llvm dev - Apr 2013 - [LLVMdev] Inconsistent use of is_stmt flag in .debug_line

Hello,

A recent series of commits, ending with r169304 and relating to PR13303, add
is_stmt to line entries for functions. This appears to be to work around
problems with gdb.

However, I observe is_stmt is not always applied to line entries for functions.
This may only affect the arm backend. Compiling the same code with the aarch64
backend does not demonstrate this problem.

It seems that DwarfDebug::beginFunction always generates a line table entry with
is_stmt for each function. Later, in DwarfDebug::beginInstruction, we create
more line table entries. If we have an instruction in a function which:

1) Is associated with the line number of the beginning of the function
2) Comes before the end of the prologue

then we overwrite the earlier line table entry with a similar entry which lacks
the is_stmt flag. The relevant code is:

unsigned Flags = 0;
PrevInstLoc = DL;
if (DL == PrologEndLoc) {
  Flags |= DWARF2_FLAG_PROLOGUE_END;
  PrologEndLoc = DebugLoc();
}
if (PrologEndLoc.isUnknown())
  Flags |= DWARF2_FLAG_IS_STMT;

  if (!DL.isUnknown()) {
    const MDNode *Scope =
DL.getScope(Asm->MF->getFunction()->getContext());
    recordSourceLine(DL.getLine(), DL.getCol(), Scope, Flags);

It's easy to change this code to apply is_stmt for every line table entry
but I think that this is the wrong approach. The real question is, why do we
have an inconsistency where some function prologues contain instructions that
can be mapped to the source line where the function begins, while others do not?

Could anyone point me the right way? As I mentioned, the problem may lie in the
arm backend.

Thanks in advance,

Bernie

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