[Engine] ExpertSource::prefetch has two overloads and each source overrides one, so the other call form is a silent no-op (#611-D) #5101

Area

Engine

Desktop app, Terminal or source?

Source (START-HERE.bat, setup.sh or a build) — a build from ./update.sh

What happened

ExpertSource has two prefetch overloads, and each of the two real sources overrides only one of them. nvcc says so
four times a build:

/home/miki/Projects/git/Strata/include/strata/core/expert_source.hpp(710): warning #611-D: overloaded virtual function "strata::core::ExpertSource::prefetch" is only partially overridden in class "strata::core::FileExpertSource"
/home/miki/Projects/git/Strata/include/strata/core/expert_source.hpp(1261): warning #611-D: overloaded virtual function "strata::core::ExpertSource::prefetch" is only partially overridden in class "strata::core::ArenaExpertSource"

Reading it, both sides look like they miss the other half:

• the base: include/strata/core/expert_source.hpp:226 virtual void prefetch(int64_t layer, int64_t expert) — body
  is a (void) no-op; and include/strata/core/expert_source.hpp:262
virtual void prefetch(int64_t layer, const int64_t* experts, int64_t n) — also a no-op.
  prefetch_begin's default (:268-269) delegates to the batch form.
• FileExpertSource (:710) overrides only the batch one (:929, implemented at src/core/expert_source.cpp:2140).
• ArenaExpertSource (:1261) overrides only the single-expert one (:1288, implemented at
  src/core/expert_source.cpp:6254, an madvise(..., MADV_WILLNEED)).

Which call sites then land on the base no-op:

• The single-expert form is called from the expert-arena / adaptive-swap code:
  src/program/generate.cpp:9104, :12585, :12600, :12984, :19919 — for example
  :12600 srcp->prefetch(s.layer, s.out); // a file-backed arena released its pages: read them back ahead.
  srcp is &src when the file source is on (src/program/generate.cpp:6989 declares
  strata::core::FileExpertSource src;, :7203 srcp = &src;) or &arena_src when the arena is on (:7065 declares
  strata::core::ArenaExpertSource arena_src;, :7387 srcp = &arena_src;). So with the file source those five
  calls do nothing.
• The batch form is called from the CPU dispatch: src/core/expert_source.cpp:5122
d.src->prefetch(d.layers, miss.data(), (int64_t) miss.size()); (CS-T, d.cold_mode == 0), with drive.d.src = srcp
  at src/program/generate.cpp:8448. So with the arena source that call, and prefetch_begin's default at
  :268-269, do nothing.

Smallest steps or command that reproduce it

The warning appears in the CUDA pass of the engine build:

./update.sh           # or: cmake --build build --target strata -j 16 | grep '#611-D'

The call-site reading above is from the source at ffa80d73.

Reproduction status

Unconfirmed static analysis. I read the code and the compiler warning; I did not see a runtime failure and I
measured no timing difference (no A/B of a decode with and without the missing read-ahead). It may be that the
no-op path is deliberate, or that the file source's own read-ahead elsewhere covers those five call sites — I did not
check that.

Suggested smallest fix

One line per class, delegating to the half that exists, and #611-D goes away:

// FileExpertSource
void prefetch(int64_t layer, int64_t expert) override { prefetch(layer, &expert, 1); }
// ArenaExpertSource
void prefetch(int64_t layer, const int64_t* experts, int64_t n) override {
    for (int64_t i = 0; i < n; ++i) prefetch(layer, experts[i]);
}

If a no-op is intended for one of them, override on the missing overload with a one-line comment saying so would
also silence the warning and make the intent readable.

Strata version

Strata 0.2.10 · engine 0.1.56 · source checkout at ffa80d73

Graphics card, memory and system

RTX 5080 16 GB · 96 GB RAM · Linux (Arch), gcc 16.2.1, CMake 4.4.3, ninja 1.13.2, CUDA/nvcc 13.4
(-DSTRATA_ENABLE_CUDA=ON -DCMAKE_CUDA_ARCHITECTURES=120)