[llvm] [BPI] Transfer value-handles when assign/move constructing BPI (PR #77774)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse created https://github.com/llvm/llvm-project/pull/77774

Background: BPI stores a collection of edge branch-probabilities, and also a set of Callback value-handles for the blocks in the edge-collection. When a block is deleted, BPI's eraseBlock method is called to clear the edge-collection of references to that block, to avoid dangling pointers.

However, when move-constructing or assigning a BPI object, the edge-collection gets moved, but the value-handles are discarded. This can lead to to stale entries in the edge-collection when blocks are deleted without the callback -- not normally a problem, but if a new block is allocated with the same address as an old block, spurious branch probabilities will be recorded about it. The fix is to transfer the handles from the source BPI object.

This was exposed by an unrelated debug-info change, it probably just shifted around allocation orders to expose this. Detected as nondeterminism and reduced by Zequan Wu:

https://github.com/llvm/llvm-project/commit/f1b0a544514f3d343f32a41de9d6fb0b6cbb6021#commitcomment-136737090

(No test because IMHO testing for a behaviour that varies with memory allocators is likely futile; I can add the reproducer with a CHECK for the relevant branch weights if it's desired though)

>From 6d1b54d4fc657af13b1cd2ddd063ab5c8f38d95c Mon Sep 17 00:00:00 2001
From: Jeremy Morse <jeremy.morse at sony.com>
Date: Thu, 11 Jan 2024 14:13:33 +0000
Subject: [PATCH] [BPI] Transfer value-handles when assign/move constructing
 BPI

Background: BPI stores a collection of edge branch-probabilities, and also
a set of Callback value-handles for the blocks in the edge-collection. When
a block is deleted, BPI's eraseBlock method is called to clear the
edge-collection of references to that block, to avoid dangling pointers.

However, when move-constructing or assigning a BPI object, the
edge-collection gets moved, but the value-handles are discarded. This can
lead to to stale entries in the edge-collection when blocks are deleted
without the callback -- not normally a problem, but if a new block is
allocated with the same address as an old block, spurious branch
probabilities will be recorded about it. The fix is to transfer the handles
from the source BPI object.

This was exposed by an unrelated debug-info change, it probably just
shifted around allocation orders to expose this. Detected as nondeterminism
and reduced by Zequan Wu:

https://github.com/llvm/llvm-project/commit/f1b0a544514f3d343f32a41de9d6fb0b6cbb6021#commitcomment-136737090
---
 llvm/include/llvm/Analysis/BranchProbabilityInfo.h | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/llvm/include/llvm/Analysis/BranchProbabilityInfo.h b/llvm/include/llvm/Analysis/BranchProbabilityInfo.h
index 6b9d1781820111..91e1872e9bd6ff 100644
--- a/llvm/include/llvm/Analysis/BranchProbabilityInfo.h
+++ b/llvm/include/llvm/Analysis/BranchProbabilityInfo.h
@@ -122,16 +122,23 @@ class BranchProbabilityInfo {
   }
 
   BranchProbabilityInfo(BranchProbabilityInfo &&Arg)
-      : Probs(std::move(Arg.Probs)), LastF(Arg.LastF),
-        EstimatedBlockWeight(std::move(Arg.EstimatedBlockWeight)) {}
+      : Handles(std::move(Arg.Handles)), Probs(std::move(Arg.Probs)),
+        LastF(Arg.LastF),
+        EstimatedBlockWeight(std::move(Arg.EstimatedBlockWeight)) {
+    for (auto &Handle : Handles)
+      Handle.setBPI(this);
+  }
 
   BranchProbabilityInfo(const BranchProbabilityInfo &) = delete;
   BranchProbabilityInfo &operator=(const BranchProbabilityInfo &) = delete;
 
   BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) {
     releaseMemory();
+    Handles = std::move(RHS.Handles);
     Probs = std::move(RHS.Probs);
     EstimatedBlockWeight = std::move(RHS.EstimatedBlockWeight);
+    for (auto &Handle : Handles)
+      Handle.setBPI(this);
     return *this;
   }
 
@@ -279,6 +286,8 @@ class BranchProbabilityInfo {
     }
 
   public:
+    void setBPI(BranchProbabilityInfo *BPI) { this->BPI = BPI; }
+
     BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI = nullptr)
         : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {}
   };