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Rust Precise GC Implementation Notes


These are my notes from implementing precise GC for Rust at Mozilla Research during my internship over the summer of 2012. An overview of my work follows in the rest of this file.

Quick Links:


At the beginning of the summer, Rust used reference counting plus a cycle collector to manage the task-local heap. The cycle collector was slow and buggy, so in practice it was never actually used.

The Rust team had considered upgrading to a proper GC for some time. Patrick (pcwalton) had even done some initial work to check the feasibility of GC. Patrick found that LLVM's existing GC support forced all GC'd pointers to be pinned on the stack. Thus, if we wanted a fast, precise GC, we would have to hack LLVM's GC infrastructure to support optimization of GC pointers.

Approaches Considered


While not precise, it is worth noting conservative GC because it is relatively simple and easy to implement, and it requires absolutely no modifications to LLVM, and only minimal support from the Rust compiler. Patrick has worked on a conservative GC for Rust, and it looks promising so far, but whether or not it can perform well is still an unknown.

Explicit LLVM register roots

But if we really wanted to implement a precise GC in Rust, then one way or another we'd need LLVM's cooperation in telling us where GC roots live on the stack. The most obvious way to do that would be to add an LLVM intrinsic to parallel LLVM's existing gcroot intrinsic, but for registers (a gcregroot, in effect).

Unfortunately, LLVM's optimization infrastructure wasn't built with maintaining GC invariants in mind. In addition to teaching LLVM's mem2reg and reg2mem passes how to move pointers between gcroot and gcregroot, we would have to go through all the LLVM optimization passes to make sure none of the GC invariants are violated. Perhaps even worse, different GC algorithms might need to maintain different invariants, and so this would be potentially language-specific. The LLVM team wasn't enthusiastic about this approach, and it would be much more than a summer's work anyway, so we looked for other approaches.

Automatic roots

So rather than explicitly marking roots at the LLVM IR level, we decided to automatically infer roots in an LLVM pass. By positioning this pass after other LLVM IR optimizations, we could free LLVM from having to maintain GC invariants in its optimizations.

The disadvantage was that this would limit what GC algorithms we could make use of. Specifically, LLVM would be free to make copies of pointers and put them anywhere, so we wouldn't necessarily know about all copies of given pointer. So we wouldn't be able to implement any moving GC algorithms with this approach, leaving primarily mark-and-sweep GC algorithms on the table.

Perhaps more troubling was the additional hacking that would be required to lower LLVM's IR into machine code. Since we ran the automatic root pass after optimizations, some GC pointers would be in virtual registers in the IR. And LLVM had no way to represent GC register roots at the machine level.

We decided to try adding a fake LLVM machine instruction to represent a GC register roots. The automatic root pass would add these fake instructions into the IR, and then the machine translation layers would pass these on down to the GC assembly printer which would spit out the stack map into the resulting module. This would require, among other things, changes to the LLVM DAG-based instruction selector, and to each machine-specific backend. It sounded painful, but we hoped it would be feasible.

Beyond GC: Cleanup

A GC, beyond just collecting garbage, also happens to useful for other purposes. We realized that a stack walker with detailed information about what lives on the stack could also potentially serve as a replacement for the C++ exception handler. Since in Rust failure happens at the level of entire tasks, we would need to worry about restoring the stack to a particular state; we could just cleanup anything living on the stack and wipe out the entire task-local heap in one go.

The difficulty would be running destructors on non-pointer types stored on the stack. Take, for example, a file descriptor, stored in a stack-allocated struct with a destructor. Missing the struct would mean leaking the file descriptor. But no pointer to the struct actually lives on the stack. So to combat this we would need our compiler to explicitly emit pointers to these stack-allocated structs, so that the automatic root pass could find those structs on the stack.

Better Static Type Information

Unfortunately, even with explicit pointer roots to stack allocations, LLVM's type information is too low-level to know what destructors to run for each type. E.g. a file descriptor and a refcount would both be represented as an int in LLVM's type system. For heap-allocated objects, this wouldn't matter, because in Rust each heap allocation has a header describing the type that lives within. But for stack allocations, we had no such header.

