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[mips] Begin reworking instruction predicates for ISAs/encodings (1/N)
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Authored by sdardis on Dec 20 2017, 3:42 AM.

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Summary

The MIPS backend has inconsistent usage of instruction predicates
for assembly and code generation. The issue arises from supporting three
encodings, two (MIPS and microMIPS) of which have a near 1:1 instruction
mapping across ISA revisions and a third encoding with a more restricted
set of instructions (MIPS16e).

To enforce consistent usage, each of the ISA_* adjectives has (or will
have) the relevant encoding attached to it along the relevant ISA revision
where the instruction is defined.

Each instruction, pattern or alias will then have the correct ISA adjective
attached to it, and the base instruction description classes will have any
predicates relating to ISA encoding or revision removed.

Pseudo instructions will also be guarded for the encoding or ABI that they are
supported in.

Finally, the hasStandardEncoding() / inMicroMipsMode() / inMips16Mode() methods
of MipsSubtarget will be changed such that only one can be true at any one time.

The result of this is that code generation and assembly will produce the
correct encoding up front, while code generated from pseudo instructions
and other inserted sequences of instructions will be able to rely on the mapping
tables to produce the correct encoding. This should fix numerous bugs where
the result 'happens' to be correct but has edge cases where microMIPS and MIPS
have subtle differences (e.g. microMIPSR6 using 'j', 'jal' instructions.)

This patch starts the process by changing most of the ISA adjectives to make
use of the EncodingPredicate member of PredicateControl. Follow on patches
will annotate instructions with their correct ISA adjective and eliminate
the usage of "let Predicates = [..]", "let AdditionalPredicates = [..]" and
"isCodeGenOnly = 1" in the cases where it was used to control instruction
availability.

Contributions from Nitesh Jain.

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