llvm dev - Oct 2017 - Status of llvm.invariant.{start|end}

Hi,
>From LangRef, these intrinsics seems really useful for letting LLVM
know about certain higher level immutability guarantee, e.g. for
objects that are not allowed to be mutated after construction.
However, it doesn't seem to work[1] and a quick code search suggests
that there's not a single optimization pass that's currently using it
for store to load forwarding, only very few that use it to eliminate
stores. The issue linked is marked as resolved-later and mentioned
that it "probably have to be redesigned before they work out right".
What has to be redesigned to make it work and is there a better way
that works currently to mark an object as immutable after a certain
point/in certain region?

Yichao Yu

[1] https://bugs.llvm.org/show_bug.cgi?id=5441

Hi Yichao,

We at Azul have been using invariant.start for marking objects as immutable
after a certain point.
Also, upstream changes to teach relevant optimizations about invariant.start and
end were added
last year.

With respect to store to load forwarding, this is handled in GVN. I think the
test cases in test/Transforms/GVN/invariant.start.ll
handle what you’re looking for.


Hope this helps,
Anna 
> On Oct 29, 2017, at 9:01 PM, Yichao Yu via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi,
> 
> From LangRef, these intrinsics seems really useful for letting LLVM
> know about certain higher level immutability guarantee, e.g. for
> objects that are not allowed to be mutated after construction.
> However, it doesn't seem to work[1] and a quick code search suggests
> that there's not a single optimization pass that's currently using
it
> for store to load forwarding, only very few that use it to eliminate
> stores. The issue linked is marked as resolved-later and mentioned
> that it "probably have to be redesigned before they work out
right".
> What has to be redesigned to make it work and is there a better way
> that works currently to mark an object as immutable after a certain
> point/in certain region?
> 
> Yichao Yu
> 
> [1] https://bugs.llvm.org/show_bug.cgi?id=5441
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> We at Azul have been using invariant.start for marking objects as immutable
after a certain point.
> Also, upstream changes to teach relevant optimizations about
invariant.start and end were added
> last year.
>
> With respect to store to load forwarding, this is handled in GVN. I think
the test cases in test/Transforms/GVN/invariant.start.ll
> handle what you’re looking for.
Hmm, I'm pretty sure I checked that.
It seems that none of the test cases in there actually requires
invariant.start for store-to-load forwarding? (they need
`invariant.start|end` to not be marked as modifying the memory but
should all work without the intrinsics.) AFAICT the simple case in the
issue I linked still doesn't work

```
declare void @g(i8*)

declare {}* @llvm.invariant.start.p0i8(i64, i8* nocapture) #0

define i8 @f() {
  %a = alloca i8
  store i8 0, i8* %a
  %i = call {}* @llvm.invariant.start.p0i8(i64 1, i8* %a)
  call void @g(i8* %a)
  %r = load i8, i8* %a
  ret i8 %r
}

attributes #0 = { argmemonly nounwind }
```

A related note, can this be marked as inaccessiblemem_or_argmemonly
plus readonly on the argument? As far as not messing with the alias
analysis goes, this works very well without LLVM knowning anything
about such a function. Of course the store to load forwarding still
need to be handled separately but it seems that this can avoid some of
the special cases on this intrinsics.
>
>
> Hope this helps,
> Anna
>
>> On Oct 29, 2017, at 9:01 PM, Yichao Yu via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>>
>> Hi,
>>
>> From LangRef, these intrinsics seems really useful for letting LLVM
>> know about certain higher level immutability guarantee, e.g. for
>> objects that are not allowed to be mutated after construction.
>> However, it doesn't seem to work[1] and a quick code search
suggests
>> that there's not a single optimization pass that's currently
using it
>> for store to load forwarding, only very few that use it to eliminate
>> stores. The issue linked is marked as resolved-later and mentioned
>> that it "probably have to be redesigned before they work out
right".
>> What has to be redesigned to make it work and is there a better way
>> that works currently to mark an object as immutable after a certain
>> point/in certain region?
>>
>> Yichao Yu
>>
>> [1] https://bugs.llvm.org/show_bug.cgi?id=5441
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>