[PATCH] D91183: Added GDB pretty printer for StringMap

[David Blaikie via Phabricator via llvm-commits]

dblaikie added inline comments.


================
Comment at: llvm/utils/gdb-scripts/prettyprinters.py:215
+    entry_ty = gdb.lookup_type('llvm::StringMapEntry<{}>'.format(value_ty.name))
+    tombstone = ((1 << (64 - 3)) - 1) << 3
+
----------------
MoritzS wrote:
> dblaikie wrote:
> > Any chance we could avoid hardcoding the tombstone value here? I guess maybe if the source were modified to store the tombstone value as a constexpr (not sure if it could be constexpr - with the pointer bitfiddling, etc - but maybe) member of StringMapImpl, would that be accessible from the pretty printer reliably? 
> I could call StringMapImpl::getTombstoneVal() from gdb to get it, but that depends on that function to not be optimized out. But this kind of function is pretty likely to be inlined and optimized out even with -O1. Even with a constexpr member I think that would have the same problems.
Yeah, relying on a function call would be problematic for the reason you mentioned & for the fact that then you can't debug a core dump (because you can't execute the function) and otherwise risk corrupting the target process by running code when trying to pretty print.

But I don't think Clang will optimize out a constexpr member & the value could be queried without executing code:
```
$ clang++-tot constexpr.cpp -g -c -O3 && llvm-dwarfdump-tot constexpr.o
constexpr.o:    file format elf64-x86-64

.debug_info contents:
0x00000000: Compile Unit: length = 0x00000052, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at 0x00000056)

0x0000000b: DW_TAG_compile_unit
              DW_AT_producer    ("clang version 12.0.0 (git at github.com:llvm/llvm-project.git 2c196bbc6bd897b3dcc1d87a3baac28e1e88df41)")
              DW_AT_language    (DW_LANG_C_plus_plus_14)
              DW_AT_name        ("constexpr.cpp")
              DW_AT_stmt_list   (0x00000000)
              DW_AT_comp_dir    ("/usr/local/google/home/blaikie/dev/scratch")

0x0000001e:   DW_TAG_variable
                DW_AT_name      ("f")
                DW_AT_type      (0x00000033 "foo")
                DW_AT_external  (true)
                DW_AT_decl_file ("/usr/local/google/home/blaikie/dev/scratch/constexpr.cpp")
                DW_AT_decl_line (4)
                DW_AT_location  (DW_OP_addr 0x0)

0x00000033:   DW_TAG_structure_type
                DW_AT_calling_convention        (DW_CC_pass_by_value)
                DW_AT_name      ("foo")
                DW_AT_byte_size (0x01)
                DW_AT_decl_file ("/usr/local/google/home/blaikie/dev/scratch/constexpr.cpp")
                DW_AT_decl_line (1)

0x0000003c:     DW_TAG_member
                  DW_AT_name    ("i")
                  DW_AT_type    (0x00000049 "const int")
                  DW_AT_decl_file       ("/usr/local/google/home/blaikie/dev/scratch/constexpr.cpp")
                  DW_AT_decl_line       (2)
                  DW_AT_external        (true)
                  DW_AT_declaration     (true)
                  DW_AT_const_value     (7)

0x00000048:     NULL

0x00000049:   DW_TAG_const_type
                DW_AT_type      (0x0000004e "int")

0x0000004e:   DW_TAG_base_type
                DW_AT_name      ("int")
                DW_AT_encoding  (DW_ATE_signed)
                DW_AT_byte_size (0x04)

0x00000055:   NULL
blaikie at blaikie-linux2:~/dev/scratch$ cat constexpr.cpp
struct foo {
  static constexpr int i = 7;
};
foo f;
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D91183/new/

https://reviews.llvm.org/D91183