From 697be2b6908d926bf89fcc6cd173459e29022f51 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=96mer=20Sinan=20A=C4=9Facan?= <omer@well-typed.com>
Date: Tue, 5 Feb 2019 10:40:29 -0500
Subject: rts/GC: Add an obvious assertion during block initialization

Namely ensure that block descriptors are initialized with valid
generation numbers.

Co-Authored-By: Ben Gamari <ben@well-typed.com>
---
 includes/Rts.h            | 9 +++++----
 includes/rts/Flags.h      | 6 +++++-
 includes/rts/storage/GC.h | 8 ++++++++
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/includes/Rts.h b/includes/Rts.h
index dd60726c39..56642e14c5 100644
--- a/includes/Rts.h
+++ b/includes/Rts.h
@@ -281,26 +281,27 @@ TICK_VAR(2)
 #define IF_RTSFLAGS(c,s)  if (RtsFlags.c) { s; } doNothing()
 
 #if defined(DEBUG)
+/* See Note [RtsFlags is a pointer in STG code] */
 #if IN_STG_CODE
 #define IF_DEBUG(c,s)  if (RtsFlags[0].DebugFlags.c) { s; } doNothing()
 #else
 #define IF_DEBUG(c,s)  if (RtsFlags.DebugFlags.c) { s; } doNothing()
-#endif
+#endif /* IN_STG_CODE */
 #else
 #define IF_DEBUG(c,s)  doNothing()
-#endif
+#endif /* DEBUG */
 
 #if defined(DEBUG)
 #define DEBUG_ONLY(s) s
 #else
 #define DEBUG_ONLY(s) doNothing()
-#endif
+#endif /* DEBUG */
 
 #if defined(DEBUG)
 #define DEBUG_IS_ON   1
 #else
 #define DEBUG_IS_ON   0
-#endif
+#endif /* DEBUG */
 
 /* -----------------------------------------------------------------------------
    Useful macros and inline functions
diff --git a/includes/rts/Flags.h b/includes/rts/Flags.h
index b3caf13c1f..678d556bf1 100644
--- a/includes/rts/Flags.h
+++ b/includes/rts/Flags.h
@@ -268,7 +268,11 @@ typedef struct _RTS_FLAGS {
 #if defined(COMPILING_RTS_MAIN)
 extern DLLIMPORT RTS_FLAGS RtsFlags;
 #elif IN_STG_CODE
-/* Hack because the C code generator can't generate '&label'. */
+/* Note [RtsFlags is a pointer in STG code]
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * When compiling with IN_STG_CODE the RtsFlags symbol is defined as a pointer.
+ * This is necessary because the C code generator can't generate '&label'.
+ */
 extern RTS_FLAGS RtsFlags[];
 #else
 extern RTS_FLAGS RtsFlags;
diff --git a/includes/rts/storage/GC.h b/includes/rts/storage/GC.h
index 1571975852..77dbe60297 100644
--- a/includes/rts/storage/GC.h
+++ b/includes/rts/storage/GC.h
@@ -240,9 +240,17 @@ void dirty_MUT_VAR(StgRegTable *reg, StgClosure *p);
 /* (needed when dynamic libraries are used). */
 extern bool keepCAFs;
 
+#include "rts/Flags.h"
+
 INLINE_HEADER void initBdescr(bdescr *bd, generation *gen, generation *dest)
 {
     bd->gen     = gen;
     bd->gen_no  = gen->no;
     bd->dest_no = dest->no;
+
+#if !IN_STG_CODE
+    /* See Note [RtsFlags is a pointer in STG code] */
+    ASSERT(gen->no < RtsFlags.GcFlags.generations);
+    ASSERT(dest->no < RtsFlags.GcFlags.generations);
+#endif
 }
-- 
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