Index: llvm/docs/ReleaseNotes.rst
===================================================================
--- llvm/docs/ReleaseNotes.rst
+++ llvm/docs/ReleaseNotes.rst
@@ -102,7 +102,8 @@
 Changes to the AArch64 Backend
 ------------------------------
 
-* Added support for the Cortex-A715 CPU.
+* Add support for the Cortex-A715 CPU.
+* Add support for assembly for RME MEC (Memory Encryption Contexts).
 
 Changes to the AMDGPU Backend
 -----------------------------
Index: llvm/lib/Target/AArch64/AArch64.td
===================================================================
--- llvm/lib/Target/AArch64/AArch64.td
+++ llvm/lib/Target/AArch64/AArch64.td
@@ -496,6 +496,9 @@
                                                  "Don't place a BTI instruction "
                                                  "after a return-twice">;
 
+def FeatureMEC : SubtargetFeature<"mec", "HasMEC",
+    "true", "Enable Memory Encryption Contexts Extension", [FeatureRME]>;
+
 //===----------------------------------------------------------------------===//
 // Architectures.
 //
@@ -541,7 +544,7 @@
 
 def HasV9_0aOps : SubtargetFeature<
   "v9a", "HasV9_0aOps", "true", "Support ARM v9a instructions",
-  [HasV8_5aOps, FeatureSVE2]>;
+  [HasV8_5aOps, FeatureMEC, FeatureSVE2]>;
 
 def HasV9_1aOps : SubtargetFeature<
   "v9.1a", "HasV9_1aOps", "true", "Support ARM v9.1a instructions",
Index: llvm/lib/Target/AArch64/AArch64SystemOperands.td
===================================================================
--- llvm/lib/Target/AArch64/AArch64SystemOperands.td
+++ llvm/lib/Target/AArch64/AArch64SystemOperands.td
@@ -167,6 +167,11 @@
 def : DC<"GZVA",    0b011, 0b0111, 0b0100, 0b100>;
 }
 
+let Requires = [{ {AArch64::FeatureMEC} }] in {
+def : DC<"CIGDPAE", 0b100, 0b0111, 0b1110, 0b000>;
+def : DC<"CIPAE",   0b100, 0b0111, 0b1110, 0b111>;
+}
+
 //===----------------------------------------------------------------------===//
 // IC (instruction cache maintenance) instruction options.
 //===----------------------------------------------------------------------===//
@@ -796,6 +801,18 @@
 def : RWSysReg<"GPTBR_EL3", 0b11, 0b110, 0b0010, 0b0001, 0b100>;
 }
 
