Regression for `mul!` from 1.9 to 1.10

[gdalle]

This is about in-place multiplication mul!(b, A, x) of a sparse matrix A by a vector x.
From 1.9.3 to 1.10.0-rc1, this operation

• has gotten significantly slower
• has started allocating when x is a view

MWE

using BenchmarkTools, LinearAlgebra, SparseArrays

function testmul(n)
    A = sparse(Float64, I, n, n)
    b = Vector{Float64}(undef, n)
    @btime mul!($b, $A, x) setup=(x=ones($n))
    @btime mul!($b, $A, x) setup=(x=view(ones($n, 1), :, 1))
    return nothing
end

testmul(1000)

Results

	Julia 1.9	Julia 1.10
x vector	2.135 μs (0 allocations)	3.298 μs (0 allocations)
x view	2.502 μs (0 allocations)	4.087 μs (1 allocation)

[dkarrasch]

Could you please investigate whether this is taking a different path then what it used to take? It could be that we missed some dispatch...

[gdalle]

First investigations on Discourse: https://discourse.julialang.org/t/why-does-mul-u-a-v-allocate-when-a-is-sparse-and-u-v-are-views/105995/10?u=gdalle

[gdalle]

Investigating the vector case first, and indeed the chain of functions has changed a lot.

In Julia 1.9, the call stack goes like this:

mul!(C, A, B)
mul!(C::StridedVecOrMat, A::AbstractSparseMatrixCSC, B::DenseInputVecOrMat, α::Number, β::Number)

In Julia 1.10, the call stack goes like that:

mul!(C, A, B)
mul!(y::AbstractVector, A::AbstractVecOrMat, x::AbstractVector, alpha::Number, beta::Number)
generic_matvecmul!(C::StridedVecOrMat, tA, A::SparseMatrixCSCUnion, B::DenseInputVector, _add::MulAddMul)
spdensemul!(C, tA, tB, A, B, _add)
_spmatmul!(C, A, B, α, β)

[dkarrasch]

I have absolutely no idea what's going on. When I compare, on current master I get

julia> using BenchmarkTools, LinearAlgebra, SparseArrays

julia> n = 1000;

julia> A = sparse(Float64, I, n, n);

julia> b = Vector{Float64}(undef, n);

julia> x = ones(n);

julia> @btime SparseArrays._spmatmul!($b, $A, $x, true, false);
  3.317 μs (0 allocations: 0 bytes)

julia> @btime mul!($b, $A, $x);
  3.311 μs (0 allocations: 0 bytes)
 1.568 μs (0 allocations: 0 bytes) # on v1.9

where _spmatmul! contains the multiplication kernel only (no character and dispatch overhead). So, it doesn't seem to be related to the changes in method dispatch AFAICT.

[gdalle]

That's a bad regression, right? Maybe an issue on the Julia repo is in order if SparseArrays is not to blame?

[dkarrasch]

Yes. It's also not clear to me why the rewrap works without allocation in the plain vector case, but allocates in the view case. Both run by the same code line.

[jishnub]

The allocation is fixed on master now

julia> testmul(1000)
  3.072 μs (0 allocations: 0 bytes)
  3.441 μs (0 allocations: 0 bytes)

julia> VERSION
v"1.11.0-DEV.984"

I suspect this is the fix.

[ViralBShah]

Is this resolved now?

[jishnub]

The regression is still present. Here's what I see:

julia> testmul(1000)
  2.148 μs (0 allocations: 0 bytes)
  2.178 μs (0 allocations: 0 bytes)

julia> VERSION
v"1.9.4"

vs

julia> testmul(1000)
  2.704 μs (0 allocations: 0 bytes)
  3.025 μs (0 allocations: 0 bytes)

julia> VERSION
v"1.12.0-DEV.301"

[ViralBShah]

@vtjnash @oscardssmith Any ideas what is going on here?