[AsmPrinter] fix nullptr dereference for MBBs with hasAddressTaken property without BB

[ivafanas]

Summary

Basic block pointer is dereferenced unconditionally for MBBs with hasAddressTaken property.

MBBs might have hasAddressTaken property without reference to BB. Backend developers must assign fake BB to MBB to workaround this issue and it should be fixed.

[dblaikie]

Could you add a test case for this? Or is it not reachable with any existing/in-tree target?

[ivafanas]

In D108092#2945714, @dblaikie wrote:

Could you add a test case for this? Or is it not reachable with any existing/in-tree target?

Yes, we have got a crash on custom out-of-tree target.
I've reviewed in-tree targets where setHasAddressTaken is called and have not found MBB without BB.

Summary message to this review http://reviews.llvm.org/D13774 gives me a bit of hope that fix will be accepted.

[dblaikie]

In D108092#2945743, @ivafanas wrote:

In D108092#2945714, @dblaikie wrote:

Could you add a test case for this? Or is it not reachable with any existing/in-tree target?

Yes, we have got a crash on custom out-of-tree target.
I've reviewed in-tree targets where setHasAddressTaken is called and have not found MBB without BB.

Summary message to this review http://reviews.llvm.org/D13774 gives me a bit of hope that fix will be accepted.

Fair enough - I'll leave this one to @rnk then

[rnk]

lgtm

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

3287

@dblaikie, for reference, other code checks for BB nullability, they are optional.

[ivafanas]

@rnk , @dblaikie

I do not have commit access.
Could you please help me to commit this patch ?

Thank you!

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::emitBasicBlockStart(const MachineBasicBlock &MBB) {
   // the references were generated.
   if (MBB.hasAddressTaken()) {
     const BasicBlock *BB = MBB.getBasicBlock();
     if (isVerbose())
       OutStreamer->AddComment("Block address taken");
 
     // MBBs can have their address taken as part of CodeGen without having
     // their corresponding BB's address taken in IR
-    if (BB->hasAddressTaken())
+    if (BB && BB->hasAddressTaken())
       for (MCSymbol *Sym : MMI->getAddrLabelSymbolToEmit(BB))
         OutStreamer->emitLabel(Sym);
   }
 
   // Print some verbose block comments.
   if (isVerbose()) {
     if (const BasicBlock *BB = MBB.getBasicBlock()) {
       if (BB->hasName()) {
         BB->printAsOperand(OutStreamer->GetCommentOS(),
                            /*PrintType=*/false, BB->getModule());
         OutStreamer->GetCommentOS() << '\n';
       }
     }
 
     assert(MLI != nullptr && "MachineLoopInfo should has been computed");