+// v9a Memory Encryption Contexts Extension registers
+let Requires = [{ {AArch64::FeatureMEC} }] in {
+def : ROSysReg<"MECIDR_EL2",     0b11, 0b100, 0b1010, 0b1000, 0b111>;
+def : RWSysReg<"MECID_P0_EL2",   0b11, 0b100, 0b1010, 0b1000, 0b000>;
+def : RWSysReg<"MECID_A0_EL2",   0b11, 0b100, 0b1010, 0b1000, 0b001>;
+def : RWSysReg<"MECID_P1_EL2",   0b11, 0b100, 0b1010, 0b1000, 0b010>;
+def : RWSysReg<"MECID_A1_EL2",   0b11, 0b100, 0b1010, 0b1000, 0b011>;
+def : RWSysReg<"VMECID_P_EL2",   0b11, 0b100, 0b1010, 0b1001, 0b000>;
+def : RWSysReg<"VMECID_A_EL2",   0b11, 0b100, 0b1010, 0b1001, 0b001>;
+def : RWSysReg<"MECID_RL_A_EL3", 0b11, 0b110, 0b1010, 0b1010, 0b001>;
+}
+
 // v9-a Scalable Matrix Extension (SME) registers
 //                                 Op0   Op1    CRn     CRm     Op2
 let Requires = [{ {AArch64::FeatureSME} }] in {
Index: llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
===================================================================
--- llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -3487,6 +3487,7 @@
     {"sme2", {AArch64::FeatureSME2}},
     {"hbc", {AArch64::FeatureHBC}},
     {"mops", {AArch64::FeatureMOPS}},
+    {"mec", {AArch64::FeatureMEC}},
     // FIXME: Unsupported extensions
     {"lor", {}},
     {"rdma", {}},
Index: llvm/test/MC/AArch64/armv9a-mec.s
===================================================================
--- /dev/null
+++ llvm/test/MC/AArch64/armv9a-mec.s
@@ -0,0 +1,56 @@
+// RUN: llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+mec < %s | FileCheck %s
+// RUN: llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+v9a < %s | FileCheck %s
+// RUN: not llvm-mc -triple aarch64-none-linux-gnu < %s 2>&1 | FileCheck --check-prefix=CHECK-NO-MEC %s
+
+          mrs x0, MECIDR_EL2
+// CHECK: mrs   x0, MECIDR_EL2       // encoding: [0xe0,0xa8,0x3c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          mrs x0, MECID_P0_EL2
+// CHECK: mrs   x0, MECID_P0_EL2      // encoding: [0x00,0xa8,0x3c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          mrs x0, MECID_A0_EL2
+// CHECK: mrs   x0, MECID_A0_EL2      // encoding: [0x20,0xa8,0x3c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          mrs x0, MECID_P1_EL2
+// CHECK: mrs   x0, MECID_P1_EL2      // encoding: [0x40,0xa8,0x3c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          mrs x0, MECID_A1_EL2
+// CHECK: mrs   x0, MECID_A1_EL2      // encoding: [0x60,0xa8,0x3c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          mrs x0, VMECID_P_EL2
+// CHECK: mrs   x0, VMECID_P_EL2     // encoding: [0x00,0xa9,0x3c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          mrs x0, VMECID_A_EL2
+// CHECK: mrs   x0, VMECID_A_EL2     // encoding: [0x20,0xa9,0x3c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          mrs x0, MECID_RL_A_EL3
+// CHECK: mrs   x0, MECID_RL_A_EL3   // encoding: [0x20,0xaa,0x3e,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:19: error: expected readable system register
+          msr MECID_P0_EL2,    x0
+// CHECK: msr   MECID_P0_EL2, x0      // encoding: [0x00,0xa8,0x1c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:15: error: expected writable system register or pstate
+          msr MECID_A0_EL2,    x0
+// CHECK: msr   MECID_A0_EL2, x0      // encoding: [0x20,0xa8,0x1c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:15: error: expected writable system register or pstate
+          msr MECID_P1_EL2,    x0
