[llvm-dev] GDB pretty printers for LLVM ADTs

[David Blaikie via llvm-dev]

In r271357 I've committed GDB pretty printer script for the following types

   - ArrayRef
   $1 = llvm::ArrayRef of length 3 = {1, 0, 7}
   - StringRef
   $2 = "foo\000bar"
   $3 = "fo"
   - SmallString
   $4 = "foo\000bar"
   - *SmallVector(Impl)*
   $5 = llvm::SmallVector of length 3, capacity 3 = {1, 0, 7}

All of these visualizers are pretty simple, robust, should work when
debugging a core dump (ie: non-live/not executing any code in the debug
process).

Note that the StringRef visualizer now properly terminates the string at
the specified length - so it can both include null characters, or terminate
prematurely before them. Same with StringPiece.

Syntax/styling for ArrayRef and SmallVector are based on the print styling
of std::vector. If people feel like the smallness should be included in any
of the Small* types, we could include that. And if the element type of
sequences is important, we can do that too - but since std::vector doesn't
include the element type, I figured that was a good thing to go off and as
we improve the pretty printers for more types the elements should be self
explanatory.

Patches and feedback welcome to add more visualizers, improve existing
ones, etc.

*How do you access/use these pretty printers?*

Currently, the simplest thing to do is:

  source /path/to/your/llvm/src/utils/gdb-scripts/prettyprinters.py

You can put this in your ~/.gdbinit if you want to make it more convenient.


*Possible Usability Improvements*
Just sourcing in .gdbinit doesn't scale if you have multiple LLVM source
trees or debug builds that might be out of sync with one another (the ABI
of the ADTs probably doesn't change all that often that this would be very
annoying - but Clang's ASTs, LLVM IR nodes, etc might change often enough
that this approach would come up for users in this situation). Also having
to go in and another line whenever we add other scripts - Clang pretty
printers, LLD pretty printers, etc, would be unfortunate.

.debug_gdb_scripts
<https://sourceware.org/gdb/onlinedocs/gdb/dotdebug_005fgdb_005fscripts-section.html#dotdebug_005fgdb_005fscripts-section>
 provides a way to load these scripts on demand, which would ensure that we
didn't need to update each of our local .gdbinit files every time a new set
of pretty printers (probably on a per project basis - one for clang, one
for LLVM, one for LLD, etc) was added.

Though it does still require a .gdbinit change (or manual command) to
adjust the auto-load path to allow the scripts to be automatically loaded.

Depending on how it's used, it doesn't entirely fix the "what if I have
multiple different LLVM things I'm debugging with incompatible pretty
printers". Using just the script name version doesn't help because the
paths the scripts are loaded from wouldn't distinguish between which source
tree.

But if we use the inline script version - the right version is embedded in
the binary & available every time. This gets a bit ugly - writing Python
inside inline asm isn't really comfortable (even with C++ raw string
literals to get past one level of escaping), but we could use incbin
assembler directive to include a Python source file straight into the
inline asm in the right place. Only problem: Clang's incbin support is buggy
<http://llvm.org/PR24811> in a way that makes this not work (disabling the
integrated assembler does demonstrate this working successfully).

Appendix of debug_gdb_scripts examples, so I don't have to keep these
somewhere else for reference. Obviously these aren't terribly portable, but
on platforms where they don't work (so long as we #define them away) users
are no worse off there - people can still source the python script directly
as described above:

1) basic usage:

asm(R"(.pushsection ".debug_gdb_scripts", "MS", at progbits,1
    .byte 1
    .asciz "utils/gdb-scripts/support.py"
    .popsection
    )");
2) annoying inline script version:

    .byte 4
    .ascii "llvmSupport.inlined-script\n"
.ascii "import gdb.printing\n"
.ascii "class SmallStringPrinter:\n"
.ascii "  \"\"\"Print an llvm::SmallString object.\"\"\"\n"
.ascii "\n"
.ascii "  def __init__(self, val):\n"
.ascii "    self.val = val\n"
.ascii "\n"
.ascii "  def to_string(self):\n"
.ascii "    begin = self.val['BeginX']\n"
.ascii "    end = self.val['EndX']\n"
.ascii "    return
begin.cast(gdb.lookup_type(\"char\").pointer()).string(length = end -
begin)\n"
.ascii "\n"
.ascii "  def display_hint (self):\n"
.ascii "    return 'string'\n"
...

.ascii "pp = gdb.printing.RegexpCollectionPrettyPrinter(\"LLVMSupport\")\n"
.ascii "pp.add_printer('llvm::SmallString', '^llvm::SmallString<.*>$',
SmallStringPrinter)\n"
...
.ascii "gdb.printing.register_pretty_printer(gdb.current_objfile(), pp)\n"
    .byte 0
    .popsection
    )");

2) incbin inline script version:

asm(R"(.pushsection ".debug_gdb_scripts", "MS", at progbits,1
    .byte 4
    .ascii "llvmSupport.inlined-script\n"
    .incbin "gdb-support.py"
    .byte 0
    .popsection
    )");
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