FAQ
No real surprises with the no bounds checks option (-B), it just eliminated the array bounds checks with the rest of the code the same (version 1.7beta1):

  0x00dd 00221 (main.go:37) MOVQ DI, CX
  0x00e0 00224 (main.go:37) SHRQ $5, DI
  0x00e4 00228 (main.go:37) MOVL (AX)(DI*4), R9
  0x00e8 00232 (main.go:37) MOVQ CX, R10
  0x00eb 00235 (main.go:37) ANDQ $31, CX
  0x00ef 00239 (main.go:37) MOVL R8, R11
  0x00f2 00242 (main.go:37) SHLL CX, R8
  0x00f5 00245 (main.go:37) ORL R8, R9
  0x00f8 00248 (main.go:37) MOVL R9, (AX)(DI*4)
  0x00fc 00252 (main.go:36) LEAQ 3(R10)(SI*2), DI
  0x0101 00257 (main.go:37) MOVL R11, R8
  0x0104 00260 (main.go:36) CMPQ DI, DX
  0x0107 00263 (main.go:36) JLS $0, 221

It is now almost as fast as C/C++ code, and isn't for the same reasons as explained before: excessively using registers to store things and not using the read/modify/write instruction (which also saves the use of a register).

The current beta will work not too badly with amd64 code but still doesn't use registers efficiently enough to support x86 code as it uses too many register. optimized C/C++ code only uses six or at most 7 registers, which the x86 architecture has, but not the nine registers that the above requires.

So for this tight loop, golang is still slower than optimized C/C++ code, but not by very much if array bounds checks are disabled.

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