From 3752203969bb8948c0c8115577019c44690ba8e2 Mon Sep 17 00:00:00 2001
From: mrg <mrg@NetBSD.org>
Date: Sun, 11 Feb 2018 00:01:12 +0000
Subject: [PATCH] introduce a inline function to set a value to zero while
 hiding this fact from GCC.  this allows the PPC code that writes to address
 zero to actually work rather than cause GCC to emit an explicit "trap"
 instruction, which in early boot means hang on my pegasosII.  use this in
 oae_init() for both rfid->rfi and also setting the jump-to-zero trap.

found with a lot of debugging, but GCC 6's new warning
-Wnull-dereference found it when i was informed of its
existence.  unfortunately, there are dozens of other
violations in our kernel today so simply enabling that
option for everything is not a good idea, but is a goal.
---
 sys/arch/powerpc/oea/oea_machdep.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/sys/arch/powerpc/oea/oea_machdep.c b/sys/arch/powerpc/oea/oea_machdep.c
index 43f7cad9fe6b..9b2bfc4a8af7 100644
--- a/sys/arch/powerpc/oea/oea_machdep.c
+++ b/sys/arch/powerpc/oea/oea_machdep.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: oea_machdep.c,v 1.73 2016/05/30 13:04:24 chs Exp $	*/
+/*	$NetBSD: oea_machdep.c,v 1.74 2018/02/11 00:01:12 mrg Exp $	*/
 
 /*
  * Copyright (C) 2002 Matt Thomas
@@ -33,7 +33,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: oea_machdep.c,v 1.73 2016/05/30 13:04:24 chs Exp $");
+__KERNEL_RCSID(0, "$NetBSD: oea_machdep.c,v 1.74 2018/02/11 00:01:12 mrg Exp $");
 
 #include "opt_ppcarch.h"
 #include "opt_compat_netbsd.h"
@@ -109,6 +109,19 @@ extern int dsitrap_fix_dbat5[];
 extern int dsitrap_fix_dbat6[];
 extern int dsitrap_fix_dbat7[];
 
+/*
+ * Load pointer with 0 behind GCC's back, otherwise it will
+ * emit a "trap" instead.
+ */
+static __inline__ uintptr_t
+zero_value(void)
+{
+	uintptr_t dont_tell_gcc;
+
+	__asm volatile ("li %0, 0" : "=r"(dont_tell_gcc) :);
+	return dont_tell_gcc;
+}
+
 void
 oea_init(void (*handler)(void))
 {
@@ -144,7 +157,7 @@ oea_init(void (*handler)(void))
 #ifdef PPC_HIGH_VEC
 	exc_base = EXC_HIGHVEC;
 #else
-	exc_base = 0;
+	exc_base = zero_value();
 #endif
 	KASSERT(mfspr(SPR_SPRG0) == (uintptr_t)ci);
 
@@ -289,8 +302,10 @@ oea_init(void (*handler)(void))
 	 * Install a branch absolute to trap0 to force a panic.
 	 */
 	if ((uintptr_t)trap0 < 0x2000000) {
-		*(volatile uint32_t *) 0 = 0x7c6802a6;
-		*(volatile uint32_t *) 4 = 0x48000002 | (uintptr_t) trap0;
+		uint32_t *p = (uint32_t *)zero_value();
+
+		p[0] = 0x7c6802a6;
+		p[1] = 0x48000002 | (uintptr_t) trap0;
 	}
 
 	/*