[PATCH] D83560: [DebugInfo] Added support for DW_OP_implicit_value in llvm

[Sourabh Singh Tomar via Phabricator via llvm-commits]

SouraVX created this revision.
SouraVX added reviewers: aprantl, probinson, dblaikie, jini.susan.george, alok.
SouraVX added projects: LLVM, debug-info.
Herald added subscribers: llvm-commits, hiraditya.

llvm is missing support for DW_OP_implicit_value operation.
DW_OP_implicit_value op is indispensable for cases such as
optimized out long double variables.

For intro refer: DWARFv5 Spec Pg: 40 2.6.1.1.4 Implicit Location Descriptions

Consider the following example:

  int main() {
          long double ld = 3.14;
          printf("dummy\n");
          ld *= ld;
          return 0;
  }

when compiled with tunk `clang` as
`clang test.c -g -O1` produces following location description
of variable `ld`:

  DW_AT_location        (0x00000000:
                       [0x0000000000201691, 0x000000000020169b): DW_OP_constu 0xc8f5c28f5c28f800, DW_OP_stack_value, DW_OP_piece 0x8, DW_OP_constu 0x4000, DW_OP_stack_value, DW_OP_bit_piece 0x10 0x40, DW_OP_stack_value)
                    DW_AT_name    ("ld")

Here one may notice that this representation is incorrect(DWARF4
stack could only hold integers(and only up to the size of address)).
Here the variable size itself is `128` bit.
GDB and LLDB confirms this:

  (gdb) p ld
  $1 = <invalid float value>
  (lldb) frame variable ld
  (long double) ld = <extracting data from value failed>

GCC represents/uses DW_OP_implicit_value in these sort of situations.
Based on the discussion with Jakub Jelinek regarding GCC's motivation
for using this, I concluded that DW_OP_implicit_value is most appropriate
in this case.

Link: https://gcc.gnu.org/pipermail/gcc/2020-July/233057.html

GDB seems happy after this patch:(LLDB doesn't have support
for DW_OP_implicit_value)

  (gdb) p ld
  p ld
  $1 = 3.14000000000000012434


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D83560

Files:
  llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
  llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
  llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
  llvm/test/DebugInfo/X86/DW_OP_implicit_value.ll

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