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Forward to a lower-dimensional kernel for trailing singleton dimensions #98

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1fa9461
Use while loops for the block loops in the kernel
lkdvos Oct 29, 2026
38b51de
Hoist the unit-stride check out of the loop nest for non-reducing ker…
lkdvos Oct 29, 2026
65820a4
Forward to a lower-dimensional kernel for trailing singleton dimensions
lkdvos Oct 29, 2026
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2 changes: 1 addition & 1 deletion Project.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -32,7 +32,7 @@ LinearAlgebra = "1.6"
Metal = "1.9"
PrecompileTools = "1"
Random = "1.6"
StridedViews = "0.5.1"
StridedViews = "0.5.5"
Test = "1.6"
TupleTools = "1.6"
julia = "1.10"
Expand Down
85 changes: 63 additions & 22 deletions src/mapreduce.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -297,7 +297,7 @@ function _mapreduce_threaded!(
return nothing
end

function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int)
function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int; unit::Bool = false)
@assert N > 0 "Cannot generate 0-dim kernel"

# many variables
Expand Down Expand Up @@ -388,28 +388,19 @@ function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int)
i = 1
if op == Nothing
# Non-reducing (map!/permute/...): each destination element is written once,
# so skip the reduction-only `iszero` hoist below and emit just the
# unit-stride and generic `@simd` loops.
# so skip the reduction-only `iszero` hoist below. Whether the innermost loop
# has unit stride is loop invariant, and is selected by the caller via `unit`.
ex = Expr(:(=), lhsex, fcallex)
if N >= 1
step1ex, step2ex = unit ? (unitstep1ex, unitstep2ex) : (stepstride1ex[i], stepstride2ex[i])
ex = quote
if $unitstridecond
@simd for $(innerloopvars[i]) in Base.OneTo($(blockdimvars[i]))
$ex
$unitstep1ex
$unitstep2ex
end
$(returnstride1ex[i])
$(returnstride2ex[i])
else
@simd for $(innerloopvars[i]) in Base.OneTo($(blockdimvars[i]))
$ex
$(stepstride1ex[i])
$(stepstride2ex[i])
end
$(returnstride1ex[i])
$(returnstride2ex[i])
@simd for $(innerloopvars[i]) in Base.OneTo($(blockdimvars[i]))
$ex
$step1ex
$step2ex
end
$(returnstride1ex[i])
$(returnstride2ex[i])
end
end
else
Expand Down Expand Up @@ -497,10 +488,12 @@ function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int)
if initop === Nothing
for outer i in 1:N
ex = quote
for $(blockloopvars[i]) in 1:blocks[$i]:dims[$i]
$(blockloopvars[i]) = 1
while $(blockloopvars[i]) <= dims[$i]
$(blockdimvars[i]) = min(blocks[$i], dims[$i] - $(blockloopvars[i]) + 1)
$ex
$(outerstepstrideex[i])
$(blockloopvars[i]) += blocks[$i]
end
$(outerreturnstrideex[i])
end
Expand All @@ -516,11 +509,13 @@ function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int)
for outer i in 1:N
ex = quote
$(initvars[i]) = $(initvars[i + 1])
for $(blockloopvars[i]) in 1:blocks[$i]:dims[$i]
$(blockloopvars[i]) = 1
while $(blockloopvars[i]) <= dims[$i]
$(blockdimvars[i]) = min(blocks[$i], dims[$i] - $(blockloopvars[i]) + 1)
$ex
$(initvars[i]) &= $(stridevars[i, 1]) > 0
$(outerstepstrideex[i])
$(blockloopvars[i]) += blocks[$i]
end
$(outerreturnstrideex[i])
end
Expand All @@ -537,6 +532,42 @@ function _mapreduce_kernel_expr(f, op, initop, N::Int, M::Int)
return ex
end

# Trailing singleton dimensions, e.g. those left by fusing dimensions, still cost a loop each.
# Forward to the kernel of lower dimensionality instead, with the views rewrapped such that
# this kernel is shared with arrays that have that dimensionality to begin with.
function _mapreduce_trim_expr(N::Int, ex)
N > 1 || return ex
for k in 1:(N - 1)
ex = quote
if nactive == $k
dk = _firstk(dims, Val($k))
sk = _firstk(strides, Val($k))
return _mapreduce_kernel!(
f, op, initop, dk, _firstk(blocks, Val($k)), _rewrap(arrays, dk, sk), sk, offsets
)
end
$ex
end
end
return quote
nactive = _lastnonsingleton(dims)
$ex
end
end
_firstk(t::Tuple, ::Val{k}) where {k} = ntuple(i -> t[i], Val(k))
_firstk(ts::Tuple{Vararg{Tuple}}, v::Val) = map(t -> _firstk(t, v), ts)
function _lastnonsingleton(dims::Dims)
n = length(dims)
while n > 1 && dims[n] == 1
n -= 1
end
return n
end
# the kernel only indexes the parents, so the strides of the views need no normalization
_rewrap(arrays, dims, strides) = map(arrays, strides) do a, s
return StridedViews.unsafe_stridedview(eltype(a), parent(a), dims, s, offset(a), a.op)
end

@generated function _mapreduce_kernel!(
(f), (op),
(initop), dims::NTuple{N, Int},
Expand All @@ -545,7 +576,17 @@ end
strides::NTuple{M, NTuple{N, Int}},
offsets::NTuple{M, Int}
) where {N, M}
return _mapreduce_kernel_expr(f, op, initop, N, M)
op === Nothing || return _mapreduce_trim_expr(N, _mapreduce_kernel_expr(f, op, initop, N, M))
# select the unit-stride variant of the innermost loop once, instead of in every iteration
unitcond = Expr(:call, :all, :(==(1)), Expr(:tuple, (:(strides[$j][1]) for j in 1:M)...))
ex = quote
if $unitcond
$(_mapreduce_kernel_expr(f, op, initop, N, M; unit = true))
else
$(_mapreduce_kernel_expr(f, op, initop, N, M; unit = false))
end
end
return _mapreduce_trim_expr(N, ex)
end

function indexorder(strides::NTuple{N, Int}) where {N}
Expand Down
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