perf(gnovm): avoid heap-boxed byte access in copy, range and index reads

[thehowl]

Note

Part 2 of 6 of the gnovm performance stack (split from #5800), to be merged in order:

1. perf(gnovm): parallelize test suites and add gno test -jobs #5811 — perf(gnovm): parallelize test suites and add gno test -jobs
2. perf(gnovm): avoid heap-boxed byte access in copy, range and index reads #5812 — perf(gnovm): avoid heap-boxed byte access in copy, range and index reads
3. perf(gnovm): recycle runtime blocks through a per-machine pool #5813 — perf(gnovm): recycle runtime blocks through a per-machine pool
4. perf(gnovm): share interface-held values when copying arrays #5814 — perf(gnovm): share interface-held values when copying arrays (gas-visible)
5. ci(gnovm): skip print-only coverage instrumentation #5815 — ci(gnovm): skip print-only coverage instrumentation
6. perf(gnovm): reduce per-call and per-op allocations #5816 — perf(gnovm): reduce per-call and per-op allocations

Each PR is based on the previous one's branch; this one diffs against part 1. Together: ci / gnovm ~14m → 6m18s; pkg/gnolang test time −64%; VM heap allocations −84% on the heaviest suite.

Summary

Profiling the gnovm test suites (dominated by interpreted Gno) showed ~50% of CPU in Go GC/malloc, with 67% of all heap allocations (694M objects in the bytes stdlib suite alone) coming from ArrayValue.GetPointerAtIndexInt2 materializing a heap *TypedValue + boxed DataByteValue per byte accessed:

• the copy() builtin allocated two boxes per byte copied (554M objects; the code carried a TODO: consider an optimization if dstv.Data != nil);
• for i, c := range byteslice allocated three objects per iteration (the index TypedValue escaping through GetPointerAtIndex(&iv), plus the view box) that Deref immediately discarded;
• b[i] reads in doOpIndex1 did the same box-then-Deref dance.

This adds TypedValue.GetValueAtIntIndex — a read-only fast path mirroring GetPointerAtIndex's checks and panics for strings and Data-backed arrays/slices — used by doOpIndex1 and the range loop, and gives copy() direct byte copies when both sides are Data-backed (or the source is a string).

Gas is unchanged: the view boxes were raw Go allocations never charged to the VM allocator, and CPU gas for copy() was already charged before the per-element loops. Verified empirically: all 2344 filetest goldens byte-identical (including Gas: and MAXALLOC-sensitive alloc tests), gno.land/pkg/sdk/vm gas tests, txtar suite, examples, cmd/gno.

Measurements

	before	after
TestStdlibs/bytes solo	151.5s	105.2s (−31%)
heap objects allocated (bytes suite)	1.03G	0.30G (−71%)
BenchmarkOpIndex1_ByteArray	185.7ns	130.0ns
full pkg/gnolang long mode, 16 cores	245.0s	184.6s

Untouched paths benchmark flat (OpRangeIter_1000 30.9µs → 30.2µs, OpIndex1_MapHit_100 187.0ns → 187.7ns). Byte writes (b[i] = x) still box — the pointer protocol spans multiple ops; left as follow-up.

[Gno2D2]

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