Use AVX2 for dense in-place XOR

[perfloop-agent]

Description

Use AVX2 for the high-cardinality bitmap-container XOR store pass and add dense XOR coverage.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement
• Code refactoring
• Documentation update
• Test improvements
• Build/CI changes

Changes Made

What was changed?

• Added a runtime-dispatched AVX2 in-place XOR helper for dense bitmap containers.
• Added dense XOR benchmarks and tests for bitmap, threshold, copy-on-write, AVX2, and scalar fallback paths.

Why was it changed?

• The bitmap-result path counted XOR cardinality with AVX2, then rewrote the same container one word at a time.

How was it changed?

• Use unaligned AVX2 loads, XORs, and stores on supported amd64 CPUs; retain the Go loop for appengine, non-AVX2 amd64, and arm64.
• Leave cardinality counting and array-result conversion unchanged.

Testing

• go test ./...
• go test -tags=appengine
• GOARCH=arm64 go build .
• GOARCH=386 go build .
• test -z "$(gofmt -s -l $(find . -name '*.go' -type f))" && go tool unconvert ./...

Checks: 5 passed.

Formatting

• Checked with gofmt -s -l as part of the formatting check above.

Fuzzing

• Not run; this only changes fixed-width bitmap XOR storage.

Performance Impact

The AVX2 path is used only for the high-cardinality bitmap result on supported amd64 CPUs. Other paths retain the portable store loop.

Workload: Bitmap.Xor on 50 dense matching containers

Metric	Before	After	Change
ns/op	65345	21612	66.9% lower

Workload: Bitmap.Xor with 1 matching dense container on amd64 AVX2

Metric	Before	After	Change
ns/op	1250	374.9	70% lower

Workload: Bitmap.Xor with 50 matching dense containers on amd64 AVX2

Metric	Before	After	Change
ns/op	64389	20054	68.9% lower

Workload: Bitmap.Xor with 256 matching dense containers on amd64 AVX2

Metric	Before	After	Change
ns/op	383903	157206	59.1% lower

Breaking Changes

• None.

Related Issues

• None.

Additional Notes

• None.

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