From 98c67dff7f493c94a15bad425b2c9caeb1670572 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Wed, 4 Mar 2026 16:14:13 +0800
Subject: [PATCH] fixes #25566; `{.align.}` pragma where each 16-byte-aligned
 (#25570)

fixes #25566

(cherry picked from commit 8e2547a5e2a616380b71cdcbc922770fa11aafb8)
---
 doc/manual.md           |  2 +-
 lib/system/alloc.nim    |  4 +++-
 lib/system/cellsets.nim | 16 ++++++++++++++++
 lib/system/gc.nim       |  4 ++--
 lib/system/mmdisp.nim   | 15 ---------------
 tests/align/talign.nim  |  1 -
 tests/align/talign2.nim | 10 ++++++++++
 7 files changed, 32 insertions(+), 20 deletions(-)
 create mode 100644 tests/align/talign2.nim

diff --git a/doc/manual.md b/doc/manual.md
index 556fba538b..55fdd83a33 100644
--- a/doc/manual.md
+++ b/doc/manual.md
@@ -7906,7 +7906,7 @@ alignment requirement of the type are ignored.
   main()
   ```
 
-This pragma has no effect on the JS backend.
+This pragma has no effect on the JavaScript backend and may significantly increase memory usage with the `--mm:refc` option.
 
 
 Noalias pragma
diff --git a/lib/system/alloc.nim b/lib/system/alloc.nim
index 1c2706120e..c40f808b88 100644
--- a/lib/system/alloc.nim
+++ b/lib/system/alloc.nim
@@ -104,6 +104,8 @@ type
       zeroField: int       # 0 means cell is not used (overlaid with typ field)
                           # 1 means cell is manually managed pointer
                           # otherwise a PNimType is stored in there
+      when sizeof(int) == 4:  # 32-bit only
+        headerAlignPad: array[8, byte]  # so addr(data) ≡ 8 (mod 16)
     else:
       alignment: int
 
@@ -854,7 +856,7 @@ proc bigChunkAlignOffset(alignment: int): int {.inline.} =
   if alignment == 0:
     result = 0
   else:
-    result = align(sizeof(BigChunk) + sizeof(Cell), alignment) - sizeof(BigChunk) - sizeof(Cell)
+    result = align(sizeof(BigChunk) + sizeof(FreeCell), alignment) - sizeof(BigChunk) - sizeof(FreeCell)
 
 proc rawAlloc(a: var MemRegion, requestedSize: int, alignment: int = 0): pointer =
   when defined(nimTypeNames):
diff --git a/lib/system/cellsets.nim b/lib/system/cellsets.nim
index f68bfc9703..5348773107 100644
--- a/lib/system/cellsets.nim
+++ b/lib/system/cellsets.nim
@@ -48,7 +48,23 @@ when defined(gcOrc) or defined(gcArc) or defined(gcAtomicArc):
 
   when not declaredInScope(PageShift):
     include bitmasks
+else:
+  type
+    RefCount = int
 
+    Cell {.pure.} = object
+      refcount: RefCount  # the refcount and some flags
+      typ: PNimType
+
+      when trackAllocationSource:
+        filename: cstring
+        line: int
+      when useCellIds:
+        id: int
+      when (not trackAllocationSource) and (not useCellIds) and sizeof(int) == 4:  # 32-bit only
+        headerAlignPad: array[8, byte]  # so addr(data) ≡ 8 (mod 16)
+
+    PCell = ptr Cell
 
 type
   PPageDesc = ptr PageDesc
diff --git a/lib/system/gc.nim b/lib/system/gc.nim
index 861e0704f5..8cc9c63878 100644
--- a/lib/system/gc.nim
+++ b/lib/system/gc.nim
@@ -460,7 +460,7 @@ proc rawNewObj(typ: PNimType, size: int, gch: var GcHeap): pointer =
   collectCT(gch)
   # Use alignment from typ.base if available, otherwise use MemAlign
   let alignment = if typ.kind == tyRef and typ.base != nil and
-        typ.base.align >= MemAlign: typ.base.align else: 0
+        typ.base.align > 16: typ.base.align else: 0
   var res = cast[PCell](rawAlloc(gch.region, size + sizeof(Cell), alignment))
   #gcAssert typ.kind in {tyString, tySequence} or size >= typ.base.size, "size too small"
   # Check that the user data (after the Cell header) is properly aligned
@@ -517,7 +517,7 @@ proc newObjRC1(typ: PNimType, size: int): pointer {.compilerRtl, noinline, raise
 
   # Use alignment from typ.base if available, otherwise use MemAlign
   let alignment = if typ.kind == tyRef and typ.base != nil and
-        typ.base.align >= MemAlign: typ.base.align else: 0
+        typ.base.align > 16: typ.base.align else: 0
   var res = cast[PCell](rawAlloc(gch.region, size + sizeof(Cell), alignment))
   sysAssert(allocInv(gch.region), "newObjRC1 after rawAlloc")
   # Check that the user data (after the Cell header) is properly aligned
diff --git a/lib/system/mmdisp.nim b/lib/system/mmdisp.nim
index ce935ff8af..7fd61e0dc3 100644
--- a/lib/system/mmdisp.nim
+++ b/lib/system/mmdisp.nim
@@ -38,21 +38,6 @@ type
   PByte = ptr ByteArray
   PString = ptr string
 
-when not defined(nimV2):
-  type
-    RefCount = int
-
-    Cell {.pure.} = object
-      refcount: RefCount  # the refcount and some flags
-      typ: PNimType
-      when trackAllocationSource:
-        filename: cstring
-        line: int
-      when useCellIds:
-        id: int
-
-    PCell = ptr Cell
-
 when declared(IntsPerTrunk):
   discard
 else:
diff --git a/tests/align/talign.nim b/tests/align/talign.nim
index 6397e31214..e1cb1539b2 100644
--- a/tests/align/talign.nim
+++ b/tests/align/talign.nim
@@ -168,4 +168,3 @@ for q in 0..100:
     new topArr[i]
     topArr[i].m.di.b = q
     doAssert(cast[uint](addr topArr[i].m.di) mod uint(alignof(DeepInner)) == 0)
-
diff --git a/tests/align/talign2.nim b/tests/align/talign2.nim
new file mode 100644
index 0000000000..528ae4d134
--- /dev/null
+++ b/tests/align/talign2.nim
@@ -0,0 +1,10 @@
+discard """
+  matrix: "--mm:refc -d:useGcAssert -d:useSysAssert; --mm:orc"
+"""
+
+block:
+  type U = object
+    d {.align: 16.}: int8
+  var e: seq[ref U]
+  for i in 0 ..< 10000: e.add(new U)
+  doAssert getTotalMem() <= 1052672 * 2
\ No newline at end of file