JIT: Fix heap allocations

[jakobbotsch]

Also override the global new operators to assert unconditionally, to catch this common pitfall proactively.
Ideally that would be a compile time diagnostic, but I could not figure out how to make that happen.

Fix #120285

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[SingleAccretion]

In the single-file host, the Jit is going to be linked statically with the rest of the native code. Is the fact this override is not causing problems in that case by construction or accidental?

[jakobbotsch]

In the single-file host, the Jit is going to be linked statically with the rest of the native code. Is the fact this override is not causing problems in that case by construction or accidental?

Accidental. Let me hide it under a define set for the standalone jits.

[jakobbotsch]

PTAL @dotnet/jit-contrib. I want to backport this to GA, but might not backport the "diagnostic" new override.

[Copilot]

Pull Request Overview

This PR aims to eliminate unintended heap allocations in the JIT by routing allocations through the JIT's custom allocators and by asserting on any accidental use of the global operator new in standalone builds.

• Replaces raw global new usages with allocator-aware placement new for LSRA structures.
• Introduces overridden global operator new/delete (guarded by JIT_STANDALONE_BUILD) that assert to catch unintended allocations.
• Adds a build define to enable the new checks.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
src/coreclr/jit/lsra.cpp	Switches allocations to use compiler memory kinds via placement new.
src/coreclr/jit/alloc.cpp	Adds global operator new/delete overrides to assert on unintended allocations.
src/coreclr/jit/CMakeLists.txt	Defines JIT_STANDALONE_BUILD to enable the new allocation assertions.

[EgorBo]

LGTM assuming CI passes

[jakobbotsch]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/18188174550