LLVM - What does '!NodePtr->isKnownSentinel(), function operator*' means?

问题

When I execute following IR:

declare void @_puts(i32, ...)

define void @main() {
entry:
  %name = alloca i32
  br i1 true, label %then, label %else

then:                                             ; preds = %entry
  call void (i32, ...) @_puts(i32 1, i32 1234)
  br label %end

else:                                             ; preds = %entry
  br label %end

end:                                              ; preds = %else, %then
  %if_val = phi i32 [ 1234, %then ], [ 0, %else ]

entry1:                                           ; No predecessors!
  store i32 %if_val, i32* %name
  %name2 = load i32, i32* %name
  call void (i32, ...) @_puts(i32 1, i32 %name2)
  ret void
}

I got following error message:

Assertion failed: (!NodePtr->isKnownSentinel()), function operator*, file /Users/Mac/llvm-source/llvm/include/llvm/ADT/ilist_iterator.h, line 139.

Abort trap: 6

What does this message means?

Can anyone explain this for me?

Thanks a lot.

回答1:

The message refers to a sentinel node of a simple_ilist, which is a data structure used for representing lists of functions in a module, basic blocks in a function, instructions in a basic blocks, and so on. A sentinel node represents the end of the list and is the only data member of such lists — the rest is inside the objects that constitute a list ("i" is for "intrusive").

I would guess that this message is caused by iterating over the end of a simple_ilist. Most likely the one holding the instructions in the end block, because this is the only block that is malformed. You can fix it by adding a terminator:

end:                                              ; preds = %else, %then
  %if_val = phi i32 [ 1234, %then ], [ 0, %else ]
  br label %entry1

来源：https://stackoverflow.com/questions/43255774/llvm-what-does-nodeptr-isknownsentinel-function-operator-means

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