[all-commits] [llvm/llvm-project] 3707c5: Make ValueObject::Cast work for casts from smaller...

[jimingham via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3707c540d23a5684a1c37b0f7e41c1d8ed7f1f8a
      https://github.com/llvm/llvm-project/commit/3707c540d23a5684a1c37b0f7e41c1d8ed7f1f8a
  Author: jimingham <jingham at apple.com>
  Date:   2024-03-11 (Mon, 11 Mar 2024)

  Changed paths:
    M lldb/source/Core/ValueObject.cpp
    M lldb/test/API/python_api/value/TestValueAPI.py
    M lldb/test/API/python_api/value/main.c

  Log Message:
  -----------
  Make ValueObject::Cast work for casts from smaller to larger structs in the cases where this currently can work. (#84588)

The ValueObjectConstResult classes that back expression result variables
play a complicated game with where the data for their values is stored.
They try to make it appear as though they are still tied to the memory
in the target into which their value was written when the expression is
run, but they also keep a copy in the Host which they use after the
value is made (expression results are "history values" so that's how we
make sure they have "the value at the time of the expression".)

However, that means that if you ask them to cast themselves to a value
bigger than their original size, they don't have a way to get more
memory for that purpose. The same thing is true of ValueObjects backed
by DataExtractors, the data extractors don't know how to get more data
than they were made with in general.

The only place where we actually ask ValueObjects to sample outside
their captured bounds is when you do ValueObject::Cast from one
structure type to a bigger structure type. In
https://reviews.llvm.org/D153657 I handled this by just disallowing
casts from one structure value to a larger one. My reasoning at the time
was that the use case for this was to support discriminator based C
inheritance schemes, and you can't directly cast values in C, only
pointers, so this was not a natural way to handle those types. It seemed
logical that since you would have had to start with pointers in the
implementation, that's how you would write your lldb introspection code
as well.

Famous last words...

Turns out there are some heavy users of the SB API's who were relying on
this working, and this is a behavior change, so this patch makes this
work in the cases where it used to work before, while still disallowing
the cases we don't know how to support.

Note that if you had done this Cast operation before with either
expression results or value objects from data extractors, lldb would not
have returned the correct results, so the cases this patch outlaws are
ones that actually produce invalid results. So nobody should be using
Cast in these cases, or if they were, this patch will point out the bug
they hadn't yet noticed.



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