llvm dev - Jul 2016 - RFC: Strong GC References in LLVM

On Fri, Jul 15, 2016 at 5:25 PM, Andrew Trick <atrick at apple.com> wrote:
>
> On Jul 15, 2016, at 4:48 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
> Why? A decision was made to give pointers types, and we've decided to
> change that. It is not clear to me that the decision to allow implicit
> early exits was, in retrospect, optimal. I think it is completely healthy
> for the project to reevaluate these kinds of decisions. We now have many
> years of experience, bug reports, and we should have a good ability to
> evaluate the compile-time impact of a potential change.
>
>
> Let me rephrase: It didn’t seem to me like the fundamental problem we were
> up against in this discussion, and it’s definitely very difficult to change
> given the burden it would place on intrinsics.
>
>
Yes, this discussion has gone a bit off track, which i will apologize for.

> FWIW, for a long time I was a very strong proponent of explicit control
> flow because I like making it easy to reason about CFG transforms and code
> motion. But I gradually came around to realize it’s a legitimate design
> either way. In some ways it works well not to have a CFG edge for exits
> where it’s illegal to insert code.
>
> I think LLVM passes have also been biased toward algorithms that scale in
> the number of blocks.
>
If this scaling is a design goal that's really interesting. Most of the
memory opt and code motion algorithms i've worked on in LLVM are
block/instruction-capped in the same way (and most have lower caps than GCC
due to compile time impact on llvm).
There certainly was a time when that wasn't true (i remember back in the
day :P), but it's no longer true.

Of course, I don't actually think either design necessarily makes scaling
easier or harder, that's just "the state of the world".
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Joining in very late, but the tangent here has been interesting (if 
rather OT for the original thread).

I agree with Danny that we might want to take a close look at how we 
model things like maythrow calls, no return, and other implicit control 
flow.  I'm not convinced that moving to a pure explicit model is the 
right idea because we get a lot of gain in practice from being able to 
reason about what are essentially a limited form of extended basic 
blocks.  I would welcome a design discussion about this, preferably in 
person, but also don't want to block any current (or future honestly) 
work on this until we have a reasonable firm plan of action.

One idea would be to explicitly acknowledge that our "basic blocks"
are
actually "extended basic blocks" with internal exits due to exception 
propagation, noreturn, and (recently) guards.  I could see a couple of 
implementation strategies here:
1) Extend BasicBlock to explicitly track potential early exiting 
instructions.  This would probably have to be conservative so that 
things like nothrow inference aren't required to update all callers in 
one go.
2) Conservative assume that BasicBlock has an unknown number of early 
exiting instructions and write an analysis pass which answers questions 
about where those early exits are.  Any pass which does code motion 
would require this analysis.  (This is essentially the principled 
version of our current hacks.)

