diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
index f472e915b068..6eba3479612b 100644
--- a/clang/include/clang/Basic/Builtins.def
+++ b/clang/include/clang/Basic/Builtins.def
@@ -1687,6 +1687,7 @@ BUILTIN(__builtin_cheri_address_set, "v*mvC*mz", "nct")
 BUILTIN(__builtin_cheri_base_get, "zvC*m", "nct")
 BUILTIN(__builtin_cheri_bounds_set, "v*mvC*mz", "nct")
 BUILTIN(__builtin_cheri_bounds_set_exact, "v*mvC*mz", "nct")
+BUILTIN(__builtin_cheri_bounds_set_round_down, "v*mvC*mz", "nct")
 BUILTIN(__builtin_cheri_equal_exact, "bvC*mvC*m", "nct")
 BUILTIN(__builtin_cheri_flags_set, "v*mvC*mz", "nct")
 BUILTIN(__builtin_cheri_flags_get, "zvC*m", "nct")
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index d130c4bca415..78f433c1bf68 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -5032,6 +5032,15 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
                                 {SizeTy}, {Cap, Length}),
         Cap->getType()));
   }
+  case Builtin::BI__builtin_cheri_bounds_set_round_down: {
+    Value *Cap = EmitScalarExpr(E->getArg(0));
+    Value *Length = EmitScalarExpr(E->getArg(1));
+    return RValue::get(Builder.CreateBitCast(
+        Builder.CreateIntrinsic(
+            llvm::Intrinsic::cheri_cap_bounds_set_round_down, {SizeTy},
+            {Cap, Length}),
+        Cap->getType()));
+  }
   case Builtin::BI__builtin_cheri_flags_get:
     return RValue::get(
         Builder.CreateIntrinsic(llvm::Intrinsic::cheri_cap_flags_get, {SizeTy},
diff --git a/llvm/include/llvm/IR/IntrinsicsCHERICap.td b/llvm/include/llvm/IR/IntrinsicsCHERICap.td
index 00d5a201de1d..305d0d281873 100644
--- a/llvm/include/llvm/IR/IntrinsicsCHERICap.td
+++ b/llvm/include/llvm/IR/IntrinsicsCHERICap.td
@@ -57,6 +57,10 @@ def int_cheri_cap_bounds_set_exact :
   Intrinsic<[llvm_cap_ty],
             [llvm_cap_ty, llvm_anyint_ty],
             [IntrNoMem, IntrWillReturn]>;
+def int_cheri_cap_bounds_set_round_down :
+  Intrinsic<[llvm_cap_ty],
+            [llvm_cap_ty, llvm_anyint_ty],
+            [IntrNoMem, IntrWillReturn]>;
 def int_cheri_cap_type_get :
   Intrinsic<[llvm_anyint_ty],
             [llvm_cap_ty],
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
index d734c6181856..4ad702a786e7 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
@@ -1494,6 +1494,7 @@ bool RISCVInstrInfo::isSetBoundsInstr(const MachineInstr &I,
   case RISCV::CSetBounds:
   case RISCV::CSetBoundsExact:
   case RISCV::CSetBoundsImm:
+  case RISCV::CSetBoundsRoundDown:
     Base = &I.getOperand(1);
     Size = &I.getOperand(2);
     return true;
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfoXCheri.td b/llvm/lib/Target/RISCV/RISCVInstrInfoXCheri.td
index 2290355d8882..229be959e128 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfoXCheri.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfoXCheri.td
@@ -408,6 +408,7 @@ let mayTrap = 1 in {
 def CSetBounds      : Cheri_rr<0x8, "csetbounds">;
 def CSetBoundsExact : Cheri_rr<0x9, "csetboundsexact">;
 def CSetBoundsImm   : Cheri_ri<0x2, "csetbounds", 0>;
+def CSetBoundsRoundDown : Cheri_rr<0xA, "csetboundsrounddown">;
 } // mayTrap = 1
 } // let Constraints = "@traps_if_sealed $rs1"
 def CClearTag       : Cheri_r<0xb, "ccleartag", GPCR>;
@@ -1301,6 +1302,7 @@ def : PatGpcrGpr<cptradd, CIncOffset>;
 def : PatGpcrSimm12<cptradd, CIncOffsetImm>;
 def : PatGpcrGpr<int_cheri_cap_bounds_set, CSetBounds>;
 def : PatGpcrGpr<int_cheri_cap_bounds_set_exact, CSetBoundsExact>;
+def : PatGpcrGpr<int_cheri_cap_bounds_set_round_down, CSetBoundsRoundDown>;
 def : PatGpcrGpr<riscv_cap_bounds_set, CSetBounds>;
 def : PatGpcrUimm12<riscv_cap_bounds_set, CSetBoundsImm>;
 def : PatGpcrUimm12<int_cheri_cap_bounds_set, CSetBoundsImm>;