[LLVMdev] LLVM IR is a compiler IR
gohman at apple.com
Tue Oct 4 11:53:18 PDT 2011
In this email, I argue that LLVM IR is a poor system for building a
Platform, by which I mean any system where LLVM IR would be a
format in which programs are stored or transmitted for subsequent
use on multiple underlying architectures.
LLVM IR initially seems like it would work well here. I myself was
once attracted to this idea. I was even motivated to put a bunch of
my own personal time into making some of LLVM's optimization passes
more robust in the absence of TargetData a while ago, even with no
specific project in mind. There are several things still missing,
but one could easily imagine that this is just a matter of people
writing some more code.
However, there are several ways in which LLVM IR differs from actual
platforms, both high-level VMs like Java or .NET and actual low-level
ISAs like x86 or ARM.
First, the boundaries of what capabilities LLVM provides are nebulous.
LLVM IR contains:
* Explicitly Target-specific features. These aren't secret;
x86_fp80's reason for being is pretty clear.
* Target-specific ABI code. In order to interoperate with native
C ABIs, LLVM requires front-ends to emit target-specific IR.
Pretty much everyone around here has run into this.
* Implicitly Target-specific features. The most obvious examples of
these are all the different Linkage kinds. These are all basically
just gateways to features in real linkers, and real linkers vary
quite a lot. LLVM has its own IR-level Linker, but it doesn't
do all the stuff that native linkers do.
* Target-specific limitations in seemingly portable features.
How big can the alignment be on an alloca? Or a GlobalVariable?
What's the widest supported integer type? LLVM's various backends
all have different answers to questions like these.
Even ignoring the fact that the quality of the backends in the
LLVM source tree varies widely, the question of "What can LLVM IR do?"
has numerous backend-specific facets. This can be problematic for
producers as well as consumers.
Second, and more fundamentally, LLVM IR is a fundamentally
vague language. It has:
* Undefined Behavior. LLVM is, at its heart, a C compiler, and
Undefined Behavior is one of its cornerstones.
High-level VMs typically raise predictable exceptions when they
encounter program errors. Physical machines typically document
their behavior very extensively. LLVM is fundamentally different
from both: it presents a bunch of rules to follow and then offers
no description of what happens if you break them.
LLVM's optimizers are built on the assumption that the rules
are never broken, so when rules do get broken, the code just
goes off the rails and runs into whatever happens to be in
the way. Sometimes it crashes loudly. Sometimes it silently
corrupts data and keeps running.
There are some tools that can help locate violations of the
rules. Valgrind is a very useful tool. But they can't find
everything. There are even some kinds of undefined behavior that
I've never heard anyone even propose a method of detection for.
* Intentional vagueness. There is a strong preference for defining
LLVM IR semantics intuitively rather than formally. This is quite
practical; formalizing a language is a lot of work, it reduces
future flexibility, and it tends to draw attention to troublesome
edge cases which could otherwise be largely ignored.
I've done work to try to formalize parts of LLVM IR, and the
results have been largely fruitless. I got bogged down in
edge cases that no one is interested in fixing.
* Floating-point arithmetic is not always consistent. Some backends
don't fully implement IEEE-754 arithmetic rules even without
-ffast-math and friends, to get better performance.
If you're familiar with "write once, debug everywhere" in Java,
consider the situation in LLVM IR, which is fundamentally opposed
to even trying to provide that level of consistency. And if you allow
the optimizer to do subtarget-specific optimizations, you increase
the chances that some bit of undefined behavior or vagueness will be
Third, LLVM is a low level system that doesn't represent high-level
abstractions natively. It forces them to be chopped up into lots of
small low-level instructions.
* It makes LLVM's Interpreter really slow. The amount of work
performed by each instruction is relatively small, so the interpreter
has to execute a relatively large number of instructions to do simple
tasks, such as virtual method calls. Languages built for interpretation
do more with fewer instructions, and have lower per-instruction
* Similarly, it makes really-fast JITing hard. LLVM is fast compared
to some other static C compilers, but it's not fast compared to
real JIT compilers. Compiling one LLVM IR level instruction at a
time can be relatively simple, ignoring the weird stuff, but this
approach generates comically bad code. Fixing this requires
recognizing patterns in groups of instructions, and then emitting
code for the patterns. This works, but it's more involved.
* Lowering high-level language features into low-level code locks
in implementation details. This is less severe in native code,
because a compiled blob is limited to a single hardware platform
as well. But a platform which advertizes architecture independence
which still has all the ABI lock-in of HLL implementation details
presents a much more frightening backwards compatibility specter.
* Apple has some LLVM IR transformations for Objective-C, however
the transformations have to reverse-engineer the high-level semantics
out of the lowered code, which is awkward. Further, they're
reasoning about high-level semantics in a way that isn't guaranteed
to be safe by LLVM IR rules alone. It works for the kinds of code
clang generates for Objective C, but it wouldn't necessarily be
correct if run on code produced by other front-ends. LLVM IR
isn't capable of representing the necessary semantics for this
unless we start embedding Objective C into it.
In conclusion, consider the task of writing an independent implementation
of an LLVM IR Platform. The set of capabilities it provides depends on who
you talk to. Semantic details are left to chance. There are features
which require a bunch of complicated infrastructure to implement which
are rarely used. And if you want light-weight execution, you'll
probably need to translate it into something else better suited for it
first. This all doesn't sound very appealing.
LLVM isn't actually a virtual machine. It's widely acknoledged that the
name "LLVM" is a historical artifact which doesn't reliably connote what
LLVM actually grew to be. LLVM IR is a compiler IR.
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