From bf726e821790fad6ee304c1c36bddedbfe4152ff Mon Sep 17 00:00:00 2001
From: Leo Korinth <lkorinth@openjdk.org>
Date: Mon, 22 Sep 2025 14:22:57 +0000
Subject: [PATCH] 8285984: G1: Use standard idiom for inlined payload in
 G1MonotonicArena::Segment

Reviewed-by: aboldtch, tschatzl
---
 src/hotspot/share/gc/g1/g1MonotonicArena.cpp   |  2 +-
 src/hotspot/share/gc/g1/g1MonotonicArena.hpp   | 18 ++++++++++--------
 .../share/gc/g1/g1MonotonicArena.inline.hpp    |  2 +-
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/src/hotspot/share/gc/g1/g1MonotonicArena.cpp b/src/hotspot/share/gc/g1/g1MonotonicArena.cpp
index 1ee299e2ee4..a9c6462680f 100644
--- a/src/hotspot/share/gc/g1/g1MonotonicArena.cpp
+++ b/src/hotspot/share/gc/g1/g1MonotonicArena.cpp
@@ -34,7 +34,7 @@ G1MonotonicArena::Segment::Segment(uint slot_size, uint num_slots, Segment* next
   _next(next),
   _next_allocate(0),
   _mem_tag(mem_tag) {
-  _bottom = ((char*) this) + header_size();
+  guarantee(is_aligned(this, SegmentPayloadMaxAlignment), "Make sure Segments are always created at correctly aligned memory");
 }
 
 G1MonotonicArena::Segment* G1MonotonicArena::Segment::create_segment(uint slot_size,
diff --git a/src/hotspot/share/gc/g1/g1MonotonicArena.hpp b/src/hotspot/share/gc/g1/g1MonotonicArena.hpp
index 828c7f4e86d..211820c5254 100644
--- a/src/hotspot/share/gc/g1/g1MonotonicArena.hpp
+++ b/src/hotspot/share/gc/g1/g1MonotonicArena.hpp
@@ -110,9 +110,10 @@ protected:
   void deallocate(void* slot) override { ShouldNotReachHere(); }
 };
 
+static constexpr uint SegmentPayloadMaxAlignment = 8;
 // A single segment/arena containing _num_slots blocks of memory of _slot_size.
 // Segments can be linked together using a singly linked list.
-class G1MonotonicArena::Segment {
+class alignas(SegmentPayloadMaxAlignment) G1MonotonicArena::Segment {
   const uint _slot_size;
   const uint _num_slots;
   Segment* volatile _next;
@@ -122,16 +123,15 @@ class G1MonotonicArena::Segment {
   uint volatile _next_allocate;
   const MemTag _mem_tag;
 
-  char* _bottom;  // Actual data.
-  // Do not add class member variables beyond this point
-
-  static size_t header_size() { return align_up(sizeof(Segment), DEFAULT_PADDING_SIZE); }
+  static size_t header_size() { return align_up(sizeof(Segment), SegmentPayloadMaxAlignment); }
 
   static size_t payload_size(uint slot_size, uint num_slots) {
-    // The cast (size_t) is required to guard against overflow wrap around.
-    return (size_t)slot_size * num_slots;
+    // The cast is required to guard against overflow wrap around.
+    return static_cast<size_t>(slot_size) * num_slots;
   }
 
+  void* payload(size_t octet) { return &reinterpret_cast<char*>(this)[header_size() + octet]; }
+
   size_t payload_size() const { return payload_size(_slot_size, _num_slots); }
 
   NONCOPYABLE(Segment);
@@ -156,7 +156,7 @@ public:
     _next_allocate = 0;
     assert(next != this, " loop condition");
     set_next(next);
-    memset((void*)_bottom, 0, payload_size());
+    memset(payload(0), 0, payload_size());
   }
 
   uint slot_size() const { return _slot_size; }
@@ -179,6 +179,7 @@ public:
   bool is_full() const { return _next_allocate >= _num_slots; }
 };
 
+static_assert(alignof(G1MonotonicArena::Segment) >= SegmentPayloadMaxAlignment, "assert alignment of Segment (and indirectly its payload)");
 
 // Set of (free) Segments. The assumed usage is that allocation
 // to it and removal of segments is strictly separate, but every action may be
@@ -235,6 +236,7 @@ public:
     assert(_initial_num_slots > 0, "Must be");
     assert(_max_num_slots > 0, "Must be");
     assert(_slot_alignment > 0, "Must be");
+    assert(SegmentPayloadMaxAlignment % _slot_alignment == 0, "ensure that _slot_alignment is a divisor of SegmentPayloadMaxAlignment");
   }
 
   virtual uint next_num_slots(uint prev_num_slots) const {
diff --git a/src/hotspot/share/gc/g1/g1MonotonicArena.inline.hpp b/src/hotspot/share/gc/g1/g1MonotonicArena.inline.hpp
index d4c1aa8c4e3..dd9ccae1849 100644
--- a/src/hotspot/share/gc/g1/g1MonotonicArena.inline.hpp
+++ b/src/hotspot/share/gc/g1/g1MonotonicArena.inline.hpp
@@ -39,7 +39,7 @@ inline void* G1MonotonicArena::Segment::allocate_slot() {
   if (result >= _num_slots) {
     return nullptr;
   }
-  void* r = _bottom + (size_t)result * _slot_size;
+  void* r = payload(static_cast<size_t>(result) * _slot_size);
   return r;
 }