[PATCH] Debug info: Support fragmented variables.

[Adrian Prantl]

aprantl added you to the CC list for the revision "Debug info: Support fragmented variables.".

Hi #llvm,

Reposting in Phabricator as requested.

Currently, LLVM cannot represent locations for aggregate variables that
are fragmented across multiple Values. This situation arises, e.g.,
during SROA, or even as part of the x86_64 calling convention when a struct
is passed by value. This patch adds the functionality to emit
DWARF location expressions using DW_OP(_bit)_piece to express partial
values.

This is implemented by adding a new operation type OpPiece to complex
DIVariables which accepts an offset and a size.

The canonical example would be something like this:

  typedef struct { long int a; int b;} S;

  int foo(S s) {
       return s.b;
  }

which at -O1 is now codegen’d into:

  ; Function Attrs: nounwind readnone ssp uwtable
  define i32 @foo(i64 %s.coerce0, i32 %s.coerce1) #0 {
  entry:
   tail call void @llvm.dbg.value(metadata !{i64 %s.coerce0}, i64 0, metadata !20)
   tail call void @llvm.dbg.value(metadata !{i32 %s.coerce1}, i64 0, metadata !21)
   ret i32 %s.coerce1, !dbg !24
  }


with this patch we'd emit the following DWARF:

  0x00000047:         TAG_formal_parameter [3]  
                    AT_name( "s" )
                    AT_decl_file( "struct.c" )
                    AT_decl_line( 3 )
                    AT_type( {0x00000069} ( S ) )
                    AT_location( 0x00000000
                       0x0000000000000000 - 0x0000000000000008: rdi, piece 0x00000008
                       0x0000000000000000 - 0x0000000000000006: rsi, bit-piece 32 64 
                       0x0000000000000006 - 0x0000000000000008: rax, bit-piece 32 64  )




cheers,
adrian

http://llvm-reviews.chandlerc.com/D2680

Files:
  include/llvm/DIBuilder.h
  include/llvm/DebugInfo.h
  lib/CodeGen/AsmPrinter/DwarfDebug.cpp
  lib/IR/DIBuilder.cpp
  lib/IR/DebugInfo.cpp
  lib/Transforms/Scalar/SROA.cpp
  test/DebugInfo/pieces.ll
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