llvm::DominatorTreeBase::recalculate called on empty function with if-converter

[hiraditya]

Repro

define i32 @sext_known_nonzero(i16 %xx) {
  %x = shl i16 256, %xx
  %z = sext i16 %x to i32
  %r = call i32 @llvm.cttz.i32(i32 %z, i1 false)
  ret i32 %r
}

$ llc -start-before=if-converter test.ll

:DomTreeBuilder::SemiNCAInfo<llvm::DominatorTreeBase<llvm::MachineBasicBlock, false> >::InfoRec; <template-parameter-1-2> = void; reference = llvm::DomTreeBuilder::SemiNCAInfo<llvm::DominatorTreeBase<llvm::MachineBasicBlock, false> >::InfoRec&; size_type = long unsigned int]: Assertion `idx < size()' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0.	Program arguments: ./bin/llc a.ll -start-before=if-converter -print-after-all
1.	Running pass 'Function Pass Manager' on module 'a.ll'.
2.	Running pass 'X86 Assembly Printer' on function '@sext_known_nonzero'
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  llc       0x000055784892fdf6 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 59
1  llc       0x00005578489302ba
2  llc       0x000055784892d892 llvm::sys::RunSignalHandlers() + 131
3  llc       0x000055784892f75f
4  libc.so.6 0x00007f9077449df0
5  libc.so.6 0x00007f907749e95c
6  libc.so.6 0x00007f9077449cc2 gsignal + 18
7  libc.so.6 0x00007f90774324ac abort + 34
8  libc.so.6 0x00007f9077432420 __assert_perror_fail + 0
9  llc       0x000055784729b6f0
10 llc       0x0000557847297e83
11 llc       0x00005578472999a8
12 llc       0x00005578472953be
13 llc       0x00005578472918db
14 llc       0x000055784728fc3c void llvm::DomTreeBuilder::Calculate<llvm::DominatorTreeBase<llvm::MachineBasicBlock, false>>(llvm::DominatorTreeBase<llvm::MachineBasicBlock, false>&) + 29
15 llc       0x000055784728f922 llvm::DominatorTreeBase<llvm::MachineBasicBlock, false>::recalculate(llvm::MachineFunction&) + 52
16 llc       0x0000557846e34e68 llvm::AsmPrinter::emitFunctionBody() + 292
17 llc       0x0000557845b5c481
18 llc       0x00005578472cdbbb llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 1117
19 llc       0x0000557847b5a06b llvm::FPPassManager::runOnFunction(llvm::Function&) + 639
20 llc       0x0000557847b5a2cd llvm::FPPassManager::runOnModule(llvm::Module&) + 97
21 llc       0x0000557847b5a6d8
22 llc       0x0000557847b55f22 llvm::legacy::PassManagerImpl::run(llvm::Module&) + 244
23 llc       0x0000557847b5af03 llvm::legacy::PassManager::run(llvm::Module&) + 39

[hiraditya]

Adding assert on graph trait methods

diff --git a/llvm/include/llvm/CodeGen/MachineFunction.h b/llvm/include/llvm/CodeGen/MachineFunction.h
index ef783f276b7d..cc924e79f66d 100644
--- a/llvm/include/llvm/CodeGen/MachineFunction.h
+++ b/llvm/include/llvm/CodeGen/MachineFunction.h
@@ -1492,7 +1492,10 @@ public:
 //
 template <> struct GraphTraits<MachineFunction*> :
   public GraphTraits<MachineBasicBlock*> {
-  static NodeRef getEntryNode(MachineFunction *F) { return &F->front(); }
+  static NodeRef getEntryNode(MachineFunction *F) {
+    assert(!F->empty() && "No entry node for an empty function");
+    return &F->front();
+       }
 
   // nodes_iterator/begin/end - Allow iteration over all nodes in the graph
   using nodes_iterator = pointer_iterator<MachineFunction::iterator>;
@@ -1516,7 +1519,10 @@ template <> struct GraphTraits<MachineFunction*> :
 };
 template <> struct GraphTraits<const MachineFunction*> :
   public GraphTraits<const MachineBasicBlock*> {
-  static NodeRef getEntryNode(const MachineFunction *F) { return &F->front(); }
+  static NodeRef getEntryNode(const MachineFunction *F) {
+    assert(!F->empty() && "No entry node for an empty function");
+    return &F->front();
+  }
 
   // nodes_iterator/begin/end - Allow iteration over all nodes in the graph
   using nodes_iterator = pointer_iterator<MachineFunction::const_iterator>;

And running the same test will assert here

MachineFunction*): Assertion `!F->empty() && "No entry node for an empty function"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0.	Program arguments: ./bin/llc a.ll -start-before=if-converter -print-after-all
1.	Running pass 'Function Pass Manager' on module 'a.ll'.
2.	Running pass 'X86 Assembly Printer' on function '@sext_known_nonzero'
...
#8 0x00007f576e632420 __assert_perror_fail ./assert/assert-perr.c:31:1
 #9 0x00005608b4dce6c1 llvm::GraphTraits<llvm::MachineFunction*>::getEntryNode(llvm::MachineFunction*) /llvm-project/llvm/include/llvm/CodeGen/MachineFunction.h:1497:21
#10 0x00005608b63c8ce4 llvm::DomTreeBuilder::SemiNCAInfo<llvm::DominatorTreeBase<llvm::MachineBasicBlock, false>>::GetEntryNode(llvm::DominatorTreeBase<llvm::MachineBasicBlock, false> const&) /llvm-project/llvm/include/llvm/Support/GenericDomTreeConstruction.h:347:3

[hiraditya]

I'm curious if we should add this assertion to detect this bug in general?