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Measured dispatch stack

Numbers on this page were captured, not estimated. A unit test (Test_DispatchStack, tagged TSTK, in GMPTests.cpp) calls FPlatformStackWalk::CaptureStackBackTrace from inside the listener body and counts the GMP frames sitting between the send call and the callback.

The same listener is hit two ways:

  • by-name — SendObjectMessage(MSGKEY(...), ...), the FName plus TMap lookup path
  • by-store — MSGKEY_SLOT(...) plus SendObjectMessageDirect(...), the baked-key path

Reproduce:

dotnet Engine/Binaries/DotNET/UnrealBuildTool/UnrealBuildTool.dll \
       <YourEditor> Win64 <DebugGame|Development> -Project=<Your.uproject> -Module=GMP -WaitMutex

Engine/Binaries/Win64/UnrealEditor[-Win64-DebugGame]-Cmd.exe \
       <Your.uproject> -run=GMPUnitTest -unattended -nopause -nosplash -nullrhi -NoShaderCompile

Results

Build by-name (FName + lookup) by-store (baked key)
Unoptimized (DebugGame) 9 7
Optimized (Development) 4 3

Frame counts exclude the listener callback itself.

Both rows are modular builds. GMP_WITH_INLINE_FIRE only takes effect when GMP_WITH_STATIC_STORE is on, and that requires GMP_STATIC_STORE_MONOLITHIC (IS_MONOLITHIC). What it removes is the first two of the three optimized frames: the cross-module GMP_API wrapper disappears, and the dispatch loop inlines into the caller.

Going from 9 to 3 is two separate effects:

  • key baking merges the three hub frames (SendObjectMessageWrapperEx → NotifyMessageImpl → FireMsgBodyAdapt) into a single NotifyMessageDirectRaw
  • the optimizer inlines away the type-erasure scaffolding: the dispatch lambda, FlexBackendThunk, TGMPFunction::operator() and the unpack thunk

What survives optimization is the work that actually has to happen: find the store, walk the matched listeners, call them.

DebugGame (unoptimized)

by-name — 9 frames

  1. GMP::FMessageHub::SendObjectMessageWrapperEx<0,int>()
  2. GMP::FMessageHub::NotifyMessageImpl()
  3. GMP::FireMsgBodyAdapt()
  4. GMP::GMPFireWithSigSourceDirectRaw()
  5. GMP::FSignalUtils::FireWithSigSourceCore<0, ...FireWithSigSourceRaw<0>...<lambda_1> >()
  6. GMP::FSignalUtils::FireWithSigSourceRaw<0>::<lambda_1>::operator()()
  7. GMP::FlexSig::TFlexBackendThunk<GMP::TGMPFunction<void __cdecl(FGMPTypedAddr const*, GMP::FGMPExtra const*)>, FGMPTypedAddr const*, GMP::FGMPExtra const*>::FlexThunk()
  8. GMP::TGMPFunction<void __cdecl(FGMPTypedAddr const*, GMP::FGMPExtra const*)>::operator()()
  9. GMP::Hub::RawUnpackThunk<std::tuple<int>, ...Test_DispatchStack...<lambda_1> >()

by-store (baked key) — 7 frames

  1. GMP::FMessageHub::NotifyMessageDirectRaw()
  2. GMP::GMPFireWithSigSourceDirectRaw()
  3. GMP::FSignalUtils::FireWithSigSourceCore<0, ...>()
  4. GMP::FSignalUtils::FireWithSigSourceRaw<0>::<lambda_1>::operator()()
  5. GMP::FlexSig::TFlexBackendThunk<...>::FlexThunk()
  6. GMP::TGMPFunction<...>::operator()()
  7. GMP::Hub::RawUnpackThunk<...>()

Development (optimized)

by-name — 4 frames

  1. GMP::FMessageHub::SendObjectMessageWrapperEx<0,int>()
  2. GMP::FMessageHub::NotifyMessageImpl()
  3. GMP::GMPFireWithSigSourceDirectRaw()
  4. GMP::FSignalUtils::FireWithSigSourceCore<0, ...>()

by-store (baked key) — 3 frames

  1. GMP::FMessageHub::NotifyMessageDirectRaw()
  2. GMP::GMPFireWithSigSourceDirectRaw() — GMP_API, the export the optimizer cannot inline through
  3. GMP::FSignalUtils::FireWithSigSourceCore<0, ...>() — the matching loop

Symbols are as the platform stack walker printed them; template arguments are elided above where they only repeat the enclosing signature.