Since we opted for automatic root insertion at the IR level, we wouldn't have explicit gcroot intrinsics to tell us what Rust type each alloca corresponded too. So we added static type information to LLVM's type system by giving meaning to the address space component of LLVM's pointer type.

LLVM pointers live in an address space (represented by an unsigned integer), which has meaning to various LLVM backends. By default, LLVM only gives meaning to address spaces 0 and 1. Address space 0 corresponds to normal, unmanaged pointers. Address space 1 corresponds to generic GC'd boxes.

We decided to make use of the other LLVM address spaces. Address space 2 and up would refer to stack-allocated types in need of cleanup. The Rust compiler would choose address spaces for its types, and then emit a map from address space to static type information. The LLVM automatic root pass would then look up the address space of the pointer, and insert the static type information into gcroot intrinsic it created to track the root. In this way, we would be able to get detailed type information all the way to the Rust runtime.

Status as of 2012-09-07

GC-based cleanup works, with optimizations off, on a large percentage of test cases in the Rust test suite, in addition to the Rust compiler itself. The vast majority of run-pass tests pass, and most of the run-fail tests pass.

The remaining failing tests mostly correspond to specific types which, for whatever reason, don't get rooted properly by the GC. For example, forms of the fail expression taking a string parameter will allocate a string, but then not store it into an alloca. Thus the GC in its current form is unable to find the string on the stack, leaking memory.

Optimizations don't work because I didn't have time to get around to fixing LLVM's SelectionDAG to work with our fake gcregroot machine instructions. I'm guessing it's a month project for someone who actually knows LLVM's machine layer, or a multi-month project for someone who only knows LLVM's IR layer.

In the meantime, Patrick's conservative GC has made progress. Maybe the way forward will be conservative GC, at least in the near term. I doubt that we'd want to stick with conservative GC forever, but that depends a lot on how much more resistance LLVM gives us.

Testing GC-based Cleanup

To test GC-based cleanup on an individual Rust file, invoke Rust with optimizations off and with --gc -Z no-landing-pads.

rustc --gc -Z no-landing-pads

Running the test suite is currently a little more complicated, because the test runner has a bug preventing itself from being compiled with GC on.

Apply the following patch to turn on GC in individual test cases.

diff --git a/mk/ b/mk/
index 851c5b8..8205ce9 100644
--- a/mk/
+++ b/mk/
@@ -360,7 +360,7 @@ CTEST_COMMON_ARGS$(1)-T-$(2)-H-$(3) :=						\
         --rustc-path $$(HBIN$(1)_H_$(3))/rustc$$(X)			\
         --aux-base $$(S)src/test/auxiliary/                 \
         --stage-id stage$(1)-$(2)							\
-        --rustcflags "$$(CFG_RUSTC_FLAGS) --target=$(2)"	\
+        --rustcflags "$$(CFG_RUSTC_FLAGS) --gc -Z no-landing-pads --target=$(2)"	\
 CFAIL_ARGS$(1)-T-$(2)-H-$(3) :=					\

Then build Rust as follows.

./configure --disable-optimize
make check -k

This will build libcore with GC off, but with GC on in each testcases. Necessarily this will cause some leakage, because libcore itself allocates memory.

To build with GC on in both libcore and each test, use the following invokation instead.

./configure --disable-optimize
RUSTFLAGS='--gc -Z no-landing-pads' make
make check -k

This might crash more, because libcore still has a few issues with GC which tend to pop up more frequently just because of how pervasively libcore is used.

Other Files in This Repository

  • -- A detailed log of everything I tried during the summer. I don't expect it to be especially helpful to anyone, but it might shed light on e.g. what exact configurations I was testing, etc.

  • -- The script I used to run tests with GC enabled in various configurations.

  • sanity-tests/ -- A couple simple tests that should always pass.

  • -- A script I used to summarize results from test runs, and diagnosing failures.

  • summary-2012-09-06.log -- The output of from my last test run, on a 32-bit Ubuntu machine.

  • test-gc.diff -- The same diff as above, in case you like using the patch command. Turns on GC within testcases, without enabling GC for either the test runner, or libcore.

Other Resources

I also gave a talk summarizing my work over the summer. If you've read this far, you probably won't need it, but you might find it interesting.


Notes from implementing precise GC in Rust, summer 2012



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