[LLVMdev] RFC: implicit null checks in llvm

[Andrew Trick]

> On Apr 23, 2015, at 4:07 PM, Reid Kleckner <rnk at google.com> wrote:
> 
> On Thu, Apr 23, 2015 at 3:31 PM, Andrew Trick <atrick at apple.com <mailto:atrick at apple.com>> wrote:
> It would be really nice to just emit a label next to an instruction and assume the stackmap entry will end up at the right address. I don’t think the backend can make that guarantee. Any Machine pass can insert instructions wherever it wants without looking for labels.
> 
> I think labels are only employed just before code emission. Is there any existing example of labels being used earlier (I think GCRootLowering is the earliest)?
> 
> SelectionDAGBuilder::lowerInvokable() inserts EH_LABELs into the DAG for normal invokes, and it more or less works out.
> 
> I don't know much about what transformations MI passes typically do, but my guess is that you're concerned that some pass might re-schedule possibly trapping operations inside the label range?

The scheduler itself doesn’t move anything around labels. But any pass can perform code motion or load/store optimization. Also, any pass can insert an instruction, like a copy, between the label and the load.

I’m not really sure how EH_LABEL ends up translating into exception tables, but my guess is that it’s encoding a range that may include any arbitrary instructions as long as the call is within the range. So as long as calls aren’t reordered with labels and appears to have side effects it would work.

So, you could add a different kind of stack map entry that encodes ranges instead of exact addresses, then survey all passes to ensure they don’t optimize loads across labels. I would have more confidence doing this with a pseudo instruction though.

Andy
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