diff --git a/cranelift/codegen/src/isa/aarch64/inst/emit.rs b/cranelift/codegen/src/isa/aarch64/inst/emit.rs
index 9d9d98d7c8..b014d9b22d 100644
--- a/cranelift/codegen/src/isa/aarch64/inst/emit.rs
+++ b/cranelift/codegen/src/isa/aarch64/inst/emit.rs
@@ -4,7 +4,7 @@ use crate::binemit::{CodeOffset, Reloc};
 use crate::ir::constant::ConstantData;
 use crate::ir::types::*;
 use crate::ir::TrapCode;
-use crate::isa::aarch64::inst::*;
+use crate::isa::aarch64::{inst::regs::PINNED_REG, inst::*};
 
 use regalloc::{Reg, RegClass, Writable};
 
@@ -1307,6 +1307,20 @@ impl<O: MachSectionOutput> MachInstEmit<O> for Inst {
                 }
                 _ => unimplemented!("{:?}", mem),
             },
+            &Inst::GetPinnedReg { rd } => {
+                let inst = Inst::Mov {
+                    rd,
+                    rm: xreg(PINNED_REG),
+                };
+                inst.emit(sink);
+            }
+            &Inst::SetPinnedReg { rm } => {
+                let inst = Inst::Mov {
+                    rd: Writable::from_reg(xreg(PINNED_REG)),
+                    rm,
+                };
+                inst.emit(sink);
+            }
         }
     }
 }
@@ -1315,6 +1329,7 @@ impl<O: MachSectionOutput> MachInstEmit<O> for Inst {
 mod test {
     use super::*;
     use crate::isa::test_utils;
+    use crate::settings;
 
     #[test]
     fn test_aarch64_binemit() {
@@ -4074,7 +4089,7 @@ mod test {
             "frintn d23, d24",
         ));
 
-        let rru = create_reg_universe();
+        let rru = create_reg_universe(&settings::Flags::new(settings::builder()));
         for (insn, expected_encoding, expected_printing) in insns {
             println!(
                 "AArch64: {:?}, {}, {}",
diff --git a/cranelift/codegen/src/isa/aarch64/inst/mod.rs b/cranelift/codegen/src/isa/aarch64/inst/mod.rs
index 90fce9c5fa..72300f1151 100644
--- a/cranelift/codegen/src/isa/aarch64/inst/mod.rs
+++ b/cranelift/codegen/src/isa/aarch64/inst/mod.rs
@@ -7,6 +7,7 @@ use crate::binemit::CodeOffset;
 use crate::ir::types::{B1, B16, B32, B64, B8, F32, F64, FFLAGS, I16, I32, I64, I8, IFLAGS};
 use crate::ir::{ExternalName, Opcode, SourceLoc, TrapCode, Type};
 use crate::machinst::*;
+use crate::settings;
 
 use regalloc::Map as RegallocMap;
 use regalloc::{RealReg, RealRegUniverse, Reg, RegClass, SpillSlot, VirtualReg, Writable};
@@ -714,6 +715,16 @@ pub enum Inst {
         rd: Writable<Reg>,
         mem: MemArg,
     },
+
+    /// Sets the value of the pinned register to the given register target.
+    GetPinnedReg {
+        rd: Writable<Reg>,
+    },
+
+    /// Writes the value of the given source register to the pinned register.
+    SetPinnedReg {
+        rm: Reg,
+    },
 }
 
 fn count_zero_half_words(mut value: u64) -> usize {
@@ -1099,6 +1110,12 @@ fn aarch64_get_regs(inst: &Inst, collector: &mut RegUsageCollector) {
         &Inst::LoadAddr { rd, mem: _ } => {
             collector.add_def(rd);
         }
+        &Inst::GetPinnedReg { rd } => {
+            collector.add_def(rd);
+        }
+        &Inst::SetPinnedReg { rm } => {
+            collector.add_use(rm);
+        }
     }
 }
 
@@ -1660,6 +1677,12 @@ fn aarch64_map_regs(
             map_wr(d, rd);
             map_mem(u, mem);
         }
+        &mut Inst::GetPinnedReg { ref mut rd } => {
+            map_wr(d, rd);
+        }
+        &mut Inst::SetPinnedReg { ref mut rm } => {
+            map(u, rm);
+        }
     }
 }
 
@@ -1850,8 +1873,8 @@ impl MachInst for Inst {
         }
     }
 
-    fn reg_universe() -> RealRegUniverse {
-        create_reg_universe()
+    fn reg_universe(flags: &settings::Flags) -> RealRegUniverse {
+        create_reg_universe(flags)
     }
 }
 
