From ef4ce7def1d4868d1d54832fba473f84938e40be Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20R=C3=B8nne=20Petersen?= <alex@alexrp.com>
Date: Sun, 9 Aug 2026 01:55:14 +0200
Subject: [PATCH] std: fix zeroing of lr in powerpc _start and clone assembly

PowerPC assembly is wonderful because you can't tell at a glance whether an
operand is an immediate integer or a register number. In this case, mtlr takes a
register number, but we haven't zeroed r0, and it's not a hardwired zero
register as on other architectures.

Fix this by copying from r31 which we've just zeroed.
---
 lib/std/os/linux/powerpc.zig   | 2 +-
 lib/std/os/linux/powerpc64.zig | 2 +-
 lib/std/start.zig              | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/std/os/linux/powerpc.zig b/lib/std/os/linux/powerpc.zig
index 614c981f8ff891e2e3fed978b65866da9d9a6ab3..5bfaae2767ee33b4d0b1ff32ccafab0ed5f1a1b1 100644
--- a/lib/std/os/linux/powerpc.zig
+++ b/lib/std/os/linux/powerpc.zig
@@ -247,7 +247,7 @@ pub fn clone() callconv(.naked) u32 {
     );
     asm volatile (
         \\ li 31, 0
-        \\ mtlr 0
+        \\ mtlr 31
         \\
         \\ #call funcptr: move arg (d) into r3
         \\ mr 3, 30
diff --git a/lib/std/os/linux/powerpc64.zig b/lib/std/os/linux/powerpc64.zig
index 89f3dbde7b3736d7cc021d55f0b9fb753f347789..5dd1fc9a9d8163ec50b37e58749a63d2eaf5312a 100644
--- a/lib/std/os/linux/powerpc64.zig
+++ b/lib/std/os/linux/powerpc64.zig
@@ -232,7 +232,7 @@ pub fn clone() callconv(.naked) u64 {
     );
     asm volatile (
         \\  li    31, 0
-        \\  mtlr   0
+        \\  mtlr  31
         \\
         \\  # call fn(arg)
         \\  ld     3, 16(1)
diff --git a/lib/std/start.zig b/lib/std/start.zig
index c49453246fcc59f09d717fe08a9d000a418cd891..ccfc572f0690ccbc254270b4d9c6c76bb66ccfa7 100644
--- a/lib/std/start.zig
+++ b/lib/std/start.zig
@@ -438,7 +438,7 @@ fn _start() callconv(.naked) noreturn {
             \\ stwu 1, -16(1)
             \\ stw 0, 0(1)
             \\ li 31, 0
-            \\ mtlr 0
+            \\ mtlr 31
             \\ b %[posixCallMainAndExit]
             ,
             .powerpc64, .powerpc64le =>
@@ -451,7 +451,7 @@ fn _start() callconv(.naked) noreturn {
             \\ li 0, 0
             \\ stdu 0, -32(1)
             \\ li 31, 0
-            \\ mtlr 0
+            \\ mtlr 31
             \\ b %[posixCallMainAndExit]
             \\ nop
             ,
-- 
2.54.0