[llvm-dev] Status of llvm.invariant.{start|end}

[Anna Thomas via llvm-dev]

On Oct 31, 2017, at 3:50 PM, Anna Thomas via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:


On Oct 31, 2017, at 12:17 PM, Yichao Yu <yyc1992 at gmail.com<mailto:yyc1992 at gmail.com>> wrote:

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 }
```
Yes, you’re right. We don’t forward across the call even in presence of invariant.start.

By definition, invariant.start represents constant physical memory, so I would think this is a legal transform
to do. Ofcourse, this can lead to miscompiles if g is a special function that can modify %a in some way,
but those are things the front end needs to identify.
Just to clarify: If g can modify %a in some way, the front end needs to identify it and avoid adding
invariant.start.

Once the invariant.start has been added by the front end,this is a perfectly legal transform to do
based on the LLVM spec.

In fact we should be implementing this in LLVM, patches welcome :)



Anna




Hope this helps,
Anna

On Oct 29, 2017, at 9:01 PM, Yichao Yu via llvm-dev <llvm-dev at lists.llvm.org<mailto: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
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