Philip

On 07/15/2016 06:57 PM, Daniel Berlin wrote:
>
>
> On Fri, Jul 15, 2016 at 5:25 PM, Andrew Trick <atrick at apple.com 
> <mailto:atrick at apple.com>> wrote:
>
>
>>     On Jul 15, 2016, at 4:48 PM, Hal Finkel <hfinkel at anl.gov
>>     <mailto:hfinkel at anl.gov>> wrote:
>>
>>     Why? A decision was made to give pointers types, and we've
>>     decided to change that. It is not clear to me that the decision
>>     to allow implicit early exits was, in retrospect, optimal. I
>>     think it is completely healthy for the project to reevaluate
>>     these kinds of decisions. We now have many years of experience,
>>     bug reports, and we should have a good ability to evaluate the
>>     compile-time impact of a potential change.
>
>     Let me rephrase: It didn’t seem to me like the fundamental problem
>     we were up against in this discussion, and it’s definitely very
>     difficult to change given the burden it would place on intrinsics.
>
>
> Yes, this discussion has gone a bit off track, which i will apologize for.
>
>     FWIW, for a long time I was a very strong proponent of explicit
>     control flow because I like making it easy to reason about CFG
>     transforms and code motion. But I gradually came around to realize
>     it’s a legitimate design either way. In some ways it works well
>     not to have a CFG edge for exits where it’s illegal to insert code.
>
>
>     I think LLVM passes have also been biased toward algorithms that
>     scale in the number of blocks.
>
> If this scaling is a design goal that's really interesting. Most of 
> the memory opt and code motion algorithms i've worked on in LLVM are 
> block/instruction-capped in the same way (and most have lower caps 
> than GCC due to compile time impact on llvm).
> There certainly was a time when that wasn't true (i remember back in 
> the day :P), but it's no longer true.
>
> Of course, I don't actually think either design necessarily makes 
> scaling easier or harder, that's just "the state of the
world".
>
>
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> On Jul 21, 2016, at 7:45 AM, Philip Reames <listmail at
philipreames.com> wrote:
> 
> Joining in very late, but the tangent here has been interesting (if rather
OT for the original thread).
> 
> I agree with Danny that we might want to take a close look at how we model
things like maythrow calls, no return, and other implicit control flow.  I'm
not convinced that moving to a pure explicit model is the right idea because we
get a lot of gain in practice from being able to reason about what are
essentially a limited form of extended basic blocks.  I would welcome a design
discussion about this, preferably in person, but also don't want to block
any current (or future honestly) work on this until we have a reasonable firm
plan of action.
> 
> One idea would be to explicitly acknowledge that our "basic
blocks" are actually "extended basic blocks" with internal exits
due to exception propagation, noreturn, and (recently) guards.  I could see a
couple of implementation strategies here:
> 1) Extend BasicBlock to explicitly track potential early exiting
instructions.  This would probably have to be conservative so that things like
nothrow inference aren't required to update all callers in one go.
> 2) Conservative assume that BasicBlock has an unknown number of early
exiting instructions and write an analysis pass which answers questions about
where those early exits are.  Any pass which does code motion would require this
analysis.  (This is essentially the principled version of our current hacks.)
This analysis can be lazy/incremental. Most passes only do “safe” speculation
and code sinking without side effects. They don’t need to run the analysis.

Thanks Philip. I forgot to mention that LLVM passes historically have taken
advantage of implicit extended basic blocks (ISel) and were never improved to
handle general EBBs. It took years for loop opts to finally handle early exits
well.
-Andy
> 
> Philip
> 
> On 07/15/2016 06:57 PM, Daniel Berlin wrote:
>> 
>> 
>> On Fri, Jul 15, 2016 at 5:25 PM, Andrew Trick <atrick at apple.com
<mailto:atrick at apple.com>> wrote:
>> 
>>> On Jul 15, 2016, at 4:48 PM, Hal Finkel < <mailto:hfinkel at
anl.gov>hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
>>> 
>>> Why? A decision was made to give pointers types, and we've
decided to change that. It is not clear to me that the decision to allow
implicit early exits was, in retrospect, optimal. I think it is completely
healthy for the project to reevaluate these kinds of decisions. We now have many
years of experience, bug reports, and we should have a good ability to evaluate
the compile-time impact of a potential change.
>> 
>> 
>> Let me rephrase: It didn’t seem to me like the fundamental problem we
were up against in this discussion, and it’s definitely very difficult to change
given the burden it would place on intrinsics.
>> 
>> 
>> Yes, this discussion has gone a bit off track, which i will apologize
for.
>>  
>> FWIW, for a long time I was a very strong proponent of explicit control
flow because I like making it easy to reason about CFG transforms and code
motion. But I gradually came around to realize it’s a legitimate design either
way. In some ways it works well not to have a CFG edge for exits where it’s
illegal to insert code.
>>  
>> I think LLVM passes have also been biased toward algorithms that scale
in the number of blocks.
>> If this scaling is a design goal that's really interesting. Most of
the memory opt and code motion algorithms i've worked on in LLVM are
block/instruction-capped in the same way (and most have lower caps than GCC due
to compile time impact on llvm).
>> There certainly was a time when that wasn't true (i remember back
in the day :P), but it's no longer true.
>> 
>> Of course, I don't actually think either design necessarily makes
scaling easier or harder, that's just "the state of the world".
>> 
>> 
> 
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