[LLVMdev] Adding a custom GC safe point creation phase

[Nicolas Geoffray]

2011/11/1 Gordon Henriksen <gordonhenriksen at mac.com>

> On Nov 1, 2011, at 4:47 AM, Nicolas Geoffray <nicolas.geoffray at gmail.com>
> wrote:
>
> Thanks for the review Gordon.
>
> On Tue, Nov 1, 2011 at 2:21 AM, Gordon Henriksen <gordonhenriksen at mac.com>wrote:
>
>> On 2011-10-31, at 17:21, Nicolas Geoffray wrote:
>>
>> > Here's a patch to allow a GCStrategy to customize the places where it
>> wants to insert safe points. I'm not sure who maintains the GC code today
>> in LLVM (I'd be happy to take ownership, if needed).
>> >
>> > The patch just adds up a custom safepoints flag, similar to the way the
>> GCStrategy can customize intrinsics lowering, or roots initialization. It
>> works pretty well, as I've tested it on VMKit. So the patch in itself
>> should not be controversial.
>>
>> Seems a perfectly reasonable patch on its own merits.
>>
>> I would be curious what your findCustomSafePoints implementation looks
>> like. Perhaps it's possible to extract some of it into the framework.
>>
>
> The code for my findCustomSafePoints is like this:
>
> void MyGC::findCustomSafePoints(GCFunctionInfo& FI, MachineFunction &MF) {
>   for (MachineFunction::iterator BBI = MF.begin(),
>                                  BBE = MF.end(); BBI != BBE; ++BBI) {
>     for (MachineBasicBlock::iterator MI = BBI->begin(),
>                                      ME = BBI->end(); MI != ME; ++MI) {
>       if (MI->getDesc().isCall()) {
>                /// Standard code need for adding a post call safe point;
>       } else if (MI->getDebucLoc().getCol() == 1) {
>               /// Standard code need for adding a post call safe point;
>       }
>   }
> }
>
> So it really looks like what we already have, except this special trick
> about checking the debug location. Piggy backing on the debug location is
> easy and works for me because:
> 1) Debug locations are passed from LLVM Instructions to MachineInstruction
> 2) I don't have column numbers in my front-end.
>
> The instructions that get this special marker on the debug location are
> instructions that may trigger a segmentation fault. Because I enter a
> signal handler, that may invoke a collection, I need to know the roots at
> the faulting instruction's address.
>
>
> I have my doubts about the robustness of this strategy, but if it's
> working well enough for your needs, then great.
>

I have my doubts too. I would really like to see the metadata approach take
over.

Nicolas


>
> Your patch seems appropriately factored. The alternative would seem to be
> calling a virtualized predicate on each machine instruction, as I don't
> think your debug info approach is something we would want to ‘bless’ just
> yet.
>
> You may need to align your safe point machine code analysis with
> [GCStrategy.cpp]LowerIntrinsics::CouldBecomeSafePoint. Note that this
> predicate operates on IR instructions instead of machine code.
>
> — Gordon
>
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