MDEV-39490 Inline the disabled fast path of Opt_trace_start::init()

[iMineLink]

Optimizer trace is disabled by default, yet init() was out of line, so every statement paid a call plus a full prologue/epilogue only to test FLAG_ENABLED and return.

Move the FLAG_ENABLED test into an inline init() so the disabled path collapses to a flag load and a predicted-not-taken branch. The enabled path setup stays out of line in the new init_traceable(). Behaviour and the init() signature are unchanged.

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[Copilot]

Pull request overview

This PR optimizes the common “optimizer trace disabled” path by inlining Opt_trace_start::init() so most statements avoid an out-of-line call overhead, while keeping the enabled/traceable setup in a separate out-of-line helper.

Changes:

• Inlined Opt_trace_start::init() in sql/opt_trace.h with a fast FLAG check and delegation to the slow path.
• Introduced Opt_trace_start::init_traceable() to hold the enabled-path setup previously in init().
• Updated sql/opt_trace.cc accordingly by moving the remaining traceability checks and setup into init_traceable().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sql/opt_trace.h	Inlines Opt_trace_start::init() to make the disabled trace path a cheap inline flag check.
sql/opt_trace.cc	Adds init_traceable() implementation to keep the enabled-path work out of line.

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[gemini-code-assist]

Code Review

This pull request refactors Opt_trace_start::init to optimize the common case where optimizer tracing is disabled. It inlines a fast-path check in sql/opt_trace.h and moves the full context setup to a new slow-path helper, init_traceable, in sql/opt_trace.cc. The feedback suggests removing the redundant assignment traceable = FALSE; inside the inlined init function, as traceable is already initialized to false in the constructor, which would further optimize the fast path by avoiding an unnecessary store instruction.

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[mariadb-RexJohnston]

Other than maybe chucking in an inline hint in the object definition, this looks fine.

[iMineLink]

:D right, the inline hint would have made the intent more explicit, but it should be redundant since the method definition is provided inside the class definition in the header. Thanks for reviewing.