+// CHECK: msr   MECID_P1_EL2, x0      // encoding: [0x40,0xa8,0x1c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:15: error: expected writable system register or pstate
+          msr MECID_A1_EL2,    x0
+// CHECK: msr   MECID_A1_EL2, x0      // encoding: [0x60,0xa8,0x1c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:15: error: expected writable system register or pstate
+          msr VMECID_P_EL2,   x0
+// CHECK: msr   VMECID_P_EL2, x0     // encoding: [0x00,0xa9,0x1c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:15: error: expected writable system register or pstate
+          msr VMECID_A_EL2,   x0
+// CHECK: msr   VMECID_A_EL2, x0     // encoding: [0x20,0xa9,0x1c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:15: error: expected writable system register or pstate
+          msr MECID_RL_A_EL3, x0
+// CHECK: msr   MECID_RL_A_EL3, x0   // encoding: [0x20,0xaa,0x1e,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:15: error: expected writable system register or pstate
+
+          dc cigdpae, x0
+// CHECK: dc cigdpae, x0             // encoding: [0x00,0x7e,0x0c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:14: error: DC CIGDPAE requires: mec
+          dc cipae, x0
+// CHECK: dc cipae, x0               // encoding: [0xe0,0x7e,0x0c,0xd5]
+// CHECK-NO-MEC: [[@LINE-2]]:14: error: DC CIPAE requires: mec
Index: llvm/test/MC/Disassembler/AArch64/armv9a-mec.txt
===================================================================
--- /dev/null
+++ llvm/test/MC/Disassembler/AArch64/armv9a-mec.txt
@@ -0,0 +1,55 @@
+# RUN: llvm-mc -triple=aarch64             -mattr=+mec -disassemble %s      | FileCheck %s
+# RUN: llvm-mc -triple=aarch64 -mattr=+v9a             -disassemble %s      | FileCheck %s
+# RUN: llvm-mc -triple=aarch64                         -disassemble %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO-MEC
+
+[0xe0,0xa8,0x3c,0xd5]
+# CHECK: mrs x0, MECIDR_EL2
+# CHECK-NO-MEC: mrs x0, S3_4_C10_C8_7
+[0x00,0xa8,0x3c,0xd5]
+# CHECK: mrs x0, MECID_P0_EL2
+# CHECK-NO-MEC: mrs x0, S3_4_C10_C8_0
+[0x20,0xa8,0x3c,0xd5]
+# CHECK: mrs x0, MECID_A0_EL2
+# CHECK-NO-MEC: mrs x0, S3_4_C10_C8_1
+[0x40,0xa8,0x3c,0xd5]
+# CHECK: mrs x0, MECID_P1_EL2
+# CHECK-NO-MEC: mrs x0, S3_4_C10_C8_2
+[0x60,0xa8,0x3c,0xd5]
+# CHECK: mrs x0, MECID_A1_EL2
+# CHECK-NO-MEC: mrs x0, S3_4_C10_C8_3
+[0x00,0xa9,0x3c,0xd5]
+# CHECK: mrs x0, VMECID_P_EL2
+# CHECK-NO-MEC: mrs x0, S3_4_C10_C9_0
+[0x20,0xa9,0x3c,0xd5]
+# CHECK: mrs x0, VMECID_A_EL2
+# CHECK-NO-MEC: mrs x0, S3_4_C10_C9_1
+[0x20,0xaa,0x3e,0xd5]
+# CHECK: mrs x0, MECID_RL_A_EL3
+# CHECK-NO-MEC: mrs x0, S3_6_C10_C10_1
+[0x00,0xa8,0x1c,0xd5]
+# CHECK: msr MECID_P0_EL2,    x0
+# CHECK-NO-MEC: msr S3_4_C10_C8_0, x0
+[0x20,0xa8,0x1c,0xd5]
+# CHECK: msr MECID_A0_EL2,    x0
+# CHECK-NO-MEC: msr S3_4_C10_C8_1, x0
+[0x40,0xa8,0x1c,0xd5]
+# CHECK: msr MECID_P1_EL2,    x0
+# CHECK-NO-MEC: msr S3_4_C10_C8_2, x0
+[0x60,0xa8,0x1c,0xd5]
+# CHECK: msr MECID_A1_EL2,    x0
+# CHECK-NO-MEC: msr S3_4_C10_C8_3, x0
+[0x00,0xa9,0x1c,0xd5]
+# CHECK: msr VMECID_P_EL2,   x0
+# CHECK-NO-MEC: msr S3_4_C10_C9_0, x0
+[0x20,0xa9,0x1c,0xd5]
+# CHECK: msr VMECID_A_EL2,   x0
+# CHECK-NO-MEC: msr S3_4_C10_C9_1, x0
+[0x20,0xaa,0x1e,0xd5]
+# CHECK: msr MECID_RL_A_EL3, x0
+# CHECK-NO-MEC: msr S3_6_C10_C10_1, x0
+[0x00,0x7e,0x0c,0xd5]
+# CHECK: dc cigdpae, x0
+# CHECK-NO-MEC: sys #4, c7, c14, #0, x0
+[0xe0,0x7e,0x0c,0xd5]
+# CHECK: dc cipae, x0
+# CHECK-NO-MEC: sys #4, c7, c14, #7, x0