From 5e6a1919c730f7c3ad27b9e5c3ddc8938fc2f43f Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Wed, 3 Sep 2025 13:11:25 +0100
Subject: [PATCH] fix aarch64-macos DWARF unwinding

turns out this isn't technically specific to that target at all; other
targets just don't emit mid-function 'ret' instructions as much so
certain CFI instruction patterns were only seen on aarch64.

thanks to jacob for finding the bug <3
---
 lib/std/debug/Dwarf/Unwind/VirtualMachine.zig | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/lib/std/debug/Dwarf/Unwind/VirtualMachine.zig b/lib/std/debug/Dwarf/Unwind/VirtualMachine.zig
index 66100f5eda..997af95cbd 100644
--- a/lib/std/debug/Dwarf/Unwind/VirtualMachine.zig
+++ b/lib/std/debug/Dwarf/Unwind/VirtualMachine.zig
@@ -48,7 +48,10 @@ const ColumnRange = struct {
 };
 
 columns: std.ArrayList(Column) = .empty,
-stack: std.ArrayList(ColumnRange) = .empty,
+stack: std.ArrayList(struct {
+    cfa: Column,
+    columns: ColumnRange,
+}) = .empty,
 current_row: Row = .{},
 
 /// The result of executing the CIE's initial_instructions
@@ -205,17 +208,21 @@ pub fn step(
             column.rule = .{ .register = i.target_register };
         },
         .remember_state => {
-            try self.stack.append(gpa, self.current_row.columns);
+            try self.stack.append(gpa, .{
+                .cfa = self.current_row.cfa,
+                .columns = self.current_row.columns,
+            });
             self.current_row.copy_on_write = true;
         },
         .restore_state => {
-            const restored_columns = self.stack.pop() orelse return error.InvalidOperation;
+            const restored = self.stack.pop() orelse return error.InvalidOperation;
             self.columns.shrinkRetainingCapacity(self.columns.items.len - self.current_row.columns.len);
-            try self.columns.ensureUnusedCapacity(gpa, restored_columns.len);
+            try self.columns.ensureUnusedCapacity(gpa, restored.columns.len);
 
+            self.current_row.cfa = restored.cfa;
             self.current_row.columns.start = self.columns.items.len;
-            self.current_row.columns.len = restored_columns.len;
-            self.columns.appendSliceAssumeCapacity(self.columns.items[restored_columns.start..][0..restored_columns.len]);
+            self.current_row.columns.len = restored.columns.len;
+            self.columns.appendSliceAssumeCapacity(self.columns.items[restored.columns.start..][0..restored.columns.len]);
         },
         .def_cfa => |i| {
             try self.resolveCopyOnWrite(gpa);