1_NEEDED. ISA_1_NEEDED
> uses the OR rule, which implements the behavior you want for
> ISA_1_USED. Likewise for FEATURE_2_USED and FEATURE_2_NEEDED.

I committed a typo fix.  Now it reads

GNU_PROPERTY_X86_FEATURE_2_NEEDED The x86 processor features in-
dicated by the corresponding bits are needed in program and they must be
supported by the hardware. Loader may refuse to load the program whose
GNU_PROPERTY_X86_FEATURE_2_NEEDED features aren’t supported by the
hardware.
GNU_PROPERTY_X86_ISA_1_NEEDED The x86 instruction sets indicated
by the corresponding bits are needed in program and they must be sup-
ported by the hardware.
Loader may refuse to load the program whose
GNU_PROPERTY_X86_ISA_1_NEEDED ISAs aren’t supported by the hardware.

-- 
H.J.