[LoongArch] Record the special AMO operand constraint with TableGen

The LoongArch Reference Manual says that the 3-register atomic memory
operations cannot have their rd equal to either rj or rk if not zero
[^1], and both GNU as and LLVM IAS enforce the constraint. However,
currently LoongArch AsmParser is checking for the opcode with a direct
numerical comparison on the opcode, which is enum-typed: the fact that
all AMO insns have adjacent numerical values is merely a coincidence,
and it is better to not rely on the current TableGen file layout and
the particular TableGen implementation behavior.

Instead, start to leverage the target-specific flags field of
MCInstrDesc, and record the constraint with TableGen, so we can stop
treating the opcode enum as number.

While documenting the new flag, it was found that v1.10 of the Manual
did not specify the similar constraint for the AMCAS instructions.
Experiments were done on a Loongson 3A6000 (LA664 uarch) and it turned
out that at least AMCAS will still signal INE with `rd == rj`. The
`rd == rk` case should be a no-op according to the semantics, but as it
is meaningless to perform CAS with the "old value" same as the "new
value", it is not worth special-casing. So the current behavior of also
enforcing the constraint for AMCAS is kept.

[^1]: if `rd == rj` an INE would be signaled; if `rd == rk` it is UB.

Author: xen0n

llvm/lib/Target/LoongArch/AsmParser/LoongArchAsmParser.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "MCTargetDesc/LoongArchBaseInfo.h"
 #include "MCTargetDesc/LoongArchInstPrinter.h"
 #include "MCTargetDesc/LoongArchMCExpr.h"
 #include "MCTargetDesc/LoongArchMCTargetDesc.h"
@@ -1560,9 +1561,10 @@ bool LoongArchAsmParser::processInstruction(MCInst &Inst, SMLoc IDLoc,
 
 unsigned LoongArchAsmParser::checkTargetMatchPredicate(MCInst &Inst) {
   unsigned Opc = Inst.getOpcode();
+  const MCInstrDesc &MCID = MII.get(Opc);
   switch (Opc) {
   default:
-    if (Opc >= LoongArch::AMADD_D && Opc <= LoongArch::AMXOR_W) {
+    if (LoongArchII::isSubjectToAMORdConstraint(MCID.TSFlags)) {
       MCRegister Rd = Inst.getOperand(0).getReg();
       MCRegister Rk = Inst.getOperand(1).getReg();
       MCRegister Rj = Inst.getOperand(2).getReg();

llvm/lib/Target/LoongArch/LoongArchInstrFormats.td

@@ -32,6 +32,11 @@ class LAInst<dag outs, dag ins, string opcstr, string opnstr,
   let InOperandList = ins;
   let AsmString = opcstr # "\t" # opnstr;
   let Pattern = pattern;
+
+  // Target-specific instruction info and defaults
+
+  bit IsSubjectToAMORdConstraint = 0;
+  let TSFlags{0} = IsSubjectToAMORdConstraint;
 }
 
 // Pseudo instructions

llvm/lib/Target/LoongArch/LoongArchInstrInfo.td

@@ -713,7 +713,9 @@ class STORE_2RI14<bits<32> op>
 let hasSideEffects = 0, mayLoad = 1, mayStore = 1, Constraints = "@earlyclobber $rd" in
 class AM_3R<bits<32> op>
     : Fmt3R<op, (outs GPR:$rd), (ins GPR:$rk, GPRMemAtomic:$rj),
-            "$rd, $rk, $rj">;
+            "$rd, $rk, $rj"> {
+  let IsSubjectToAMORdConstraint = 1;
+}
 
 let hasSideEffects = 0, mayLoad = 1, mayStore = 0 in {
 class LLBase<bits<32> op>

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchBaseInfo.h

@@ -56,6 +56,29 @@ enum {
   MO_DESC_CALL,
   // TODO: Add more flags.
 };
+
+// Target-specific flags of LAInst.
+// All definitions must match LoongArchInstrFormats.td.
+enum {
+  // Whether the instruction's rd is normally required to differ from rj and
+  // rk, in the way the 3-register atomic memory operations behave
+  // (Section 2.2.7.1 and 2.2.7.2, LoongArch Reference Manual Volume 1 v1.10;
+  // while Section 2.2.7.3 lacked similar description for the AMCAS
+  // instructions, at least the INE exception is still signaled on Loongson
+  // 3A6000 when its rd == rj).
+  //
+  // Used for generating diagnostics for assembler input that violate the
+  // constraint. As described on the manual, the covered instructions require
+  // rd != rj && rd != rk to work as intended.
+  IsSubjectToAMORdConstraintShift = 0,
+  IsSubjectToAMORdConstraintMask = 1 << IsSubjectToAMORdConstraintShift,
+};
+
+/// \returns true if this instruction's rd is normally required to differ
+/// from rj and rk, in the way 3-register atomic memory operations behave.
+static inline bool isSubjectToAMORdConstraint(uint64_t TSFlags) {
+  return TSFlags & IsSubjectToAMORdConstraintMask;
+}
 } // end namespace LoongArchII
 
 namespace LoongArchABI {

llvm/test/MC/LoongArch/Basic/Integer/invalid64.s

@@ -91,3 +91,14 @@ amxor.w $a0, $a1, $a0
 amadd.d $a0, $a1, $a2, $a3
 # CHECK: :[[#@LINE+1]]:24: error: optional integer offset must be 0
 amadd.d $a0, $a1, $a2, 1
+
+## According to experiment results on real LA664 HW, the AMCAS instructions
+## are subject to the same constraint as the other 3-register atomic insns.
+## This is undocumented in v1.10 of the LoongArch Reference Manual.
+
+# CHECK: :[[#@LINE+1]]:10: error: $rd must be different from both $rk and $rj
+amcas.b $a0, $a0, $a0
+# CHECK: :[[#@LINE+1]]:10: error: $rd must be different from both $rk and $rj
+amcas.h $a0, $a0, $a1
+# CHECK: :[[#@LINE+1]]:13: error: $rd must be different from both $rk and $rj
+amcas_db.w $a0, $a1, $a0