[vbtcl]

ElementSelectExpression, RangeSelectExpression and MemberAccessExpression evaluate their value operand with value().eval(). For a local variable, that returns a copy of the entire variable. Each read of one element of a large local array or packed vector in a constant function copies the whole local. Hsiao ECC matrix generation, for example, reads single bits of a 20,880-bit packed local and single elements of a 22,800-element unpacked local in its inner loops.

When the value operand names a local of the current frame, the select now reads the variable in place, through the new NamedValueExpression::evalLocalRef. This only happens when every selector is side-effect free: literals, names, unary operators other than ++/--, binary operators, conversions, and nested selects of those. Any other selector keeps the copy, so the value is still read before the selector's side effects (a[a[0]++] reads the old a). checkConstant diagnostics are issued exactly as before.

Measured with the boson native-elab driver setup, getAllDiagnostics, 1 thread:

design	before	this PR	with #2 (memo)	both
astral	112.3 s	7.8 s	8.6 s	2.2 s
carfield	27.5 s	3.2 s	4.4 s	2.0 s

The rendered diagnostics are byte-identical in every column. The new EvalTests.cpp case covers unpacked, packed and struct selects of locals, a side-effecting selector, and an out-of-bounds read. The full unittests pass: 22082 assertions in 2204 test cases.

Ships to boson as scripts/patches/slang/0011-read-locals-in-place-in-const-eval-selects.patch (partcleda/boson#13187).

🤖 Generated with Claude Code

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