In AVX2 boolean Compress, jump from 8 to 32 with 32-bit shifts
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@ -293,8 +293,7 @@ def pext_width{} = if (hasarch{'AVX2'}) 4 else 1
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def pext_popc{x:T, m:T} = {
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def w = scalwidth{T}
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def scal{v} = if (isvec{T}) T**v else v
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def mod{a} = a % (1<<w)
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def lowbits{k} = base{1<<k, cdiv{w,k}**1}
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def lowbits{w,k} = base{1<<k, cdiv{w,k}**1}
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# At each step, x and z are split into groups of length k
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# - z tells how many bits in the group are NOT used
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# - x contains the bits, with z zeros above
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@ -302,7 +301,7 @@ def pext_popc{x:T, m:T} = {
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def build{k & k > 1} = {
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def h = k>>1 # Increase size from h to k
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{x,z} := build{h}
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def low_s = lowbits{k} # Low bit in each new group
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def low_s = lowbits{w,k} # Low bit in each new group
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def low = scal{low_s}
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if (k == 2) {
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z0 := z & low
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@ -322,7 +321,7 @@ def pext_popc{x:T, m:T} = {
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if (2*sh<k/2) shift{2*sh, o>>1, s} else s
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}
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# Shift high x group down by low z, then add halves of z
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odd:T = scal{mod{low_s*(1<<k - 1<<h)}} # Top half
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odd:T = scal{low_s*(1<<k - 1<<h)} # Top half
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ze := z&~odd
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z1 := ze + scal{low_s*(1<<(k-1) - 1)} # z-1, as signed k-bit
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move := odd &~ (z1<<1) # Only groups where z>0 move
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@ -332,16 +331,25 @@ def pext_popc{x:T, m:T} = {
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}
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}
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}
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# Finally, compose groups with regular shifts
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def g = 8
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def build{k & ~isvec{T} & k > g} = {
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{x,z} := build{g}
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o := z*lowbits{g} # Offsets by prefix sum
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# Compose k/g groups with k/g-1 regular shifts
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def multi_shift{x, z, g, k, sc} = {
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o := z * sc{lowbits{k,g}} # Offsets by prefix sum
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def s = 1<<g - 1
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def gr{sh} = (x & mod{s<<sh}) >> (o>>(sh-g) & s)
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pe := fold{|, x&s, each{gr, g*slice{iota{cdiv{w,g}},1}}}
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tup{pe, o>>(w-g)}
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def s0 = sc{s}
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def oo{sh} = if (sh==g) z else o>>(sh-g) # Offset for group
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def gr{sh} = (x & sc{s<<sh}) >> (oo{sh} & s0) # Shifted group
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pe := fold{|, x&s0, each{gr, g*slice{iota{k/g},1}}}
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tup{pe, o>>(k-g)}
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}
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def build{k==32 & hasarch{'AVX2'} & isvec{T}} = {
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def S = re_el{ty_u{k}, T}
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def c{T,vs} = each{{v}=>T~~v, vs}
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c{T, multi_shift{...c{S, build{8}}, 8, k, {s}=>S**s}}
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}
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def build{k & ~isvec{T} & k > 8} = {
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multi_shift{...build{8}, 8, k, {s}=>s}
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}
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# Final result
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def {pe, z} = build{w}
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tup{pe, scal{w} - z}
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}
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