From 4329ff66989dc5b143e2524fb6143dba20f885d8 Mon Sep 17 00:00:00 2001
From: Jan Rysavy <janrysavy@users.noreply.github.com>
Date: Tue, 21 Jul 2026 11:35:48 +0200
Subject: [PATCH] Add Win64 in-place small-block realloc fast path

---
 FastMM5.pas | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/FastMM5.pas b/FastMM5.pas
index 4d6aa4f..8657799 100644
--- a/FastMM5.pas
+++ b/FastMM5.pas
@@ -8104,7 +8104,7 @@ function FastMM_ReallocMem(APointer: Pointer; ANewSize: NativeInt): Pointer;
   mov eax, r8d
   and eax, (CBlockIsFreeFlag or CIsSmallBlockFlag)
   cmp eax, CIsSmallBlockFlag
-  je FastMM_ReallocMem_ReallocSmallBlock
+  je @SmallBlock
 
   mov eax, r8d
   and eax, (not CHasDebugInfoFlag)
@@ -8120,6 +8120,29 @@ function FastMM_ReallocMem(APointer: Pointer; ANewSize: NativeInt): Pointer;
   xor edx,edx
   jmp HandleInvalidFreeMemOrReallocMem
 
+@SmallBlock:
+  {Inline the common in-place small-block downsize path.  This avoids the compiler-generated frame and saved registers
+  in FastMM_ReallocMem_ReallocSmallBlock on Win64.}
+  mov r9d, r8d
+  and r9d, CDropSmallBlockFlagsMask
+  shl r9d, CSmallBlockSpanOffsetBitShift
+  mov rax, rcx
+  and rax, -CMediumBlockAlignment
+  sub rax, r9
+  mov rax, TSmallBlockSpanHeader(rax).SmallBlockManager
+  movzx r9d, TSmallBlockManager(rax).BlockSize
+  sub r9d, CSmallBlockHeaderSize
+
+  {Upsizes and large downsizes use the existing moving path.}
+  cmp r9, rdx
+  jb FastMM_ReallocMem_ReallocSmallBlock
+  lea r10, [rdx * 4 + CSmallBlockDownsizeCheckAdder]
+  cmp r10, r9
+  jb FastMM_ReallocMem_ReallocSmallBlock
+
+  mov rax, rcx
+  ret
+
 {$endif}
 {$else}
 var