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[MC] [Win64EH] Canonicalize ARM64 unwind opcodes
Audit Required700fbe591ac0

Authored by mstorsjo on Sep 7 2020, 4:45 AM.

Description

[MC] [Win64EH] Canonicalize ARM64 unwind opcodes

Convert 2-byte opcodes to equivalent 1-byte ones.

Adjust the existing exhaustive testcase to avoid being altered by
the simplification rules (to keep that test exercising all individual
opcodes).

Fix the assembler parser limits for register pairs; for .seh_save_regp
and .seh_save_regp_x, we can allow up to x29, for a x29+x30 pair
(which gets remapped to the UOP_SaveFPLR(X) opcodes), for .seh_save_fregp
and .seh_save_fregpx, allow up to d14+d15.

Not creating .seh_save_next for float register pairs, as the
actual unwinder implementation in current versions of Windows is buggy
for that case.

This gives a minimal but measurable size reduction. (For a 6.5 MB
DLL with 300 KB .xdata, the .xdata shrinks by 48 bytes. The opcode
sequences are padded to a 4 byte boundary, so very small improvements
might not end up mattering directly.)

Differential Revision: https://reviews.llvm.org/D87367