[llvm-dev] ORC JIT Weekly #14 -- Removable code

[Lang Hames via llvm-dev]

Hi All,

A preliminary version of removable code support has been posted for review
in https://reviews.llvm.org/D79312. This patch removes all uses of
VModuleKeys (except for Legacy layers) and takes a whole-JITDylib-at-a-time
approach to removal. Removing whole JITDylibs requires more work from
clients (compared to per-module removal): Modules to be removed must be
placed into throw-away JITDylibs and re-exports used to make symbol
definitions visible at the intended locations. On the other hand
restricting removal to whole JITDylibs can help to avoid subtle dependence
bugs: existing object formats and linker rules are already designed to make
independently loading and unloading libraries relatively safe, whereas
there is no precedent for unloading individual modules from within a
library at runtime.

As an example of how unloading individual modules can lead to subtle
dependence bugs consider the following REPL for a simple language
(Kaleidoscope from https://llvm.org/docs/tutorial/). In this example the
REPL values will all be of floating point type, functions will be compiled
lazily, and anonymous expressions will be removed immediately after running
them. Under these assumptions, how does the REPL respond to the following
input?

repl> def foo(x) x * 2.0;
repl> foo(2.0) + 1.0;
// output #1 here
// anonymous expression #1 removed here
repl> foo(3.0);
// output #2 here
// anonymous expression #2 removed here

We expect the result to be:

output #1: 5.0
output #2: 6.0

And on Linux and Darwin it will be. On Windows however, the output is
likely* to be:

output #1: 5.0
output #2: segfault

(*The exact output will depend on the target options used).

The problem is that when compiling to COFF (the Windows relocatable object
file format) floating point constants may be stored in named COMDAT entries
(See e.g. https://llvm.org/PR40074). Only one copy of a constant value is
emitted, and other modules will be linked to refer to that copy. In our
example, because of lazy compilation, the first copy of the constant 2.0
that the JIT linker will encounter is the one used in anonymous expression
#1. The body of foo will link against this copy and  be left with a
dangling reference when anonymous expression #1 is deleted. Attempting to
re-run the foo function in anonymous expression #2 will then crash.

The COFF format and linker rules already ensure that dynamic libraries each
get their own copies of their floating point constants, so by following the
existing rules and only allowing per-JITDylib removal this case becomes
safe.

There's plenty more to talk about here, but it's getting late so I'll leave
it here for tonight. Questions and comments on this approach and the
initial patch are very welcome, especially from people who have use-cases
for removing code. Barring any major objections I'm hoping that we can have
this feature in-tree in the next week or so.

-- Lang.
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