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Reduce the fixed cost of the mapreduce kernel #97

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Reduce the fixed cost of the mapreduce kernel #97

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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
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48 changes: 26 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
$(outerreturnstrideex[i])
end
Expand All @@ -545,7 +540,16 @@ end
strides::NTuple{M, NTuple{N, Int}},
offsets::NTuple{M, Int}
) where {N, M}
return _mapreduce_kernel_expr(f, op, initop, N, M)
op === Nothing || return _mapreduce_kernel_expr(f, op, initop, N, M)
# select the unit-stride variant of the innermost loop once, instead of in every iteration
unitcond = Expr(:call, :all, :(==(1)), Expr(:tuple, (:(strides[$j][1]) for j in 1:M)...))
return 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
end

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