Microbenchmarks where inliner runs out of budget with TieredPGO

[AndyAyersMS]

See #84264 for context

A number of microbenchmarks—particularly those under System.Text.Json—run out of budget when run with TieredPGO.

• System.Text.Json.Tests.Perf_Get.GetByte and related
• System.Text.Json.Serialization.Tests.WriteJson<ImmutableDictionary<String, String>>.SerializeToWriter(Mode: SourceGen)
• Also, a good example is When calculating xxhash, net7.0 is slower than net6.0 #90090 (with a temp workaround in Mark XxHash64.Complete as noinline #90142) that illustrates the problem.

Suggest that we review RWC/SPMI to see how often this thing comes up outside of microbenchmarks.

Possible fixes

• increase budget (if these cases are rare)
• reduce amount of AggressiveInlining in System.Text.Json (if these cases are all json related)
• rework inliner to consider candidates in priority order

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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Issue Details

---

See #84264 for context

A number of microbenchmarks—particularly those under System.Text.Json—run out of budget when run with TieredPGO.

• System.Text.Json.Tests.Perf_Get.GetByte and related
• System.Text.Json.Serialization.Tests.WriteJson<ImmutableDictionary<String, String>>.SerializeToWriter(Mode: SourceGen)

Suggest that we review RWC/SPMI to see how often this thing comes up outside of microbenchmarks.

Possible fixes

• increase budget (if these cases are rare)
• reduce amount of AggressiveInlining in System.Text.Json (if these cases are all json related)
• rework inliner to consider candidates in priority order

Author:	AndyAyersMS
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[AndyAyersMS]

cc @EgorBo

[MichalPetryka]

While looking at dumps for #85349 I've noticed that a lot of autoproperties weren't inlined in System.Text.Json, would it make sense to always inline really small methods like autoproperties and maybe not count them into the inliner budget?

[AndyAyersMS]

While looking at dumps for #85349 I've noticed that a lot of autoproperties weren't inlined in System.Text.Json, would it make sense to always inline really small methods like autoproperties and maybe not count them into the inliner budget?

Can you give a more specific example?

Really small methods should already be bypassing budget checks.

[MichalPetryka]

Can you give a more specific example?

Here, note all the set_ calls. Since that PR let the jit inline more, a few of those were inlined and turned into a single mov like this:

[AndyAyersMS]

These look small in C#, but can you verify they're small in IL?

[MichalPetryka]

These look small in C#, but can you verify they're small in IL?

Well they're autoproperties, won't those be always compiled as ldarg.0+ldfld/ldarg.0+ldarg.1+stfld?

[AndyAyersMS]

These look small in C#, but can you verify they're small in IL?

Well they're autoproperties, won't those be always compiled as ldarg.0+ldfld/ldarg.0+ldarg.1+stfld?

I don't know about always, but it looks like these are indeed small:

.method public hidebysig specialname instance void modreq([System.Runtime]System.Runtime.CompilerServices.IsExternalInit) 
        set_IsPublic(bool 'value') cil managed
{
  .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 ) 
  // Code size       8 (0x8)
  .maxstack  8
  IL_0000:  ldarg.0
  IL_0001:  ldarg.1
  IL_0002:  stfld      bool class System.Text.Json.Serialization.Metadata.JsonPropertyInfoValues`1<!T>::'<IsPublic>k__BackingField'
  IL_0007:  ret
} // end of method JsonPropertyInfoValues`1::set_IsPublic

Now, do you know for sure why these didn't get inlined?

[EgorBo]

I guess when we run out of budget (or too many locals?) we give up even on "below ALWAYS_INLINE" stuff

[MichalPetryka]

Now, do you know for sure why these didn't get inlined?

I haven't checked thoroughly, but the change from the PR made the assignments to delegate typed properties take less of the budget, so that's why I assume it's more budget being available that let it inline more.