From c9474064dc39a53fac9edc2529d9e2d9b397a3a5 Mon Sep 17 00:00:00 2001
From: Gareth Rees <gdr@garethrees.org>
Date: Tue, 27 Dec 2022 16:12:34 +0000
Subject: [PATCH] Ensure that the jump buffer is appropriately aligned.

---
 code/config.h | 10 ++++++++++
 code/ss.h     | 10 +++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/code/config.h b/code/config.h
index 83f985a885..d9d71e31cf 100644
--- a/code/config.h
+++ b/code/config.h
@@ -314,6 +314,16 @@
 #define ATTRIBUTE_UNUSED
 #endif
 
+/* Attribute for data structures that need to be allocated at
+ * addresses with a particular alignment.
+ * GCC: <https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-aligned-function-attribute>
+ */
+#if defined(MPS_BUILD_GC) || defined(MPS_BUILD_LL)
+#define ATTRIBUTE_ALIGNED(ALIGNMENT) __attribute__((__aligned__(ALIGNMENT)))
+#else
+#define ATTRIBUTE_ALIGNED(ALIGNMENT)
+#endif
+
 
 /* Compiler extensions */
 
diff --git a/code/ss.h b/code/ss.h
index f05ff67b62..cd46990ebc 100644
--- a/code/ss.h
+++ b/code/ss.h
@@ -19,12 +19,20 @@
  * The jumpBuffer is used to capture most of the mutator's state on
  * entry to the MPS, but can't capture it all.  See
  * <design/stack-scan#.sol.setjmp.scan>.
+ *
+ * The stack capturing mechanism in STACK_CONTEXT_BEGIN puts the
+ * StackContextStruct on the stack, where it will later be scanned
+ * using TraceScanArea. The jumpBuffer must be allocated on the stack
+ * at an address with suitable alignment so that TraceScanArea will
+ * correctly fix any addresses therein. This is vital on platform XCA6
+ * where jmp_buf is declared as an array of int, which has 4-byte
+ * alignment, but we need it to have 8-byte alignment.
  */
 
 #include <setjmp.h>
 
 typedef struct StackContextStruct {
-  jmp_buf jumpBuffer;
+  ATTRIBUTE_ALIGNED(MPS_PF_ALIGN) jmp_buf jumpBuffer;
 } StackContextStruct;