Inlining in shared generic code leaves unused temp behind

[jkotas]

Forked from dotnet/coreclr#9541 (comment).

The code for List<__Canon>::Clear() after dotnet/coreclr#9540:

G_M58440_IG01:
       56                   push     rsi
       4883EC30             sub      rsp, 48
       48894C2428           mov      qword ptr [rsp+28H], rcx
       488BF1               mov      rsi, rcx

G_M58440_IG02:
       488B0E               mov      rcx, qword ptr [rsi]        ;** ?? --- redundant load?        
       448B4618             mov      r8d, dword ptr [rsi+24]     ; int size = _size;
       33C9                 xor      ecx, ecx                    ;** 0 ----^ clears earlier move
...

The redundant load is left over temp from optimized out dictionary lookup. Can we get rid of it?

Jit Dump: ListClear.txt

[jkotas]

cc @dotnet/jit-contrib

[jkotas]

Notice that the frame setup is unnecessary as well because of the method is leaf method now. Not sure whether it is side-effect of the unused dictionary temp, or whether it is separate issue.

[mikedn]

It looks like rsi contains this. As far as the JIT is concerned this may be null so that load cannot be eliminated because it has an associated side effect.

[AndyAyersMS]

As for why there's a frame, it looks like the jit sets lvaGenericsContextUsed when it creates the initial runtime lookup and does not reconsider it later on when the use is deleted. Setting this flag triggers reporting of the context in the gc info (via a copy of this, kept alive in a stack slot) which in turn requires a frame.

It might be possible to recompute this flag post-optimization, or perhaps ref count the uses.

[mikedn]

As for why there's a frame, it looks like the jit sets lvaGenericsContextUsed when it creates the initial runtime lookup and does not reconsider it later on when the use is deleted

Similar to #4549

[VSadov]

Yes. this seems a dupe of #4549

It is kind of annoying. It did push me in some cases to use instance methods instead of static ones, even though there was no use of this.

Like in: https://github.com/VSadov/NonBlocking/blob/master/src/NonBlocking/ConcurrentDictionary/DictionaryImpl.cs#L57

[jkotas]

Ok, let's use #4549 to track it then.