diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index 6a05028db34e55..4106f9e60d8afe 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -137,6 +137,13 @@ jobs:
         run: echo "DFLTCC=0" >> $GITHUB_ENV
         if: ${{ endsWith(matrix.os, 's390x') }}
 
+      # Emulated ppc64le/s390x runners are slow enough that subprocess
+      # assertion timeouts (e.g. assert_separately) flake. Scale them up, as
+      # macos.yml does for slower/contended runners.
+      - name: Set timeout scale for emulated runners
+        run: echo "RUBY_TEST_TIMEOUT_SCALE=10" >> $GITHUB_ENV
+        if: ${{ endsWith(matrix.os, 'ppc64le') || endsWith(matrix.os, 's390x') }}
+
       - name: make ${{ matrix.test_task }}
         run: |
           test -n "${LAUNCHABLE_STDOUT}" && exec 1> >(tee "${LAUNCHABLE_STDOUT}")
diff --git a/configure.ac b/configure.ac
index 2de91209d61919..8d020771c6d505 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4163,7 +4163,7 @@ AC_SUBST(JIT_CARGO_SUPPORT)dnl "no" or the cargo profile of the rust code
 [begin]_group "build section" && {
 AC_CACHE_CHECK([for prefix of external symbols], rb_cv_symbol_prefix, [
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[extern void conftest_external(void) {}]], [[]])],[
-	rb_cv_symbol_prefix=`$NM conftest.$ac_objext |
+	rb_cv_symbol_prefix=`$NM conftest.$ac_objext 2>/dev/null |
 			     sed -n ['/.*T[ 	]\([^ 	]*\)conftest_external.*/!d;s//\1/p;q']`
 	],
 	[rb_cv_symbol_prefix=''])
diff --git a/ext/json/parser/parser.c b/ext/json/parser/parser.c
index c216faf4b5a56e..136aab6ae764f5 100644
--- a/ext/json/parser/parser.c
+++ b/ext/json/parser/parser.c
@@ -2300,16 +2300,19 @@ static VALUE cResumableParser_feed(VALUE self, VALUE str)
 
         const size_t size = parser->state.end - parser->state.start;
         const size_t consumed = size - remaining;
-        char *old_ptr = RSTRING_PTR(parser->buffer);
 
         if (RB_OBJ_FROZEN_RAW(parser->buffer)) {
             VALUE new_buffer = rb_obj_hide(rb_str_buf_new(remaining + RSTRING_LEN(str)));
-            MEMCPY(RSTRING_PTR(new_buffer), old_ptr + consumed, char, remaining);
+
+            char *old_ptr = RSTRING_PTR(parser->buffer);
+            memcpy(RSTRING_PTR(new_buffer), old_ptr + consumed, remaining);
             rb_str_set_len(new_buffer, remaining);
             offset = 0;
             parser->buffer = new_buffer;
         } else if (consumed > (size / 2) && size >= 512) {
-            MEMMOVE(old_ptr, old_ptr + consumed, char, remaining);
+            rb_str_modify(parser->buffer);
+            char *old_ptr = RSTRING_PTR(parser->buffer);
+            memmove(old_ptr, old_ptr + consumed, remaining);
             rb_str_set_len(parser->buffer, remaining);
             offset = 0;
         }
diff --git a/gc/default/default.c b/gc/default/default.c
index 18d7a05b336430..66df18d879a101 100644
--- a/gc/default/default.c
+++ b/gc/default/default.c
@@ -1047,27 +1047,15 @@ gc_malloc_counters_increase_unsigned(rb_objspace_t *objspace, const struct gc_ma
     return (size_t)inc;
 }
 
-static inline int64_t
+static inline void
 gc_malloc_counters_snapshot(rb_objspace_t *objspace, struct gc_malloc_bytes *c)
 {
     MALLOC_COUNTERS_LOCK(objspace);
     gc_counter_t malloc_now = gc_counter_load_relaxed(&c->malloc);
     gc_counter_t free_now   = gc_counter_load_relaxed(&c->free);
-    gc_counter_t malloc_at  = gc_counter_load_relaxed(&c->malloc_at_last_gc);
-    gc_counter_t free_at    = gc_counter_load_relaxed(&c->free_at_last_gc);
     gc_counter_store_release(&c->malloc_at_last_gc, malloc_now);
     gc_counter_store_release(&c->free_at_last_gc,   free_now);
     MALLOC_COUNTERS_UNLOCK(objspace);
-
-    gc_counter_t malloc_delta = malloc_now - malloc_at;
-    gc_counter_t free_delta   = free_now   - free_at;
-
-    if (malloc_delta >= free_delta) {
-        return (int64_t)(malloc_delta - free_delta);
-    }
-    else {
-        return -(int64_t)(free_delta - malloc_delta);
-    }
 }
 
