Poorman's 5-arg mul! (#723)

Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>

Author: oxinabox

Project.toml

@@ -1,6 +1,6 @@
 name = "Compat"
 uuid = "34da2185-b29b-5c13-b0c7-acf172513d20"
-version = "3.18.0"
+version = "3.19.0"
 
 [deps]
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"

README.md

@@ -55,6 +55,8 @@ changes in `julia`.
 
 ## Supported features
 
+* `mul!(C, A, B, alpha, beta)` now performs in-place multiply add ([#29634]). (since Compat 3.19).
+
 * `reinterpret(reshape, T, a::AbstractArray{S})` reinterprets `a` to have eltype `T` while
   inserting or consuming the first dimension depending on the ratio of `sizeof(T)` and `sizeof(S)`.
   ([#37559]). (since Compat 3.18)
@@ -208,3 +210,4 @@ Note that you should specify the correct minimum version for `Compat` in the
 [#37244]: https://github.com/JuliaLang/julia/pull/37244
 [#37517]: https://github.com/JuliaLang/julia/pull/37517
 [#37559]: https://github.com/JuliaLang/julia/pull/37559
+[#29634]: https://github.com/JuliaLang/julia/pull/29634

src/Compat.jl

@@ -739,6 +739,29 @@ if VERSION < v"1.6.0-DEV.1083"
     end
 end
 
+if VERSION < v"1.3.0-alpha.115"
+    # https://github.com/JuliaLang/julia/pull/29634
+    # Note this is much less performant than real 5-arg mul!, but is provided so old versions of julia don't error at least
+
+    function _mul!(C, A, B, alpha, beta)
+        Y = similar(C)
+        LinearAlgebra.mul!(Y, A, B)
+        C .= Y .* alpha .+ C .* beta
+        return C
+    end
+
+    # all combination of Number and AbstractArray for A and B except both being Number
+    function LinearAlgebra.mul!(C::AbstractArray, A::Number, B::AbstractArray, alpha::Number, beta::Number)
+        return _mul!(C, A, B, alpha, beta)
+    end
+    function LinearAlgebra.mul!(C::AbstractArray, A::AbstractArray, B::Number, alpha::Number, beta::Number)
+        return _mul!(C, A, B, alpha, beta)
+    end
+    function LinearAlgebra.mul!(C::AbstractArray, A::AbstractArray, B::AbstractArray, alpha::Number, beta::Number)
+        return _mul!(C, A, B, alpha, beta)
+    end
+end
+
 include("iterators.jl")
 include("deprecated.jl")
 

test/runtests.jl

@@ -679,6 +679,25 @@ end
     @test sum(ars) == 8010003
 end
 
+# https://github.com/JuliaLang/julia/pull/29634
+@testset "5-arg mul!" begin
+    A = [1.0 2.0; 3.0 4.0]
+    B = [10.0, 2.0]
+    C = [100.0, 1000.0]
+    x = -20.0
+    alpha = 0.1
+    beta = 0.1
+
+    Cmut = copy(C)
+    @test  Cmut == mul!(Cmut, A, B, alpha, beta) ≈ ((A * B * alpha) + (C * beta))
+
+    Cmut = copy(C)
+    @test  Cmut == mul!(Cmut, x, B, alpha, beta) ≈ ((x * B * alpha) + (C * beta))
+
+    Cmut = copy(C)
+    @test  Cmut == mul!(Cmut, B, x, alpha, beta) ≈ ((B * x * alpha) + (C * beta))
+end
+
 include("iterators.jl")
 
 nothing