Batch no-run container reads during decoding

[perfloop-agent]

Description

Batch no-run container payload reads during decoding from ordinary io.Reader values. The decoder now coalesces exact whole-container payloads without consuming bytes after the bitmap. It also keeps bitmap payloads aligned when an array payload precedes it in a shared read.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Performance improvement
• Test improvements

Changes Made

What was changed?

• Batch no-run payloads in roaringArray.readFrom using cardinalities from the descriptive header, with a bounded group size and exact container slices.
• Keep direct ByteInput, run-container, and copy-on-write paths on their existing behavior.
• End a group before a bitmap when its current byte offset is not 8-byte aligned, and add regression coverage for an array container followed by a bitmap container.
• Add counting-reader benchmark coverage for sparse files and short reads.

Why was it changed?

• Avoid one small reader operation per sparse no-run container for ordinary reader inputs.
• Preserve the caller's stream boundary and the alignment required by bitmap containers backed by []uint64.

How was it changed?

• Calculate each payload length from the key/cardinality header and read groups only at serialized container boundaries.
• Install bounded sub-slices for each container and start bitmap payloads in aligned groups.

Testing

• go test ./...
• go test -tags appengine
• go tool unconvert ./...
• GOARCH=386 go build
• GOARCH=arm go build
• GOARCH=arm64 go build

Checks: 8 passed.

Formatting

• go fmt ./...

Fuzzing

• go test -tags=gofuzz -run=TestGenerateSmatCorpus
• go test -run='^$' -fuzz=FuzzSmat -fuzztime=300s -timeout=60s

Performance Impact

The sparse-reader benchmark covers unbuffered file and short-read reader cases. The existing BenchmarkUnserializeReadFrom row remains a guard for the ordinary bytes.Reader decode path.

Workload: Bitmap.ReadFrom of a no-run sparse bitmap with 1024 singleton containers from an unbuffered file

Metric	Before	After	Change
read-calls/op	1028	5	99.5% lower

Workload: Bitmap.ReadFrom of a no-run sparse bitmap with 4096 singleton containers from an unbuffered file

Metric	Before	After	Change
read-calls/op	4100	5	99.9% lower

Workload: Bitmap.ReadFrom of a no-run sparse bitmap with 16384 singleton containers from an unbuffered file

Metric	Before	After	Change
read-calls/op	16388	5	100% lower

Workload: Bitmap.ReadFrom of a no-run sparse bitmap with 1024 singleton containers through a 257-byte short-read file reader

Metric	Before	After	Change
read-calls/op	1058	42	96% lower

Workload: Bitmap.ReadFrom of a no-run sparse bitmap with 4096 singleton containers through a 257-byte short-read file reader

Metric	Before	After	Change
read-calls/op	4226	162	96.2% lower

Workload: Bitmap.ReadFrom of a no-run sparse bitmap with 16384 singleton containers through a 257-byte short-read file reader

Metric	Before	After	Change
read-calls/op	16898	642	96.2% lower

Breaking Changes

None.

Related Issues

None.

Additional Notes

No public API changes.

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