[llvm-dev] ORC JIT Weekly #6 -- General initializer support and JITLink optimizations

Lang Hames via llvm-dev
2020-02-25T00:34:44Z

Hi Bjoern,

> I wonder, will your changes be in the upcoming LLVM10 release?

It’s too late to cherry pick these changes for LLVM 10, but they will be in LLVM
11.

> Also, do your changes allow to get the actual addresses of the constructor
and destructors - or are they fully managed by the ORC? I hope this makes sense!

In general you will want to let ORC take care of running the initializers for
you since the problem is non-trivial, but if you want to get the raw initializer
data it is possible to do so with some legwork.

Background: I simplified the description of what was included in the initializer
patch to keep it short, but there are actually two major parts to the GenericIR
and MachO init schemes that are now available in LLJIT. The first part is the
Platform subclass (see lib/ExecutionEngine/Orc/LLJIT.cpp for the
GenericLLVMIRPlatform class, and
include/llvm/ExecutionEngine/Orc/MachOPlatform.h for the MachOPlatform class).
The second part is the LLJIT::PlatformSupport subclass (both PlatformSupport
subclasses are in lib/ExecutionEngine/Orc/LLJIT.cpp). The Platform subclass
aggregates and exposes the raw initialization data, and the PlatformSupport
subclass hides the handling of that initialization data behind a common
interface for LLJIT — the LLJIT::PlatformSupport::initialize method.

If you want to get the raw data yourself you could write your own
PlatformSupport subclass to do so by following the examples that are available
in-tree. However, note that your PlatformSupport class(es) will be platform
specific, as will be the initializer data. In general you will have to deal with
things other than constructors and destructors, e.g. Objective-C metadata on
Darwin.

Regards,
Lang.

> On Feb 23, 2020, at 11:58 PM, Gaier, Bjoern <Bjoern.Gaier at
horiba.com> wrote:
>
> ﻿
> Hello Lang,
>
> This sounds interesting even tough I’m not able to understand everything.
> I wonder, will your changes be in the upcoming LLVM10 release? Also, do
your changes allow to get the actual addresses of the constructor and
destructors - or are they fully managed by the ORC? I hope this makes sense!
>
> Kind greetings and wishing you a nice vacation
> Björn
>
> From: Lang Hames <lhames at gmail.com>
> Sent: 24 February 2020 08:47
> To: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
> Cc: Stefan Gränitz <stefan.graenitz at gmail.com>; David Blaikie
<dblaikie at gmail.com>; Geoff Levner <glevner at gmail.com>; Jacob
Lifshay <programmerjake at gmail.com>; Christian Schafmeister <meister
at temple.edu>; Andres Freund <andres at anarazel.de>; Gaier, Bjoern
<Bjoern.Gaier at horiba.com>; guangnan he <gnhe2009 at gmail.com>;
preejackie <praveenvelliengiri at gmail.com>
> Subject: ORC JIT Weekly #6 -- General initializer support and JITLink
optimizations
>
> Hi All,
>
> The general initializer support patch has landed (see 85fb997659b plus
follow up fixes).
>
> Some quick background:
>
> Until now ORC, like MCJIT, has handled static initializer discovery by
searching for llvm.global_ctors and llvm.global_dtors arrays in the IR added to
the JIT. This approach suffers from several drawbacks:
> 1) It provides no built-in support for other program representations:
Object files and custom program representations added to the JIT require manual
intervention from the user to run their initializers.
> 2) It requires naming and promoting the linkage of initializer functions,
since they have to be looked up by name to be run.
> 3) It doesn't handle platform-specific initializers, e.g. Objective-C
registration, which are described by globals in specific sections.
>
> The general initializer support patch has changed how initialization is
handled. Now all MaterializationUnits, regardless of what kind of program
representation they wrap (IR, object files, ASTs, etc.) can now declare an
optional "initializer symbol". Instances of the new Platform class
(see include/llvm/ExecutionEngine/Orc/Core.h) are notified whenever
MaterializationUnits are added to a JITDylib, and can record the presence of any
declared initializers. By issuing lookups for initializers, the Platform can
force their materialization and arrange for them to be run in a platform
specific way (See https://reviews.llvm.org/D74300 for more discussion on this).
>
> This new system is flexible enough to permit two very different platform
implementations for LLJIT that are already available in tree:
GenericLLVMIRPlatform and MachOPlatform. The former essentially re-implements
the existing llvm.global_ctor scanning scheme: It promotes functions that appear
in the llvm.global_ctors array, then looks them up by name and executes them
when requested. On the other hand, MachOPlatform implements a scheme that mimics
the behavior of the Darwin dynamic loader, dyld: By installing an
ObjectLinkingLayer::Plugin, the MachOPlatform can scan all objects as they are
materialized to discover known special sections (E.g. __mod_init_func,
__objc_classlist, and __objc_selref), then handle them according the usual
platform rules (__mod_init_func pointers are executed, Objective-C classes and
selectors are registered with the Objective-C runtime).
>
> While this system is still very new, it is far enough along that the lli
command line tool, when run with the -jit-mode=orc-lazy option, can now execute
simple IR compiled from simple Objective C and Swift programs on Darwin.
>
> Also of interest this week: JITLink has a new "GOT and Stub
bypass" optimization for x86-64. When linking position independent code,
JITLink must conservatively build global offset table entries and stubs to
access/call external symbols that may be out of range of the JIT'd code.
With this new optimization, these indirect accesses may be bypassed if the
JIT'd code ends up being allocated within range of the target. Coupled with
a slab allocator for your JIT this optimization can eliminate a layer of
indirection and may improve performance for some use cases. See 27a79b72162.
>
> Just a heads up: I expect next week to be a quiet one, as I'm out on
vacation from Wednesday.
>
> -- Lang.
>
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