[llvm-branch-commits] [BOLT] Eliminate dead jump tables (PR #91666)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-bolt

Author: Amir Ayupov (aaupov)

<details>
<summary>Changes</summary>

Dead jump tables such as those arising from FIXED_PIC_BRANCH
optimization don't need to be updated or moved. Further, if any jump
table entry points to a block removed by unreachable code elimination,
we would be unable to update it and jump table emission would fail.

Identify non-referenced jump tables and delete them to prevent that.

Test Plan: NFC for existing tests.


---
Full diff: https://github.com/llvm/llvm-project/pull/91666.diff


1 Files Affected:

- (modified) bolt/lib/Core/BinaryFunction.cpp (+20) 


``````````diff
diff --git a/bolt/lib/Core/BinaryFunction.cpp b/bolt/lib/Core/BinaryFunction.cpp
index 11103f7bdce8..f74ecea8ac0a 100644
--- a/bolt/lib/Core/BinaryFunction.cpp
+++ b/bolt/lib/Core/BinaryFunction.cpp
@@ -1697,6 +1697,26 @@ void BinaryFunction::postProcessEntryPoints() {
 }
 
 void BinaryFunction::postProcessJumpTables() {
+  // Set of JTs accessed from this function.
+  std::unordered_set<uint64_t> LiveJTs;
+  for (auto &JTSite : JTSites)
+    LiveJTs.emplace(JTSite.second);
+
+  // Remove dead jump tables (reference removed as a result of
+  // POSSIBLE_PIC_FIXED_BRANCH optimization).
+  for (auto JTI = JumpTables.begin(), JTE = JumpTables.end(); JTI != JTE; ) {
+    const uint64_t Address = JTI->first;
+    JumpTable *JT = JTI->second;
+    bool HasOneParent = JT->Parents.size() == 1;
+    if (LiveJTs.count(Address) == 0 && HasOneParent) {
+      BC.deregisterJumpTable(Address);
+      delete JT;
+      JTI = JumpTables.erase(JTI);
+      continue;
+    }
+    ++JTI;
+  }
+
   // Create labels for all entries.
   for (auto &JTI : JumpTables) {
     JumpTable &JT = *JTI.second;

``````````

</details>


https://github.com/llvm/llvm-project/pull/91666