Read a constant left operand in place: no shared count bump per render (0149)

[rmorozov]

Requested by Ruslan · project thread

Before: in '<|start_header_id|>' + message['role'], the long-string constant on the left was copied on every evaluation because the right operand is not "pure". Each copy is an atomic increment and decrement on the constant's count, which every thread rendering the template shares. In a chat-style loop that count is contended: 4 threads rendered only 1.5x as fast as 1.

After: a ConstantExpression on the left is always read in place, because evaluating the right operand cannot change it. Nothing is borrowed beyond the expression, so no value can escape the template. The same loop now scales 3.6x on 4 threads (24k to 42k renders/s).

How: BinaryExpression's constructor sets m_leftByRef when the left node is a ConstantExpression, as well as when both operands are pure (src/expression_evaluator.cpp:469). I added six rows to ConstantSubstitutionTest with a constant left operand and an impure right one; they match Python Jinja2.

Not built: the static heap kind 0149 proposed. I measured it as a flag on ValueObject that intrusive_ptr_add_ref/release skip. It cost Render +0.3..+1.4% Ir on every case and gave no measurable MT gain on the suite. The suite copies at most 18 constants per render, and those copies come from 3 sites. The numbers, the sites and a reopen condition are in docs/tasks/0149 (status: done). The remaining constant copies are stores, such as list-literal items and macro arguments, and they must own what they hold.

Counts vs 5ce1786 (local count.py):

• Render: chat_llama -0.13%, chat_mistral -0.09%, chat_qwen -0.11%, html_autoescape -0.18%; every other case ±0.00%.
• Load: +0.00..+0.35%. The largest is expressions (+156 Ir), from one kind check per binary operator.

Roles: one verifier round with ASan+UBSan, the full suite and 102 templates compared with Python. It found no blocking issues, and its one nit (the comment wording) is fixed. Pushes red in CI: 0 so far.

The one mismatch it found, ~ with a Markup right operand under autoescape, is the same on base and already tracked as autoescape.env_on_concat_markup_var (0051).

Touches src/expression_evaluator.{h,cpp} (the comment and ctor lines only). 0158 is changing expression_evaluator.h in parallel; git merge-tree will be checked before merge.

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