[LLVMdev] Adding safe-point code generation

Jeffrey Yasskin jyasskin at google.com
Tue Jun 23 12:18:57 PDT 2009

On Tue, Jun 23, 2009 at 4:18 AM, Gordon Henriksen <gordonhenriksen at me.com>wrote:

> Hi Jeffrey,
> On 2009-06-22, at 01:49, Jeffrey Yasskin wrote:
> I need to add thread-switching support to Unladen Swallow's JIT, and LLVM's
> safe point support looks like a good way to get code into all the right
> places. However,
> http://llvm.org/docs/GarbageCollection.html#collector-algos points
> out that there's no way to emit code at safe points yet, and there are
> no loop safe points at all. So I'll be trying to implement them.
> Is there anything I should know before starting?
> Sounds like you've got the right idea.
> One way to do this might be to add a FunctionPass
> to LLVMTargetMachine::addCommonCodeGenPasses()
> alongside createGCLoweringPass(), which would insert user-defined code for
> safe points. Unfortunately, code inserted there would stick around in the IR
> after the machine code was emitted, and if the function were JITted again,
> we'd get duplicate safe points.
> Unfortunately, I don't believe this is workable. It would make this work
> much easier if it were.
> Another way to do it might be to add a MachineFunction pass next
> to createGCMachineCodeAnalysisPass() (or instead of it), which could
> emit appropriate MachineInstructions to implement the safe point. This,
> of course, forces safe points to be written in MachineInstructions instead
> of IR instructions, which isn't ideal.
> I think this is the way to go, though it's also the most involved. My
> primary rationale is that code generation can hack on the CFG, even
> introducing loops where there were none expressed in the IR. It could be
> that I'm being unnecessarily pessimistic on this point, though.
> As a specific example of the code generator hacking on the CFG, take atomic
> operations which expand to loops on architectures which use
> load-reserved/store-conditional to implement these primitives. It may not be
> necessary or desirable to add safe points to these loops, but perhaps should
> be preferred on the basis o correctness.

As another example, consider a 64-bit integer divide on a 32-bit
> architecture expanding to a libcall. Some, but perhaps not all, collection
> algorithms would want to emit safe point code for this call, but it simply
> does not exist in the IR to instrument.

I'm worried that it'll be necessary for some garbage collectors and hurt
performance too much for others. Should it be configurable? I don't need to
insert safe points in such loops and calls for my use, so I'd try to make it
extensible, but then leave support for these loops and calls to someone who
actually needs them.

Also, code injection of the form 'give me 8 bytes of noops at each safe
> point' and 'insert a cold instruction sequence at the end of the function'
> are best expressed at the machine code level. Safe points are hot code and
> unusual, target-specific techniques are regularly used with them if you
> survey the literature, so a design which accommodates that reality is
> preferred, even though hacking on the MachineFunction representation is less
> pleasant than the IR.

I'd like to allow target-specific techniques, but also allow
target-independent techniques, and the target-independent techniques are
more important for what I'm actually doing. Now that I've looked into
MachineInstructions more, it looks like it's impossible to use them in a
target-independent way.

> One element of this design that is desirable from a design perspective is
> that it preserves the original IR. Chris has said that it's a long-term goal
> of LLVM to not mangle the Function during code generation, and this moves in
> that direction instead of regressing.

I definitely like that feature.

Another way might be to run a pass over the IR inserting llvm.safepoint()
> calls, which could be implemented as a function in the module. Then we'd
> want a MachineFunction pass to inline this for us during codegen. The
> llvm.safepoint() calls could be easily identified and removed if the IR
> needs to be re-used.
> I see this as fairly equivalent to the first option.

I don't, because the user can control how they lower llvm.safepoint(). For
uses like mine, where I'd rather write the safe point in IR, I can implement
it as a function (if I have a selectiondag-level inliner). For more advanced
garbage collectors, they can implement it as a custom lowering in their
target. The biggest downside of an llvm.safepoint intrinsic would be that it
couldn't capture loops and calls introduced by lowering, unless those
lowerings explicitly introduced new safepoints.

> Also, regardless, stop point markers (a label is actually generated) need
> to survive as such into the MachineFunction representation else we'll never
> be able to generate a register map.
> Hope that helps,
> Gordon
> P.S. There's an interesting circularity which I may not have accounted for
> in the original design: If code is injected at each safe point, and a call
> instruction is injected, do we need to generate *another* safe point for
> that call? Clearly, the expansion of a safe point cannot be recursive with
> itself; but I think that we should allow generating a register map at the
> return address of that call, just as some collectors may want to instrument
> the libcall case discussed above.
> Actually, this distinction between safe points for inserting code and safe
> points for frame maps is probably is a critical design issue for your use
> case. Our current definition of a safe point is at the return address of a
> call instruction, which is precisely what's required to walk the stack. This
> is NOT the location where you want to add a call to your runtime.

I think you're saying that we need two kinds of safe points: points at which
to insert code, and points that the GCFunctionInfo::iterator traverses. The
code-inserting safe points could insert traversed safe points.
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