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bitoperations.jl's Issues

add bflip, mention Int32 bit indexing

It would be helpful to toggle bit[s].

v0.5:
@inline bflip(x::Integer, bit::Integer) = x $ bmask(typeof(x), bit)
@inline bflip{U <: Integer}(x::Integer, bits::UnitRange{U}) = x $ bmask(typeof(x), bits))
v0.6:
@inline bflip(x::Integer, bit::Integer) = xor(x, bmask(typeof(x), bit))
@inline bflip{U <: Integer}(x::Integer, bits::UnitRange{U}) = xor(x, bmask(typeof(x), bits))

Julia codes tighter when the bit, bits, nbits args are Int32 (or Int16 or Int8).

On a 64bit [32bit] system, bmask(Int32, 5) is bmask(Int32, 5::Int64) [bmask(Int32, 5::32)].

#                                                                               using bit::Int32

julia> code_llvm(bmask, (Type{Int32}, Int32))
define i32 @julia_bmask_62946(%jl_value_t*, i32) #0 !dbg !5 {
top:
  %2 = call i32 @"julia_<<_62947"(i32 1, i32 %1) #0
  ret i32 %2
}

julia> code_native(bmask, (Type{Int32}, Int32))
	.text
Filename: bitops.jl
	pushq	%rbp
	movq	%rsp, %rbp
Source line: 32
	movabsq	$"<<", %rax
	movl	$1, %edi
	callq	*%rax
	popq	%rbp
	retq
	nopw	(%rax,%rax)


#                                                                               using bit::Int64

julia> code_llvm(bmask, (Type{Int32}, Int64))
define i32 @julia_bmask_62948(%jl_value_t*, i64) #0 !dbg !5 {
top:
  %2 = icmp slt i64 %1, 0
  %3 = trunc i64 %1 to i32
  %4 = shl i32 1, %3
  %5 = icmp ugt i64 %1, 31
  %6 = select i1 %5, i32 0, i32 %4
  %7 = sub i64 0, %1
  %8 = trunc i64 %7 to i32
  %9 = lshr i32 1, %8
  %10 = icmp ugt i64 %7, 31
  %11 = select i1 %10, i32 0, i32 %9
  %12 = select i1 %2, i32 %11, i32 %6
  ret i32 %12
}

julia> code_native(bmask, (Type{Int32}, Int64))
	.text
Filename: bitops.jl
	pushq	%rbp
	movq	%rsp, %rbp
Source line: 32
	movl	$1, %eax
	movl	$1, %edx
	movl	%esi, %ecx
	shll	%cl, %edx
	xorl	%edi, %edi
	cmpq	$31, %rsi
	cmoval	%edi, %edx
	movq	%rsi, %rcx
	negq	%rcx
	shrl	%cl, %eax
	cmpq	$31, %rcx
	cmoval	%edi, %eax
	testq	%rsi, %rsi
	cmovnsl	%edx, %eax
	popq	%rbp
	retq
	nopw	%cs:(%rax,%rax)

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UnitRange is not entirely transparent on usage, and StepRanges don't work.

First issue: arrays start at one in julia, but using a range starting with one with bitops returns a range starting with the second bit.

julia> bits = 0b1010100010101100
0xa8ac

julia> bget(bits, 1:4)
0x0006

julia> bget(bits, 0:4)
0x000c

julia> (bitstring(bget(bits, 1:4)), bitstring(bget(bits, 0:4)))
("0000000000000110", "0000000000001100")

Additionally, StepRanges (declared with start:step:stop) do not work. This would be useful to have properly working, for xample with a backwards-stepped range, such as 16:-1:0

julia> bget(bits, 1:2:4)
ERROR: MethodError: no method matching bget(::UInt16, ::StepRange{Int64, Int64})

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