Validate CPU kernel registrations at compile time

[GopalakrishnanN]

Description

• Mark the ONNX, optional FP16, and ONNX-ML CPU kernel factory tables constexpr.
• Add a consteval invariant that requires the disabled BuildKernelCreateInfo<void> entry first and rejects null factory pointers.
• Preserve reduced-operator behavior: disabled registrations may legitimately repeat the non-null disabled entry.
• Add independent compile-time tests for valid, missing-sentinel, null-factory, and null-sentinel tables, plus a runtime check that the real CPU registry initializes non-empty.

Motivation and Context

CPU kernel registration is a fixed build-time descriptor set, but malformed tables were only discoverable while constructing the runtime registry. These checks make sentinel placement and factory-pointer validity compile-time requirements across the normal, reduced-ops, optional FP16, and optional ONNX-ML table variants.

The existing compact runtime registration loops are intentionally unchanged. A templated shared loop increased the provider library by 5,006 bytes, and an inline span version increased it by 33,970 bytes; keeping the loops produced the smallest measured result.

Measurement

Windows RelWithDebInfo, MSVC 19.51, touched-file rebuild of cpu_execution_provider.cc:

Artifact	Before	After	Delta
cpu_execution_provider.obj	2,033,704 B	2,033,836 B	+132 B
onnxruntime_providers.lib	678,492,398 B	678,492,530 B	+132 B

The single baseline rebuild was 8.55 s; final post-change samples were 6.78 s, 6.54 s, and 6.44 s. Timing is noisy, so this PR does not claim a compile-time improvement.

Testing

• cmake --build build/Windows/RelWithDebInfo --config RelWithDebInfo --target onnxruntime_test_all --parallel 8
• onnxruntime_test_all.exe --gtest_filter=CpuKernelRegistrationTest.*:KernelRegistryTests.* from the test output directory: 9 tests from 2 suites passed.
• lintrunner -a on all three changed files: no lint issues.

[Copilot]

Pull request overview

This PR makes malformed CPU kernel factory tables a compile-time error by adding a consteval validation helper and applying it to the CPU EP’s ONNX, optional FP16, and ONNX-ML registration tables. It also adds compile-time unit tests for the validation logic plus a small runtime sanity test that the real CPU kernel registry is non-empty.

Changes:

• Add cpu::registration_internal::IsKernelRegistrationTableValid (consteval) to enforce that the disabled BuildKernelCreateInfo<void> entry is first and that no table entries are null.
• Mark the CPU EP’s ONNX / FP16 / ONNX-ML function_table arrays as constexpr and add static_assert validation for each.
• Add unit tests covering valid/invalid tables and a runtime check that the default CPU kernel registry initializes non-empty.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
onnxruntime/test/framework/cpu_kernel_registration_test.cc	Adds compile-time table-validation tests plus a runtime non-empty registry check.
onnxruntime/core/providers/cpu/cpu_kernel_registration.h	Introduces a consteval helper to validate kernel registration tables at compile time.
onnxruntime/core/providers/cpu/cpu_execution_provider.cc	Makes the registration tables constexpr and enforces compile-time invariants via static_assert.