[llvm] [CAS] Fix alignment error from MappedFileRegionArena (PR #159128)

[Steven Wu via llvm-commits]

https://github.com/cachemeifyoucan created https://github.com/llvm/llvm-project/pull/159128

Fix a bug that when an alignment error can happen when reading a slice
of file from existing MappedFileRegionArena.


>From f478a8f5a6a5c55f48a50004f87b258c69e72e5f Mon Sep 17 00:00:00 2001
From: Steven Wu <stevenwu at apple.com>
Date: Tue, 16 Sep 2025 09:49:22 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.6
---
 llvm/lib/CAS/MappedFileRegionArena.cpp | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/CAS/MappedFileRegionArena.cpp b/llvm/lib/CAS/MappedFileRegionArena.cpp
index 2deb87d7adecf..472843d78af6e 100644
--- a/llvm/lib/CAS/MappedFileRegionArena.cpp
+++ b/llvm/lib/CAS/MappedFileRegionArena.cpp
@@ -216,17 +216,18 @@ Expected<MappedFileRegionArena> MappedFileRegionArena::create(
     if (!Size)
       return Size.takeError();
 
-    Header *H = reinterpret_cast<Header *>(HeaderContent.data());
-    if (H->HeaderOffset != HeaderOffset)
+    Header H;
+    memcpy(&H, HeaderContent.data(), sizeof(H));
+    if (H.HeaderOffset != HeaderOffset)
       return createStringError(
           std::make_error_code(std::errc::invalid_argument),
           "specified header offset (" + utostr(HeaderOffset) +
-              ") does not match existing config (" + utostr(H->HeaderOffset) +
+              ") does not match existing config (" + utostr(H.HeaderOffset) +
               ")");
 
     // If the capacity doesn't match, use the existing capacity instead.
-    if (H->Capacity != Capacity)
-      Capacity = H->Capacity;
+    if (H.Capacity != Capacity)
+      Capacity = H.Capacity;
   }
 
   // If the size is smaller than capacity, we need to resize the file.