[PATCH] D83468: [Debuginfo] Fix for PR46653

[Jaydeep Chauhan via Phabricator via llvm-commits]

Jac1494 added a comment.

>> Also possible it's not a bug at all & GlobalISel just ends up doing some optimization that does end up creating a necessary/correct line 0 at the start of the function where SelectionDAG/FastISel does not.

@dblaikie  SelectionDAG/FastISel is also creating necessary/correct line zero. Also this line zero is user can control using "-use-unknown-locations=Disable" option as per below code.But this options is not fully working for Global-isel because of incorrect line zeros.

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp:1964

  if (!DL) {
     // We have an unspecified location, which might want to be line 0.
     // If we have already emitted a line-0 record, don't repeat it.
     if (LastAsmLine == 0)
       return;
     // If user said Don't Do That, don't do that.
     if (UnknownLocations == Disable)
       return;
     // See if we have a reason to emit a line-0 record now.
     // Reasons to emit a line-0 record include:
     // - User asked for it (UnknownLocations).
     // - Instruction has a label, so it's referenced from somewhere else,
     //   possibly debug information; we want it to have a source location.
     // - Instruction is at the top of a block; we don't want to inherit the
     //   location from the physically previous (maybe unrelated) block.
     if (UnknownLocations == Enable || PrevLabel ||
         (PrevInstBB && PrevInstBB != MI->getParent())) {
       // Preserve the file and column numbers, if we can, to save space in
       // the encoded line table.
       // Do not update PrevInstLoc, it remembers the last non-0 line.
       const MDNode *Scope = nullptr;
       unsigned Column = 0;
       if (PrevInstLoc) {
         Scope = PrevInstLoc.getScope();
         Column = PrevInstLoc.getCol();
       }
       recordSourceLine(/*Line=*/0, Column, Scope, /*Flags=*/0);
     }
     return;
   }

List of SelectionDAG/FastIsel base test cases are given below which creating meaningful/correct line zero.

  CodeGen/X86/dbg-line-0-no-discriminator.ll
  CodeGen/X86/stack-protector.ll
  CodeGen/X86/unknown-location.ll
  DebugInfo/AArch64/line-header.ll
  DebugInfo/MIR/X86/debug-loc-0.mir
  DebugInfo/X86/dbg-prolog-end.ll
  DebugInfo/X86/dwarf-no-source-loc.ll
  DebugInfo/X86/tail-merge.ll



>> Best to look at the assembly/optimizations/etc and see why it's getting line zero and decide whether it's dropping a usable location, or doing the right thing with line 0.

Because of below code Global-isel is creating line zero and This is still looks incorrect to me,Because it is not solving jump behavior but it is creating jump behavior by adding line zero.

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp:2183

  // We only emit constants into the entry block from here. To prevent jumpy
   // debug behaviour set the line to 0.
   if (const DebugLoc &DL = Inst.getDebugLoc())
     EntryBuilder->setDebugLoc(
         DebugLoc::get(0, 0, DL.getScope(), DL.getInlinedAt()));
   else
     EntryBuilder->setDebugLoc(DebugLoc());


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  https://reviews.llvm.org/D83468/new/

https://reviews.llvm.org/D83468