Do not call SET_HDR() to remove forwarding counts if none exists in hblk

Issue #237 (bdwgc).

This also prevents writing zeros to GC_all_nils by GC_remove_counts().

* headers.c (GC_remove_counts): If HDR(h+1) is zero (and sz is at least
one block) then assert that HDR() of other hbp elements is zero too and
then return (w/o SET_HDR(hbp,0) invocation).
* ptr_chck.c (GC_is_valid_displacement): Remove comment that if(p) is
needed to avoid a data race.

2 files changed, 9 insertions, 4 deletions

diff --git a/headers.c b/headers.c
index d2d3da8a..69c9f210 100644
--- a/headers.c
+++ b/headers.c
@@ -310,6 +310,15 @@ GC_INNER void GC_remove_counts(struct hblk *h, size_t sz/* bytes */)
 {
     struct hblk * hbp;
 
+    if (sz <= HBLKSIZE) return;
+    if (HDR(h+1) == 0) {
+#     ifdef GC_ASSERTIONS
+        for (hbp = h+2; (word)hbp < (word)h + sz; hbp++)
+          GC_ASSERT(HDR(hbp) == 0);
+#     endif
+      return;
+    }
+
     for (hbp = h+1; (word)hbp < (word)h + sz; hbp += 1) {
         SET_HDR(hbp, 0);
     }
diff --git a/ptr_chck.c b/ptr_chck.c
index a4298a81..88affde3 100644
--- a/ptr_chck.c
+++ b/ptr_chck.c
@@ -123,11 +123,7 @@ GC_API void * GC_CALL GC_is_valid_displacement(void *p)
     word sz;
 
     if (!EXPECT(GC_is_initialized, TRUE)) GC_init();
-
-    /* A quick check to avoid TSan report about the data race   */
-    /* between GC_find_header() and GC_remove_counts().         */
     if (NULL == p) return NULL;
-
     hhdr = HDR((word)p);
     if (hhdr == 0) return(p);
     h = HBLKPTR(p);