[PATCH] D91183: Added GDB pretty printer for StringMap

[Moritz Sichert via Phabricator via llvm-commits]

MoritzS 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
+
----------------
dblaikie wrote:
> 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;
> ```
You are right, it also works with gcc! I changed it.


Repository:
  rG LLVM Github Monorepo

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

https://reviews.llvm.org/D91183