 #define heap_pages_lomem	objspace->heap_pages.range[0]
@@ -4224,10 +4212,10 @@ gc_sweep_finish(rb_objspace_t *objspace)
         }
     }
 
-    (void)gc_malloc_counters_snapshot(objspace, &objspace->malloc_counters.counters);
+    gc_malloc_counters_snapshot(objspace, &objspace->malloc_counters.counters);
 #if RGENGC_ESTIMATE_OLDMALLOC
     if (objspace->profile.latest_gc_info & GPR_FLAG_MAJOR_MASK) {
-        (void)gc_malloc_counters_snapshot(objspace, &objspace->malloc_counters.oldcounters);
+        gc_malloc_counters_snapshot(objspace, &objspace->malloc_counters.oldcounters);
     }
 #endif
 
diff --git a/include/ruby/internal/abi.h b/include/ruby/internal/abi.h
index 7ceb8c40b70eef..322761fc683341 100644
--- a/include/ruby/internal/abi.h
+++ b/include/ruby/internal/abi.h
@@ -24,7 +24,7 @@
  * In released versions of Ruby, this number is not defined since teeny
  * versions of Ruby should guarantee ABI compatibility.
  */
-#define RUBY_ABI_VERSION 3
+#define RUBY_ABI_VERSION 4
 
 /* Windows does not support weak symbols so ruby_abi_version will not exist
  * in the shared library. */
diff --git a/pathname_builtin.rb b/pathname_builtin.rb
index 60ae6d34129b92..8fdf871201cc61 100644
--- a/pathname_builtin.rb
+++ b/pathname_builtin.rb
@@ -533,7 +533,19 @@ def parent
     self + '..'
   end
 
-  # Returns +true+ if +self+ points to a mountpoint.
+  # :markup: markdown
+  #
+  # call-seq:
+  #   mountpoint? -> true or false
+  #
+  # Returns whether the path in `self` points to a mountpoint:
+  #
+  # ```ruby
+  # Pathname('/').mountpoint?      # => true
+  # Pathname('/etc').mountpoint?   # => false
+  # Pathname('nosuch').mountpoint? # => false
+  # ```
+  #
   def mountpoint?
     begin
       stat1 = self.lstat
diff --git a/test/json/resumable_parser_test.rb b/test/json/resumable_parser_test.rb
index 600cd33e5ecf28..52f1356a704f9a 100644
--- a/test/json/resumable_parser_test.rb
+++ b/test/json/resumable_parser_test.rb
@@ -244,6 +244,21 @@ def test_spill_frames_stack
     assert_equal expected, @parser.value
   end
 
+  def test_buffer_shrink
+    doc1 = '{"a":"' + ("x" * 800) + '"} {'   # >= 512 bytes
+    doc2 = '"b":1} '
+
+    parser = JSON::ResumableParser.new({})
+
+    parser << doc1 # internal buffer becomes a *shared* string here
+    parser.parse # consume doc1 -> >50% of a >=512B buffer is now consumed
+    parser.value
+
+    parser << doc2 # buffer is shrinked
+    parser.parse
+    parser.value
+  end
+
   private
 
   def assert_partial_value(expected, json)
diff --git a/tool/leaked-globals b/tool/leaked-globals
index 73da769318ad37..c1a3fb42b8869c 100755
--- a/tool/leaked-globals
+++ b/tool/leaked-globals
@@ -60,6 +60,23 @@ end
 # extern, so we allow the Rust one.
 REPLACE.push("rust_eh_personality") if RUBY_PLATFORM.include?("darwin")
 
+# Under LTO the compiler emits raw LLVM bitcode objects (magic "BC\xC0\xDE", or
+# the bitcode wrapper magic).  nm carries --no-llvm-bc (added in configure so it
+# ignores Rust's newer-version bitcode) and so cannot read them, only emitting
+# "not recognized" noise.  Drop them; leaked symbols are still checked on the
+# linked library and any non-bitcode objects.
+if defined?(NM) and NM.include?("--no-llvm-bc")
+  bitcode = 0
+  ARGV.reject! do |n|
+    next false unless File.file?(n)
+    case File.binread(n, 4)&.bytes
+    when [0x42, 0x43, 0xC0, 0xDE], [0xDE, 0xC0, 0x17, 0x0B]
+      bitcode += 1
+    end
+  end
+  puts "Skip #{col.skip("#{bitcode} LLVM bitcode object(s)")}" if bitcode > 0
+end
+
 print "Checking leaked global symbols..."
 STDOUT.flush
 if soext
@@ -105,7 +122,7 @@ Pipe.new(NM + ARGV).each do |line|
   puts col.fail("leaked") if count.zero?
   count += 1
   puts "  #{n}"
-end
+end unless ARGV.empty?
 case count
 when 0
   puts col.pass("none")