[LLVMdev] [PROPOSAL] Attach debugging information with LLVM instruction

[Devang Patel]

Hi All,

Today, debugging information is encoded in LLVM IR using various
llvm.dbg intrinsics, such as llvm.dbg.stoppoint. For exmaple,

!1 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c", metadata
!"/tmp", metadata !"clang 1.0", i1 true, i1 false, metadata !"", i32
0}

  ...
  call void @llvm.dbg.stoppoint(i32 5, i32 5, metadata !1)
  store i32 42, i32* %i
  call void @llvm.dbg.stoppoint(i32 6, i32 5, metadata !1)
  store i32 1, i32* %j.addr
  br label %if.end
  ...


This approach has several disadvantages.
- The llvm.dbg.stoppoint()s act like hurdles to the optimizer. The
LLVM customers expect that the optimizer does not trip over these
hurdles. They expect LLVM to produce same high quality code
irrespective of the presence of debug info. It is a tedious and never
ending task to ensure that the optimizer safely ignores these llvm.dbg
intrinsics.
- The instructions lose original location info when the optimizer
moves them around.
- It is extremely error prone to keep track of lexical scopes and
inlined functions using a pair of llvm.dbg intrinsics.


The proposed solution is to optionally attach debug information with
llvm instruction directly. A new keyword 'dbg' is used to identify
debugging information associated with an instruction. The debugging
information, if available, is printed after the last instruction
operand. The debugging information entry uses MDNode and it is not
counted as an instruction operand. For example,

!1 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c", metadata
!"/tmp", metadata !"clang 1.0", i1 true, i1 false, metadata !"", i32
0}
!7 = metadata !{i32 5, i32 5, metadata !1, metadata !1}
!8 = metadata !{i32 6, i32 5, metadata !1, metadata !1}

  ...
  store i32 42, i32* %i, dbg metadata !7
  store i32 1, i32* %j.addr, dbg metadata !8
  br label %if.end, dbg metadata !8
  ...


Now, the optimizer does not need to worry about those llvm.dbg
hurdles. Instructions do not lose their location information when they
are rearranged in instruction stream. And the stage is set to produce,
preserve and emit accurate debug information for inlined functions.

Any thoughts/suggestions/questions ?

-
Devang