diff --git a/include/jemalloc/internal/ehooks.h b/include/jemalloc/internal/ehooks.h
index bae468b3..064ecf5a 100644
--- a/include/jemalloc/internal/ehooks.h
+++ b/include/jemalloc/internal/ehooks.h
@@ -61,8 +61,7 @@ bool ehooks_default_split_impl();
 bool ehooks_default_merge(extent_hooks_t *extent_hooks, void *addr_a,
     size_t size_a, void *addr_b, size_t size_b, bool committed,
     unsigned arena_ind);
-bool ehooks_default_merge_impl(tsdn_t *tsdn, void *addr_a, bool head_a,
-    void *addr_b, bool head_b);
+bool ehooks_default_merge_impl(tsdn_t *tsdn, void *addr_a, void *addr_b);
 void ehooks_default_zero_impl(void *addr, size_t size);
 
 /*
@@ -338,21 +337,10 @@ ehooks_split(tsdn_t *tsdn, ehooks_t *ehooks, void *addr, size_t size,
 
 static inline bool
 ehooks_merge(tsdn_t *tsdn, ehooks_t *ehooks, void *addr_a, size_t size_a,
-    bool head_a, void *addr_b, size_t size_b, bool head_b, bool committed) {
+    void *addr_b, size_t size_b, bool committed) {
 	extent_hooks_t *extent_hooks = ehooks_get_extent_hooks_ptr(ehooks);
-	/*
-	 * The definition of extent_hooks merge function doesn't know about
-	 * extent head state, but the implementation does.  As a result, it
-	 * needs to call iealloc again and walk the rtree.  Since the cost of an
-	 * iealloc is large relative to the cost of the default merge hook
-	 * (which on posix-likes is just "return false"), we go even further
-	 * when we short-circuit; we don't just check if the extent hooks
-	 * generally are default, we check if the merge hook specifically is.
-	 */
-	if (extent_hooks == &ehooks_default_extent_hooks
-	    || extent_hooks->merge == &ehooks_default_merge) {
-		return ehooks_default_merge_impl(tsdn, addr_a, head_a, addr_b,
-		    head_b);
+	if (extent_hooks == &ehooks_default_extent_hooks) {
+		return ehooks_default_merge_impl(tsdn, addr_a, addr_b);
 	} else if (extent_hooks->merge == NULL) {
 		return true;
 	} else {
diff --git a/include/jemalloc/internal/emap.h b/include/jemalloc/internal/emap.h
index 5a5dbb6d..364aefac 100644
--- a/include/jemalloc/internal/emap.h
+++ b/include/jemalloc/internal/emap.h
@@ -224,6 +224,26 @@ extent_assert_can_expand(const edata_t *original, const edata_t *expand) {
 	assert(edata_past_get(original) == edata_base_get(expand));
 }
 
+JEMALLOC_ALWAYS_INLINE bool
+edata_neighbor_head_state_mergeable(bool edata_is_head,
+    bool neighbor_is_head, bool forward) {
+	/*
+	 * Head states checking: disallow merging if the higher addr extent is a
+	 * head extent.  This helps preserve first-fit, and more importantly
+	 * makes sure no merge across arenas.
+	 */
+	if (forward) {
+		if (neighbor_is_head) {
+			return false;
+		}
+	} else {
+		if (edata_is_head) {
+			return false;
+		}
+	}
+	return true;
+}
+
 JEMALLOC_ALWAYS_INLINE edata_t *
 emap_edata_lookup(tsdn_t *tsdn, emap_t *emap, const void *ptr) {
 	EMAP_DECLARE_RTREE_CTX;
diff --git a/src/ehooks.c b/src/ehooks.c
index e1815ee8..ca3ca209 100644
--- a/src/ehooks.c
+++ b/src/ehooks.c
@@ -188,11 +188,10 @@ ehooks_default_split(extent_hooks_t *extent_hooks, void *addr, size_t size,
 }
 
