[PATCH] D12923: Add support for function attribute "notail"
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Wed Sep 23 11:13:52 PDT 2015
On 09/23/2015 08:48 AM, Akira Hatanaka wrote:
> On Tue, Sep 22, 2015 at 8:31 AM, Philip Reames
> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
> To be clear, this is a debuging aid only? It's not something
> required for correctness? I'm somewhat bothered by that because
> it seems like it would be a useful implementation tool for higher
> level languages.
> It's not purely a debugging aid that helps when you are using the
> debugger. There are projects (that are not debuggers) that rely on not
> missing frames to produce results that are useful.
If it's not simply best effort, that constrains our choices.
> A couple of thoughts in no particular order:
> 1) Can we always annotate the call site rather than the function?
> That removes the unpredictability due to optimization.
> Annotating the call site should be fine. For the use cases that we
> care about, it probably isn't important to prevent tail calls on
> indirect calls.
Given this, I would lean towards a notail value being added as an
alternative to "tail" and "musttail". This seems to fit the existing
uses, doesn't have any obvious loop holes or best effort semantics, and
solves the problem at hand.
> 2) Calling it something like "no-direct-tail-call" or
> "prefer-no-tail" would remove some of the confusion value. When I
> see "notail", I expect that to always be respected; the best
> effort semantics come as a bit of a surprise.
> I agree. A name that indicates it's only a best effort option or it's
> an option that affects only direct calls would be good.
(This only applies if we're talking about a function annotation. The
call site annotation applies to both direct and indirect calls.)
> 3) This seems analogous to the "tail" marker in that it indicates
> a preference/option. Whatever we end up with, it needs to be a
> verifier option to have a "tail" or "musttail" call site which is
> also "notail". It also needs to be an error to have a mustail
> callsite to a notail function (if such ends up existing.)
> If we are going to annotate the function, I think we should have the
> verifier catch incompatibilities between the markers on the call sites
> and the function attribute on the called functions.
> If we are annotating the call site, the verifier check isn't needed
> since the tail-call related markers are enums that are mutually exclusive.
> 4) It somewhat feels like there are two concepts being intermixed
> here. 1) A call site which will never be a tail call. 2) A
> function which we prefer not to tail call to. Does it make sense
> to separate them?
> Yes, it makes sense to separate them. For the use case we care about,
> either 1) or 2) will do. We don't have to have support for both.
I would lean toward doing (1) for now. We can come back and implement
(2) at a later time if we find it's needed. After (1), each call site
will have four states:
- "notail" - Can not be a tail call.
- "" - May be a tail call if analysis finds it legal, profitable, and
- "tail" - May be a tail call, profitability hinted
- "musttail" - Must be a tail call.
* (2) would basically just change the desirability of moving from "" to
> On 09/21/2015 06:22 PM, Akira Hatanaka wrote:
>> Several users have been asking for this function attribute to
>> prevent losing the calling functions's information in the
>> backtrace. If we attach the attribute to a function, ideally we
>> would want to prevent tail call optimization on all call sites
>> that call the function. However, the compiler cannot always tell
>> which function is called from a call site if it's an indirect
>> call, so it's fine if an indirect call to the marked function
>> ends up being tail-call optimized. For direct calls, we want the
>> function attribute to prevent tail call 100% of the time.
>> We can also use a "notail" marker on the call instruction instead
>> of using a function attribute. The only downside of using a
>> marker is that we probably will never be able to prevent tail
>> call optimization on indirect calls even when the compiler can
>> turn it into a direct call (for example, via inlining). I'm not
>> sure at the moment how important this is.
>> On Thu, Sep 17, 2015 at 9:47 AM, Philip Reames via llvm-commits
>> <llvm-commits at lists.llvm.org
>> <mailto:llvm-commits at lists.llvm.org>> wrote:
>> Can you give a bit of background on what you're trying to
>> address here? After reading through the discussion and
>> seeing that this is a best effort flag, I'm not sure that a
>> function attribute is the best way to describe this. I'm
>> open to being convinced it is, but I'd like to hear a bit
>> more about the use case and get broader visibility on the
>> proposal first.
>> On 09/16/2015 07:27 PM, Akira Hatanaka via llvm-commits wrote:
>>> ahatanak created this revision.
>>> ahatanak added a subscriber: llvm-commits.
>>> This patch adds support for a new IR function attribute "notail". The attribute is used to disable tail call optimization on calls to functions marked with the attribute.
>>> This attribute is different from the existing attribute "disable-tail-calls", which disables tail call optimizations on all call sites within the marked function.
>>> The patch to add support for the corresponding source-level function attribute is here:
>>> llvm-commits mailing list
>>> llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>
>> llvm-commits mailing list
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