[llvm] [LFI][AArch64] Add rewrites for memory accesses (PR #195167)

[Nick Desaulniers via llvm-commits]

================
@@ -64,6 +66,110 @@ static bool isPrivilegedTPAccess(const MCInst &Inst) {
   return false;
 }
 
+// Classification functions are limited to Armv8.1-A. Instructions outside of
+// this subset are not guaranteed to be rewritten and as a result may fail LFI
+// verification after compilation.
+
+// Instructions that have mayLoad/mayStore set in TableGen but don't actually
+// perform memory accesses.
+static bool isFakeMemAccess(const MCInst &Inst) {
+  switch (Inst.getOpcode()) {
+  case AArch64::DMB:
+  case AArch64::DSB:
+  case AArch64::ISB:
+  case AArch64::HINT:
+    // The range of sub-architectures supported by LFI do not include any load
+    // or store instructions in the HINT space.
+    return true;
+  default:
+    return false;
+  }
+}
+
+static bool mayPrefetch(const MCInst &Inst) {
+  switch (Inst.getOpcode()) {
+  case AArch64::PRFMl:
+  case AArch64::PRFMroW:
+  case AArch64::PRFMroX:
+  case AArch64::PRFMui:
+  case AArch64::PRFUMi:
+    return true;
+  default:
+    return false;
+  }
+}
+
+// User-mode DC/IC instructions that take a virtual address operand. Encoded as
+// SYSxt with op1=3, Cn=7, op2=1 where the Cm field selects the operation.
+static bool isVASysOp(const MCInst &Inst) {
+  if (Inst.getOpcode() != AArch64::SYSxt)
+    return false;
+  if (Inst.getOperand(0).getImm() != 3 || Inst.getOperand(1).getImm() != 7 ||
+      Inst.getOperand(3).getImm() != 1)
+    return false;
+  switch (Inst.getOperand(2).getImm()) {
+  case 4:  // DC ZVA
+  case 5:  // IC IVAU
+  case 10: // DC CVAC
+  case 11: // DC CVAU
+  case 12: // DC CVAP
+  case 13: // DC CVADP
+  case 14: // DC CIVAC
+    return true;
+  default:
+    return false;
+  }
+}
+
+static MCInst replaceRegAt(const MCInst &Inst, unsigned Idx,
+                           MCRegister NewReg) {
+  MCInst New;
+  New.setOpcode(Inst.getOpcode());
+  New.setLoc(Inst.getLoc());
+  for (unsigned I = 0, E = Inst.getNumOperands(); I != E; ++I) {
+    if (I == Idx) {
+      assert(Inst.getOperand(I).isReg());
+      New.addOperand(MCOperand::createReg(NewReg));
+    } else {
+      New.addOperand(Inst.getOperand(I));
+    }
+  }
+  return New;
+}
----------------
nickdesaulniers wrote:

Do we really need a whole new `MCInst`? Why not modify `Inst`?

https://github.com/llvm/llvm-project/pull/195167