[BOLT] Eliminate dead jump tables

[aaupov]

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: ninja check-bolt

[llvmbot]

@llvm/pr-subscribers-bolt

Author: Amir Ayupov (aaupov)

Changes

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 --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;

[rafaelauler]

Looks like this is dangerous and crashes binaries in our tests? Do you still want to move forward with this stack? Or are you working on it? If this stack is work in progress, maybe mark it as "changes planned" or the equivalent in github.

[aaupov]

Looks like this is dangerous and crashes binaries in our tests? Do you still want to move forward with this stack? Or are you working on it? If this stack is work in progress, maybe mark it as "changes planned" or the equivalent in github.

Yes, we'll need to discuss how to move forward with it, as it's unsafe as is. Marking as draft for now.