[WebAssembly] Use SetVector instead of SmallPtrSet in FixBrTableDefaults

[dschuff]

This pass inserts all the MBBs into a set and then iterates over them. But when
the number of elements gets large enough, SmallPtrSet expands into a hash table
which wouldn't have a deterministic iteration order since the elements are
pointers. This results in nondeterministic jump table layouts.
Use SetVector instead for a deterministic iteration order.

[llvmbot]

@llvm/pr-subscribers-backend-webassembly

Author: Derek Schuff (dschuff)

Changes

This pass inserts all the MBBs into a set and then iterates over them. But when
the number of elements gets large enough, SmallPtrSet expands into a hash table
which wouldn't have a deterministic iteration order since the elements are
pointers. This results in nondeterministic jump table layouts.
Use SetVector instead for a deterministic iteration order.

---

Full diff: https://github.com/llvm/llvm-project/pull/84418.diff

1 Files Affected:

• (modified) llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp (+5-3)

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp
index 495f19a7ccde55..4252fc1f55fc9f 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp
@@ -159,19 +159,21 @@ bool WebAssemblyFixBrTableDefaults::runOnMachineFunction(MachineFunction &MF) {
                     << MF.getName() << '\n');
 
   bool Changed = false;
-  SmallPtrSet<MachineBasicBlock *, 16> MBBSet;
+  SetVector<MachineBasicBlock *, SmallVector<MachineBasicBlock *, 16>,
+            DenseSet<MachineBasicBlock *>, 16>
+      MBBSet;
   for (auto &MBB : MF)
     MBBSet.insert(&MBB);
 
   while (!MBBSet.empty()) {
     MachineBasicBlock *MBB = *MBBSet.begin();
-    MBBSet.erase(MBB);
+    MBBSet.remove(MBB);
     for (auto &MI : *MBB) {
       if (WebAssembly::isBrTable(MI.getOpcode())) {
         fixBrTableIndex(MI, MBB, MF);
         auto *Fixed = fixBrTableDefault(MI, MBB, MF);
         if (Fixed != nullptr) {
-          MBBSet.erase(Fixed);
+          MBBSet.remove(Fixed);
           Changed = true;
         }
         break;