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New attribute: Tag_RISCV_reserved_register #195

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New attribute: Tag_RISCV_reserved_register #195

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628c0b2
New attribute: Tag_RISCV_reserved_register
kito-cheng Jan 9, 2022
425426a
Tweaks to Tag_RISCV_reserved_register description
edward-jones Jan 9, 2022
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19 changes: 19 additions & 0 deletions riscv-elf.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -896,6 +896,7 @@ value if the tag number is even.
| Tag_RISCV_priv_spec | 8 | uleb128 | Indicates the major version of the privileged specification.
| Tag_RISCV_priv_spec_minor | 10 | uleb128 | Indicates the minor version of the privileged specification.
| Tag_RISCV_priv_spec_revision | 12 | uleb128 | Indicates the revision version of the privileged specification.
| Tag_RISCV_reserved_registers | 14 | uleb128 | Indicates the extra reserved register information.
| Reserved for non-standard attribute | >= 32768 | - | -
|===

Expand Down Expand Up @@ -942,6 +943,24 @@ Tag_RISCV_priv_spec contains the major/minor/revision version information of
the privileged specification. It will report errors if object files of different
privileged specification versions are merged.

===== Tag_RISCV_reserved_registers, 14, uleb128=register bit-vector

Tag_RISCV_reserved_registers contains the information about the reserved
registers which are not reserved by ABI. Registers might be reserved by specific
compiler option like `-ffixed-x4` or a special purpose ABI variant like
overlay ABI.

The encoding of Tag_RISCV_reserved_registers is a bit-vector, each bit
corresponding to a register. The bit number for a register is the same as DWARF
register numbers.

First bit is must set, used to distinguish between attribute is unset or having
empty reserved register set.

It will report errors if object files are linked with different
Tag_RISCV_reserved_registers values, or if the tag is missing from any object
when the tag is set in at least one linked object.

== Code relaxation

At link time, when all the memory objects have been resolved, the code sequence
Expand Down