@@ -2589,6 +2612,14 @@ impl ShowWithRRU for Inst {
                 }
                 _ => unimplemented!("{:?}", mem),
             },
+            &Inst::GetPinnedReg { rd } => {
+                let rd = rd.show_rru(mb_rru);
+                format!("get_pinned_reg {}", rd)
+            }
+            &Inst::SetPinnedReg { rm } => {
+                let rm = rm.show_rru(mb_rru);
+                format!("set_pinned_reg {}", rm)
+            }
         }
     }
 }
diff --git a/cranelift/codegen/src/isa/aarch64/inst/regs.rs b/cranelift/codegen/src/isa/aarch64/inst/regs.rs
index b675d7f4d7..f4f19cf517 100644
--- a/cranelift/codegen/src/isa/aarch64/inst/regs.rs
+++ b/cranelift/codegen/src/isa/aarch64/inst/regs.rs
@@ -2,6 +2,7 @@
 
 use crate::isa::aarch64::inst::InstSize;
 use crate::machinst::*;
+use crate::settings;
 
 use regalloc::{RealRegUniverse, Reg, RegClass, RegClassInfo, Writable, NUM_REG_CLASSES};
 
@@ -10,6 +11,11 @@ use std::string::{String, ToString};
 //=============================================================================
 // Registers, the Universe thereof, and printing
 
+/// The pinned register on this architecture.
+/// It must be the same as Spidermonkey's HeapReg, as found in this file.
+/// https://searchfox.org/mozilla-central/source/js/src/jit/arm64/Assembler-arm64.h#103
+pub const PINNED_REG: u8 = 21;
+
 #[rustfmt::skip]
 const XREG_INDICES: [u8; 31] = [
     // X0 - X7
@@ -22,8 +28,12 @@ const XREG_INDICES: [u8; 31] = [
     47, 48,
     // X18
     60,
-    // X19 - X28
-    49, 50, 51, 52, 53, 54, 55, 56, 57, 58,
+    // X19, X20
+    49, 50,
+    // X21, put aside because it's the pinned register.
+    58,
+    // X22 - X28
+    51, 52, 53, 54, 55, 56, 57,
     // X29
     61,
     // X30
@@ -131,14 +141,13 @@ pub fn writable_spilltmp_reg() -> Writable<Reg> {
 }
 
 /// Create the register universe for AArch64.
-pub fn create_reg_universe() -> RealRegUniverse {
+pub fn create_reg_universe(flags: &settings::Flags) -> RealRegUniverse {
     let mut regs = vec![];
     let mut allocable_by_class = [None; NUM_REG_CLASSES];
 
-    // Numbering Scheme: we put V-regs first, then X-regs. The X-regs
-    // exclude several registers: x18 (globally reserved for platform-specific
-    // purposes), x29 (frame pointer), x30 (link register), x31 (stack pointer
-    // or zero register, depending on context).
+    // Numbering Scheme: we put V-regs first, then X-regs. The X-regs exclude several registers:
+    // x18 (globally reserved for platform-specific purposes), x29 (frame pointer), x30 (link
+    // register), x31 (stack pointer or zero register, depending on context).
 
     let v_reg_base = 0u8; // in contiguous real-register index space
     let v_reg_count = 32;
@@ -159,9 +168,12 @@ pub fn create_reg_universe() -> RealRegUniverse {
 
     let x_reg_base = 32u8; // in contiguous real-register index space
     let mut x_reg_count = 0;
+
+    let uses_pinned_reg = flags.enable_pinned_reg();
+
     for i in 0u8..32u8 {
         // See above for excluded registers.
-        if i == 15 || i == 18 || i == 29 || i == 30 || i == 31 {
+        if i == 15 || i == 18 || i == 29 || i == 30 || i == 31 || i == PINNED_REG {
             continue;
         }
         let reg = Reg::new_real(
@@ -188,13 +200,24 @@ pub fn create_reg_universe() -> RealRegUniverse {
     });
 
