[llvm] 27c8596 - [win][x64] Unwind v2: When making headroom leave space for the header (#188368)

[via llvm-commits]

Author: Daniel Paoliello
Date: 2026-04-01T09:21:58-07:00
New Revision: 27c85964006ec45bd8215ec3271c79fbda435735

URL: https://github.com/llvm/llvm-project/commit/27c85964006ec45bd8215ec3271c79fbda435735
DIFF: https://github.com/llvm/llvm-project/commit/27c85964006ec45bd8215ec3271c79fbda435735.diff

LOG: [win][x64] Unwind v2: When making headroom leave space for the header (#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.

Added: 
    

Modified: 
    llvm/lib/MC/MCWin64EH.cpp

Removed: 
    


################################################################################
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)) {