[llvm-dev] Visitation of declarations in FunctionAtrs with new and old pass managers?

[Johannes Doerfert via llvm-dev]

Passes had to deal with Decl. vs Def. before and they managed.
That said, there is only that much you can do with declarations
and I would suggest we do it early as part of a module pass.

Attributor run early as module pass will right now *not* annotate
the declaration but still derive the information for the call sites:
https://godbolt.org/z/b9a8Gheqb

We could make it annotate declarations as well with little effort.

~ Johannes


On 4/11/21 1:39 AM, Arthur Eubanks wrote:
> I think it makes more sense to do something like that in a pass
> like InferFunctionAttrsPass. We should decide to visit declarations
> consistently with function pass managers, which currently don't visit
> declarations. Adding declarations to the new PM CGSCC infra would add
> complexity and passes would have to check if they're working with a
> declaration vs definition.
>
> On Fri, Apr 9, 2021 at 1:41 PM Philip Reames <listmail at philipreames.com>
> wrote:
>
>> I ran across a dependency in the way the new and old pass managers
>> interact with function-attrs.  I'm not sure whether this is expected
>> behavior or not, but with some digging, I couldn't find a clear
>> motivation.  Anyone have context on this?
>>
>> Essentially, under the old pass manager, FunctionAttrs appears to visit
>> declarations, and under the new one, it doesn't.   Here's an example
>> that shows how this can change the output of function-attrs:
>>
>> $ cat decl.ll
>>
>> declare void @readnone() readnone
>>
>> $ ./opt -S -function-attrs decl.ll -enable-new-pm=1
>> ; ModuleID = 'decl.ll'
>> source_filename = "decl.ll"
>>
>> ; Function Attrs: readnone
>> declare void @readnone() #0
>>
>> attributes #0 = { readnone }
>>
>> $ ./opt -S -function-attrs decl.ll -enable-new-pm=0
>> ; ModuleID = 'decl.ll'
>> source_filename = "decl.ll"
>>
>> ; Function Attrs: nofree nosync readnone
>> declare void @readnone() #0
>>
>> attributes #0 = { nofree nosync readnone }
>>
>> (The example uses nofree and nosync, but please don't focus on the
>> semantics of those attributes.  That's a separate discussion.)
>>
>> To me, it seems odd to not have declarations be SCCs of their own, and
>> thus passed to function-attrs.  Does anyone have a good explanation for
>> why we made this change?  And in particular, what the "right" way of
>> inferring attributes for a partially annotated declaration might be in
>> our new world?
>>
>> Philip
>>
>>