     // Other regs, not available to the allocator.
-    let allocable = regs.len();
+    let allocable = if uses_pinned_reg {
+        // The pinned register is not allocatable in this case, so record the length before adding
+        // it.
+        let len = regs.len();
+        regs.push((xreg(PINNED_REG).to_real_reg(), "x21/pinned_reg".to_string()));
+        len
+    } else {
+        regs.push((xreg(PINNED_REG).to_real_reg(), "x21".to_string()));
+        regs.len()
+    };
+
     regs.push((xreg(15).to_real_reg(), "x15".to_string()));
     regs.push((xreg(18).to_real_reg(), "x18".to_string()));
     regs.push((fp_reg().to_real_reg(), "fp".to_string()));
     regs.push((link_reg().to_real_reg(), "lr".to_string()));
     regs.push((zero_reg().to_real_reg(), "xzr".to_string()));
     regs.push((stack_reg().to_real_reg(), "sp".to_string()));
+
     // FIXME JRS 2020Feb06: unfortunately this pushes the number of real regs
     // to 65, which is potentially inconvenient from a compiler performance
     // standpoint.  We could possibly drop back to 64 by "losing" a vector
diff --git a/cranelift/codegen/src/isa/aarch64/lower.rs b/cranelift/codegen/src/isa/aarch64/lower.rs
index 209b3d6c83..ac5adc8dd4 100644
--- a/cranelift/codegen/src/isa/aarch64/lower.rs
+++ b/cranelift/codegen/src/isa/aarch64/lower.rs
@@ -1935,9 +1935,16 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(ctx: &mut C, insn: IRInst) {
             }
         }
 
-        Opcode::GetPinnedReg
-        | Opcode::SetPinnedReg
-        | Opcode::Spill
+        Opcode::GetPinnedReg => {
+            let rd = output_to_reg(ctx, outputs[0]);
+            ctx.emit(Inst::GetPinnedReg { rd });
+        }
+        Opcode::SetPinnedReg => {
+            let rm = input_to_reg(ctx, inputs[0], NarrowValueMode::None);
+            ctx.emit(Inst::SetPinnedReg { rm });
+        }
+
+        Opcode::Spill
         | Opcode::Fill
         | Opcode::FillNop
         | Opcode::Regmove
diff --git a/cranelift/codegen/src/isa/aarch64/mod.rs b/cranelift/codegen/src/isa/aarch64/mod.rs
index 2f52e618c1..879788e32a 100644
--- a/cranelift/codegen/src/isa/aarch64/mod.rs
+++ b/cranelift/codegen/src/isa/aarch64/mod.rs
@@ -52,7 +52,7 @@ impl MachBackend for AArch64Backend {
         let frame_size = vcode.frame_size();
 
         let disasm = if want_disasm {
-            Some(vcode.show_rru(Some(&create_reg_universe())))
+            Some(vcode.show_rru(Some(&create_reg_universe(flags))))
         } else {
             None
         };
@@ -77,7 +77,7 @@ impl MachBackend for AArch64Backend {
     }
 
     fn reg_universe(&self) -> RealRegUniverse {
-        create_reg_universe()
+        create_reg_universe(&self.flags)
     }
 }
 
diff --git a/cranelift/codegen/src/machinst/compile.rs b/cranelift/codegen/src/machinst/compile.rs
index 8b09568644..2fdaa7292c 100644
--- a/cranelift/codegen/src/machinst/compile.rs
+++ b/cranelift/codegen/src/machinst/compile.rs
@@ -20,7 +20,7 @@ where
     // This lowers the CL IR.
     let mut vcode = Lower::new(f, abi).lower(b);
 
-    let universe = &B::MInst::reg_universe();
+    let universe = &B::MInst::reg_universe(vcode.flags());
 
     debug!("vcode from lowering: \n{}", vcode.show_rru(Some(universe)));
 
diff --git a/cranelift/codegen/src/machinst/mod.rs b/cranelift/codegen/src/machinst/mod.rs
index 844d0d1a4f..b9f89cfe67 100644
--- a/cranelift/codegen/src/machinst/mod.rs
+++ b/cranelift/codegen/src/machinst/mod.rs
@@ -102,6 +102,7 @@ use crate::ir::condcodes::IntCC;
 use crate::ir::{Function, Type};
 use crate::result::CodegenResult;
 use crate::settings::Flags;
+
 use alloc::boxed::Box;
 use alloc::vec::Vec;
 use core::fmt::Debug;
@@ -189,7 +190,7 @@ pub trait MachInst: Clone + Debug {
     fn with_block_offsets(&mut self, my_offset: CodeOffset, targets: &[CodeOffset]);
 
     /// Get the register universe for this backend.
-    fn reg_universe() -> RealRegUniverse;
+    fn reg_universe(flags: &Flags) -> RealRegUniverse;
 
     /// Align a basic block offset (from start of function).  By default, no
     /// alignment occurs.