[WebAssembly] Add NullifyDebugValueLists pass

`WebAssemblyDebugValueManager` does not currently handle
`DBG_VALUE_LIST`, which is a recent addition to LLVM. We tried to
nullify them within the constructor of `WebAssemblyDebugValueManager` in
D102589, but it made the class error-prone to use because it deletes
instructions within the constructor and thus invalidates existing
iterators within the BB, so the user of the class should take special
care not to use invalidated iterators. This actually caused a bug in
ExplicitLocals pass.

Instead of trying to fix ExplicitLocals pass to make the iterator usage
correct, which is possible but error-prone, this adds
NullifyDebugValueLists pass that nullifies all `DBG_VALUE_LIST`
instructions before we run WebAssembly specific passes in the backend.
We can remove this pass after we implement handlers for
`DBG_VALUE_LIST`s in `WebAssemblyDebugValueManager` and elsewhere.

Fixes https://github.com/emscripten-core/emscripten/issues/14255.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D102999

1 files changed, 5 insertions, 5 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
index 9bea203..7ed224d 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
@@ -305,11 +305,12 @@ bool WebAssemblyExplicitLocals::runOnMachineFunction(MachineFunction &MF) {
         if (!MFI.isVRegStackified(OldReg)) {
           const TargetRegisterClass *RC = MRI.getRegClass(OldReg);
           Register NewReg = MRI.createVirtualRegister(RC);
+          auto InsertPt = std::next(MI.getIterator());
           if (UseEmpty[Register::virtReg2Index(OldReg)]) {
             unsigned Opc = getDropOpcode(RC);
-            MachineInstr *Drop = BuildMI(MBB, std::next(MI.getIterator()),
-                                         MI.getDebugLoc(), TII->get(Opc))
-                                     .addReg(NewReg);
+            MachineInstr *Drop =
+                BuildMI(MBB, InsertPt, MI.getDebugLoc(), TII->get(Opc))
+                    .addReg(NewReg);
             // After the drop instruction, this reg operand will not be used
             Drop->getOperand(0).setIsKill();
             if (MFI.isFrameBaseVirtual() && OldReg == MFI.getFrameBaseVreg())
@@ -320,8 +321,7 @@ bool WebAssemblyExplicitLocals::runOnMachineFunction(MachineFunction &MF) {
 
             WebAssemblyDebugValueManager(&MI).replaceWithLocal(LocalId);
 
-            BuildMI(MBB, std::next(MI.getIterator()), MI.getDebugLoc(),
-                    TII->get(Opc))
+            BuildMI(MBB, InsertPt, MI.getDebugLoc(), TII->get(Opc))
                 .addImm(LocalId)
                 .addReg(NewReg);
           }