 bool
-ehooks_default_merge_impl(tsdn_t *tsdn, void *addr_a, bool head_a, void *addr_b,
-    bool head_b) {
+ehooks_default_merge_impl(tsdn_t *tsdn, void *addr_a, void *addr_b) {
 	assert(addr_a < addr_b);
 	/*
-	 * For non-DSS cases (first 2 branches) --
+	 * For non-DSS cases --
 	 * a) W/o maps_coalesce, merge is not always allowed (Windows):
 	 *   1) w/o retain, never merge (first branch below).
 	 *   2) with retain, only merge extents from the same VirtualAlloc
@@ -204,17 +203,23 @@ ehooks_default_merge_impl(tsdn_t *tsdn, void *addr_a, bool head_a, void *addr_b,
 	 *      disallowed if b is a head extent, i.e. no merging across
 	 *      different mmap regions.
 	 *
-	 * a2) and b2) share the implementation (the no_merge_heads branch).
+	 * a2) and b2) are implemented in emap_try_acquire_edata_neighbor, and
+	 * sanity checked in the second branch below.
 	 */
 	if (!maps_coalesce && !opt_retain) {
 		return true;
 	}
-	/*
-	 * Don't merge across mappings when retain is on -- this preserves
-	 * first-fit ordering.
-	 */
-	if (opt_retain && head_b) {
-		return true;
+	if (config_debug) {
+		edata_t *a = emap_edata_lookup(tsdn, &arena_emap_global,
+		    addr_a);
+		bool head_a = edata_is_head_get(a);
+		edata_t *b = emap_edata_lookup(tsdn, &arena_emap_global,
+		    addr_b);
+		bool head_b = edata_is_head_get(b);
+		emap_assert_mapped(tsdn, &arena_emap_global, a);
+		emap_assert_mapped(tsdn, &arena_emap_global, b);
+		assert(edata_neighbor_head_state_mergeable(head_a, head_b,
+		    /* forward */ true));
 	}
 	if (have_dss && !extent_dss_mergeable(addr_a, addr_b)) {
 		return true;
@@ -228,14 +233,7 @@ ehooks_default_merge(extent_hooks_t *extent_hooks, void *addr_a, size_t size_a,
     void *addr_b, size_t size_b, bool committed, unsigned arena_ind) {
 	tsdn_t *tsdn = tsdn_fetch();
 
-	edata_t *a = emap_edata_lookup(tsdn, &arena_emap_global, addr_a);
-	bool head_a = edata_is_head_get(a);
-	edata_t *b = emap_edata_lookup(tsdn, &arena_emap_global, addr_b);
-	bool head_b = edata_is_head_get(b);
-	emap_assert_mapped(tsdn, &arena_emap_global, a);
-	emap_assert_mapped(tsdn, &arena_emap_global, b);
-
-	return ehooks_default_merge_impl(tsdn, addr_a, head_a, addr_b, head_b);
+	return ehooks_default_merge_impl(tsdn, addr_a, addr_b);
 }
 
 void
diff --git a/src/emap.c b/src/emap.c
index 949b53e5..0fe230ab 100644
--- a/src/emap.c
+++ b/src/emap.c
@@ -48,26 +48,6 @@ emap_update_edata_state(tsdn_t *tsdn, emap_t *emap, edata_t *edata,
 	emap_assert_mapped(tsdn, emap, edata);
 }
 
-static inline bool
-edata_neighbor_head_state_mergeable(bool edata_is_head,
-    bool neighbor_is_head, bool forward) {
-	/*
-	 * Head states checking: disallow merging if the higher addr extent is a
-	 * head extent.  This helps preserve first-fit, and more importantly
-	 * makes sure no merge across arenas.
-	 */
-	if (forward) {
-		if (neighbor_is_head) {
-			return false;
-		}
-	} else {
-		if (edata_is_head) {
-			return false;
-		}
-	}
-	return true;
-}
-
 static inline bool
 edata_can_acquire_neighbor(edata_t *edata, rtree_contents_t contents,
     extent_pai_t pai, extent_state_t expected_state, bool forward,
diff --git a/src/extent.c b/src/extent.c
index 6d9e0029..1748d98b 100644
--- a/src/extent.c
+++ b/src/extent.c
@@ -1189,8 +1189,8 @@ extent_merge_impl(tsdn_t *tsdn, pac_t *pac, ehooks_t *ehooks, edata_t *a,
 	emap_assert_mapped(tsdn, pac->emap, b);
 
 	bool err = ehooks_merge(tsdn, ehooks, edata_base_get(a),
-	    edata_size_get(a), edata_is_head_get(a), edata_base_get(b),
-	    edata_size_get(b), edata_is_head_get(b), edata_committed_get(a));
+	    edata_size_get(a), edata_base_get(b), edata_size_get(b),
+	    edata_committed_get(a));
 
 	if (err) {
 		return true;