From 0f56d7afe298ae8f2b49d6c3b14fd935c971f1bb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20R=C3=B8nne=20Petersen?= <alex@alexrp.com>
Date: Fri, 3 Oct 2025 03:24:21 +0200
Subject: [PATCH] std.debug: use correct return address offset for s390x

Makes FP-based unwinding work.
---
 lib/std/debug.zig | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 2b1ea3492f..6d705f3c86 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -952,6 +952,9 @@ const StackIterator = union(enum) {
     /// Offset of the saved return address wrt the frame pointer.
     const ra_offset = off: {
         if (native_arch == .powerpc64le) break :off 2 * @sizeOf(usize);
+        // On s390x, r14 is the link register and we need to grab it from its customary slot in the
+        // register save area (ELF ABI s390x Supplement §1.2.2.2).
+        if (native_arch == .s390x) break :off 14 * @sizeOf(usize);
         break :off @sizeOf(usize);
     };