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

[Johannes Doerfert via llvm-dev]

Passes had to check declaration vs. definition before,
I don't think that is the most important argument.
That said, there is only so much you can do for declarations
anyway.

My proposal would be, as might have been expected, to run a

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
>>
>>