[WebAssembly] Fix fixBrTableIndex removing instruction without checking uses

[aardappel]

Summary

Fixes: https://bugs.llvm.org/show_bug.cgi?id=52352

[aardappel]

There might be a more elegant way to test if an instruction still has users for its def/reg, anyone know?

Also I could add a test for this.. this particular situation does not come up in C/C++, but I could de-compile the Rust generated bitcode (see linked bug) to ll and generate a test from that. Not sure how useful that is though, as eraseFromParent was clearly wrong here and this is the code that should have been there in the first place.

[tlively]

llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp

68–71

Looks like we probably want to use MachineRegisterInfo::hasOneNonDBGUser(Register RegNo) here. Hopefully that lets us get rid of the goto ;)

[tlively]

Also, it would be nice to have a test for this, but if it would be too much trouble, then I guess it's fine.

[aardappel]

Simplified using use_nodbg_empty

[aardappel]

llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp

68–71

Thanks! Not sure how I managed to miss those functions. In this case use_nodbg_empty is the one we actually want, since the use by br_table has already been removed.

llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp

void fixBrTableIndex(MachineInstr &MI, MachineBasicBlock *MBB,
 
   assert(MI.getDesc().getOpcode() == WebAssembly::BR_TABLE_I64 &&
          "64-bit br_table pseudo instruction expected");
 
   // Find extension op, if any. It sits in the previous BB before the branch.
   auto ExtMI = MF.getRegInfo().getVRegDef(MI.getOperand(0).getReg());
   if (ExtMI->getOpcode() == WebAssembly::I64_EXTEND_U_I32) {
     // Unnecessarily extending a 32-bit value to 64, remove it.
-    assert(MI.getOperand(0).getReg() == ExtMI->getOperand(0).getReg());
+    auto ExtDefReg = ExtMI->getOperand(0).getReg();
+    assert(MI.getOperand(0).getReg() == ExtDefReg);
     MI.getOperand(0).setReg(ExtMI->getOperand(1).getReg());
+    if (MF.getRegInfo().use_nodbg_empty(ExtDefReg)) {
+      // No more users of extend, delete it.
       ExtMI->eraseFromParent();
+    }
   } else {
     // Incoming 64-bit value that needs to be truncated.
     Register Reg32 =
         MF.getRegInfo().createVirtualRegister(&WebAssembly::I32RegClass);
     BuildMI(*MBB, MI.getIterator(), MI.getDebugLoc(),
             WST.getInstrInfo()->get(WebAssembly::I32_WRAP_I64), Reg32)
         .addReg(MI.getOperand(0).getReg());
     MI.getOperand(0).setReg(Reg32);
   }