From 27185223cfaac1800e15f831109a86cd819c0d77 Mon Sep 17 00:00:00 2001
From: NullVoxPopuli <199018+NullVoxPopuli@users.noreply.github.com>
Date: Wed, 9 Sep 2026 18:46:19 -0400
Subject: [PATCH 1/2] Fix LowLevelRegisters tuple order and drop unused
 register helpers

Split out of nvp/simplify-some-vm-hot-paths.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 packages/@glimmer/runtime/lib/vm/low-level.ts | 19 ++---------------
 packages/@glimmer/runtime/lib/vm/stack.ts     | 21 +++----------------
 2 files changed, 5 insertions(+), 35 deletions(-)

diff --git a/packages/@glimmer/runtime/lib/vm/low-level.ts b/packages/@glimmer/runtime/lib/vm/low-level.ts
index a817f25548a..19bca0fc635 100644
--- a/packages/@glimmer/runtime/lib/vm/low-level.ts
+++ b/packages/@glimmer/runtime/lib/vm/low-level.ts
@@ -23,23 +23,8 @@ import type { VM } from './append';
 import '../bootstrap';
 import { APPEND_OPCODES } from '../opcodes';
 
-export type LowLevelRegisters = [$pc: number, $ra: number, $sp: number, $fp: number];
-
-export function initializeRegisters(): LowLevelRegisters {
-  return [0, -1, 0, 0];
-}
-
-export function restoreRegisters(pc: number, sp: number): LowLevelRegisters {
-  return [pc, -1, sp, 0];
-}
-
-export function initializeRegistersWithSP(sp: number): LowLevelRegisters {
-  return [0, -1, sp, 0];
-}
-
-export function initializeRegistersWithPC(pc: number): LowLevelRegisters {
-  return [pc, -1, 0, 0];
-}
+// Positional order matches the register constants: $pc=0, $ra=1, $fp=2, $sp=3.
+export type LowLevelRegisters = [$pc: number, $ra: number, $fp: number, $sp: number];
 
 export interface VmStack {
   readonly registers: LowLevelRegisters;
diff --git a/packages/@glimmer/runtime/lib/vm/stack.ts b/packages/@glimmer/runtime/lib/vm/stack.ts
index 9d736c28a64..cf88e2c309e 100644
--- a/packages/@glimmer/runtime/lib/vm/stack.ts
+++ b/packages/@glimmer/runtime/lib/vm/stack.ts
@@ -1,11 +1,9 @@
 import assert from '@glimmer/debug-util/lib/assert';
 import { LOCAL_DEBUG } from '@glimmer/local-debug-flags';
-import { $fp, $pc, $sp } from '@glimmer/vm/lib/registers';
+import { $fp, $sp } from '@glimmer/vm/lib/registers';
 
 import type { LowLevelRegisters } from './low-level';
 
-import { initializeRegistersWithSP } from './low-level';
-
 export interface EvaluationStack {
   readonly registers: LowLevelRegisters;
 
@@ -25,15 +23,10 @@ export interface EvaluationStack {
 
 export default class EvaluationStackImpl implements EvaluationStack {
   static restore(snapshot: unknown[], pc: number): EvaluationStackImpl {
-    const stack = new this(snapshot.slice(), initializeRegistersWithSP(snapshot.length - 1));
-
     assert(typeof pc === 'number', 'pc is a number');
 
-    stack.registers[$pc] = pc;
-    stack.registers[$sp] = snapshot.length - 1;
-    stack.registers[$fp] = -1;
-
-    return stack;
+    // [$pc, $ra, $fp, $sp]
+    return new this(snapshot.slice(), [pc, -1, -1, snapshot.length - 1]);
   }
 
   readonly registers: LowLevelRegisters;
@@ -100,12 +93,4 @@ export default class EvaluationStackImpl implements EvaluationStack {
   }
 
   declare snapshot?: (this: EvaluationStackImpl) => unknown[];
-
-  static {
-    if (LOCAL_DEBUG) {
-      EvaluationStackImpl.prototype.snapshot = function () {
-        return this.stack.slice(this.registers[$fp], this.registers[$sp] + 1);
-      };
-    }
-  }
 }

From 7186100856597c89e5e26e05ea5be4053f66d2ed Mon Sep 17 00:00:00 2001
From: NullVoxPopuli <199018+NullVoxPopuli@users.noreply.github.com>
Date: Mon, 21 Sep 2026 14:36:46 -0400
Subject: [PATCH 2/2] Apply suggestion from @NullVoxPopuli

---
 packages/@glimmer/runtime/lib/vm/low-level.ts | 1 -
 1 file changed, 1 deletion(-)

diff --git a/packages/@glimmer/runtime/lib/vm/low-level.ts b/packages/@glimmer/runtime/lib/vm/low-level.ts
index 19bca0fc635..9a6add0eb6c 100644
--- a/packages/@glimmer/runtime/lib/vm/low-level.ts
+++ b/packages/@glimmer/runtime/lib/vm/low-level.ts
@@ -23,7 +23,6 @@ import type { VM } from './append';
 import '../bootstrap';
 import { APPEND_OPCODES } from '../opcodes';
 
-// Positional order matches the register constants: $pc=0, $ra=1, $fp=2, $sp=3.
 export type LowLevelRegisters = [$pc: number, $ra: number, $fp: number, $sp: number];
 
 export interface VmStack {