[llvm-dev] sret read after unwind

[Nikita Popov via llvm-dev]

On Mon, Dec 6, 2021 at 10:51 PM Philip Reames <listmail at philipreames.com>
wrote:

> I think this is an interesting problem.
>
> I'd probably lean towards the use of a separate attribute, but not
> strongly so.
>
> The example which makes me prefer the separate attribute would be a
> function with an out-param.  It's very unlikely that an out-param will be
> read on the exception path.  Being able to perform DSE for such out params
> seems quite interesting.
>
Right. I think it's mainly a question of whether we'd be able to infer the
attribute in practice, in cases where it's not annotated by the frontend
(which it should do in the sret case). I think this is possible at least
for the case where all calls to the function pass in an alloca to this
parameter (or another argument with nounwindread I guess) and don't use a
landingpad for the call. However, I believe we do inference in this
direction (RPO rather than PO) only in the module optimization pipeline,
which means that DSE/MemCpyOpt might not be able to make use of the
inferred information.

> However, I'll note that the same problem can be framed as an escape
> problem.  That is, we have an annotation not that a value is dead on the
> exception path, but that it hasn't been captured on entry to the routine.
> Then, we can apply local reasoning to show that the first store can't be
> visible to may_unwind, and eliminate it.
>
I don't think this would solve the same problem. In the given examples the
pointer is already not visible to @may_unwind because it is noalias.
"noalias" here is a weaker version of "not captured before": The pointer
may be captured, but it's illegal to write through the captured pointer,
which is sufficient for alias analysis. The problem with unwinding is that
after the unwind, the calling function may read the stored value in a
landingpad, which does not require any capture of the pointer.

Regards,
Nikita

> I'd want to give the escape framing more thought as that seems potentially
> more general.  Does knowing that an argument does not point to escaped
> memory on entry help on all of your motivating examples?
>
> Philip
> On 12/4/21 2:39 AM, Nikita Popov via llvm-dev wrote:
>
> Hi,
>
> Consider the following IR:
>
> declare void @may_unwind()
> define void @test(i32* noalias sret(i32) %out) {
>     store i32 0, i32* %out
>     call void @may_unwind()
>     store i32 1, i32* %out
>     ret void
> }
>
> Currently, we can't remove the first store as dead, because the
> @may_unwind() call may unwind, and the caller might read %out at that
> point, making the first store visible.
>
> Similarly, it prevents call slot optimization in the following example,
> because the call may unwind and make an early write to the sret argument
> visible:
>
> declare void @may_unwind(i32*)
> declare void @llvm.memcpy.p0i8.p0i8.i64(i8*, i8*, i64, i1)
> define void @test(i32* noalias sret(i32) %arg) {
>     %tmp = alloca i32
>     call void @may_unwind(i32* nocapture %tmp)
>     %tmp.8 = bitcast i32* %tmp to i8*
>     %arg.8 = bitcast i32* %arg to i8*
>     call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %arg.8, i8* align 4
> %tmp.8, i64 4, i1 false)
>     ret void
> }
>
> I would like to address this in some form. The easiest way would be to
> change LangRef to specify that sret arguments cannot be read on unwind
> paths. I think that matches how sret arguments are generally used.
>
> Alternatively, this could be handled using a separate attribute that can
> be applied to any argument, something along the lines of "i32* nounwindread
> sret(i32) %arg". The benefit would be that this is decoupled from sret ABI
> semantics and could potentially be inferred (e.g. if the function is only
> ever used with call and not invoke, this should be a given).
>
> Any thoughts on this? Is this a problem worth solving, and if yes, would a
> new attribute be preferred over restricting sret semantics?
>
> Regards,
> Nikita
>
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>
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