[llvm] [X86][MC] Support encoding/decoding for APX variant MUL/IMUL/DIV/IDIV instructions (PR #76919)

[Shengchen Kan via llvm-commits]

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@@ -8,8 +8,40 @@
 
 def TruePredicate : Predicate<"true">;
 
+// Intel x86 instructions have three separate encoding spaces: legacy, VEX, and
+// EVEX. Not all X86 instructions are extended for EGPR. The following is an
+// overview of which instructions are extended and how we implement them.
+//
+// * Legacy space
+//   All instructions in legacy maps 0 and 1 that have explicit GPR or memory
+//   operands can use the REX2 prefix to access the EGPR, except XSAVE*/XRSTOR.
+//
+// * EVEX space
+//   All instructions in the EVEX space can access the EGPR in their
+//   register/memory operands.
+//
+// For the above intructions, the only difference in encoding is reflected in
+// the REX2/EVEX prefix when EGPR is used, i.e. the opcode and opcode name are
+// unchanged. We don’t add new entries in TD, and instead we extend GPR with
+// R16-R31 and make them allocatable only when the feature EGPR is available.
+//
+// Besides, some instructions in legacy space with map 2/3 and VEX space are
+// promoted into EVEX space. Encoding space changes and opcode, opcode map may
+// change after the promotion. For these instructions, we add new entries in TD
+// to avoid overcomplicating the assembler and disassembler.
+//
+// HasEGPR is for the new entries and NoEGPR is for the entries before promotion,
+// so that the promoted variant can be selected first to benefit RA.
 def HasEGPR      : Predicate<"Subtarget->hasEGPR()">;
 def NoEGPR       : Predicate<"!Subtarget->hasEGPR()">;
+// APX extends some instructions with a new form that has an extra register
+// operand called a new data destination (NDD). In such forms, NDD is the new
+// destination register receiving the result of the computation and all other
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KanRobert wrote:

No.  It 's copied from 3.1.2.2 New Data Destination. The verb here is used as a postpositional attributive and requires the ing form.

https://github.com/llvm/llvm-project/pull/76919