[llvm] [win][x64] Unwind v2: When making headroom leave space for the header (PR #188368)

[Daniel Paoliello via llvm-commits]

https://github.com/dpaoliello created https://github.com/llvm/llvm-project/pull/188368

When the `ensureHeadroom` call was added during unwind v2 info emission, it didn't take into account that there is a header of 2-bytes before all the fixups, so not enough space was actually reserved.

>From 1ba6d234d5a4aaf50a7cabdd8f8f975517d5b246 Mon Sep 17 00:00:00 2001
From: "Daniel Paoliello (HE/HIM)" <danpao at microsoft.com>
Date: Fri, 20 Mar 2026 14:46:16 -0700
Subject: [PATCH] [win][x64] Unwind v2: When making headroom leave space for
 the header

---
 llvm/lib/MC/MCWin64EH.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/MC/MCWin64EH.cpp b/llvm/lib/MC/MCWin64EH.cpp
index e88c37d22ebea..32f701019e528 100644
--- a/llvm/lib/MC/MCWin64EH.cpp
+++ b/llvm/lib/MC/MCWin64EH.cpp
@@ -335,7 +335,8 @@ static void EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
   // Emit the epilog instructions.
   if (EnableUnwindV2) {
     // Ensure the fixups and appended content apply to the same fragment.
-    OS->ensureHeadroom(info->EpilogMap.size() * 2);
+    // size byte + flags byte + 2 per epilog (for the distance).
+    OS->ensureHeadroom(2 + info->EpilogMap.size() * 2);
 
     bool IsLast = true;
     for (const auto &Epilog : llvm::reverse(info->EpilogMap)) {