[llvm] [IR] Fix assertion error in User new/delete edge case (PR #129914)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-ir

Author: Marc Auberer (marcauberer)

<details>
<summary>Changes</summary>

Fixes #<!-- -->129900

If `operator delete` was called after an unsuccessful call of `operator new` we ran into undefined behaviour.
This was discovered by our malfunction tests while preparing an upgrade to LLVM 20, that explicitly check for such kind of bugs.

---
Full diff: https://github.com/llvm/llvm-project/pull/129914.diff


1 Files Affected:

- (modified) llvm/lib/IR/User.cpp (+6) 


``````````diff
diff --git a/llvm/lib/IR/User.cpp b/llvm/lib/IR/User.cpp
index b0aa785deb9af..ab44cb4b8a3f7 100644
--- a/llvm/lib/IR/User.cpp
+++ b/llvm/lib/IR/User.cpp
@@ -146,6 +146,9 @@ void *User::allocateFixedOperandUser(size_t Size, unsigned Us,
   Use *Start = reinterpret_cast<Use *>(Storage + DescBytesToAllocate);
   Use *End = Start + Us;
   User *Obj = reinterpret_cast<User *>(End);
+  Obj->NumUserOperands = Us;
+  Obj->HasHungOffUses = false;
+  Obj->HasDescriptor = DescBytes != 0;
   for (; Start != End; Start++)
     new (Start) Use(Obj);
 
@@ -172,6 +175,9 @@ void *User::operator new(size_t Size, HungOffOperandsAllocMarker) {
   void *Storage = ::operator new(Size + sizeof(Use *));
   Use **HungOffOperandList = static_cast<Use **>(Storage);
   User *Obj = reinterpret_cast<User *>(HungOffOperandList + 1);
+  Obj->NumUserOperands = 0;
+  Obj->HasHungOffUses = true;
+  Obj->HasDescriptor = false;
   *HungOffOperandList = nullptr;
   return Obj;
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/129914