go / expert
Snippet
Guaranteed Compiler Bounds Check Elimination in Slices through Range Anchor Assertions
The Go compiler inserts runtime bounds checks before every slice index access to prevent memory corruption. By explicitly referencing the maximum index once ('_ = data[3]'), the compiler's Bounds Check Elimination (BCE) pass proves that data is at least 4 bytes long, allowing it to safely eliminate all subsequent bounds checks for index 0, 1, 2, and 3 in the generated assembly.
snippet.go
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package fastprocessfunc ProcessFourByteBlocks(data []byte) int {if len(data) < 4 {return 0}_ = data[3]b0 := uint32(data[0])b1 := uint32(data[1]) << 8b2 := uint32(data[2]) << 16b3 := uint32(data[3]) << 24return int(b0 | b1 | b2 | b3)}
Breakdown
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if len(data) < 4 { return 0 }
Ensures runtime safety before establishing explicit bounds assertions.
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_ = data[3]
Acts as a BCE hint assertion, proving to the compiler SSA compiler pass that len(data) >= 4.
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b0 := uint32(data[0])
Executes direct memory lookup without emitting instruction-level bounds check branches.