rts: Fix segfault in profiling/debug/sanity checking modewip/erikd/remove-nat

The profiling code assumes that un-used memory at the start of a block
is zero filled so the profiling code can just skip over it. However, when
compiled in DEBUG mode and with the debug sanity checking RTS flag (`-DS`)
given on the command line, the memory allocator pre-fills new blocks with
`0xaa` which causes the profiling code to segfault.

The fix is just to detect debug sanity checking and ignore any `0xaa`
filled memory. It does however mean that profiling results when debug
sanity checking is enabled may produce incorrect results.

1 files changed, 7 insertions, 0 deletions

diff --git a/rts/LdvProfile.c b/rts/LdvProfile.c
index 1dfdc56b34..42081ae129 100644
--- a/rts/LdvProfile.c
+++ b/rts/LdvProfile.c
@@ -184,7 +184,14 @@ processNurseryForDead( void )
     for (bd = MainCapability.r.rNursery->blocks; bd != NULL; bd = bd->link) {
         p = bd->start;
         while (p < bd->free) {
+            // The start of the block may be zero filled which we need to skip
+            // over.
             while (p < bd->free && !*p) p++; // skip slop
+
+            // In debug mode, with sanity checking enabled, the start of the
+            // block may be filled with `0xaa` so if we find it, we just break.
+            IF_DEBUG(sanity, if (*((StgWord32*)p) == 0xaaaaaaaa) break;);
+
             if (p >= bd->free) break;
             p += processHeapClosureForDead((StgClosure *)p);
         }