Ensure that all-zeros rhs gives all-zeros LHS

src/cg.jl

@@ -7,14 +7,18 @@ export cg, cg!
 cg(A, b; kwargs...) = cg!(zerox(A,b), A, b; kwargs...)
 
 function cg!(x, A, b;
-    tol::Real=size(A,2)*eps(), maxiter::Integer=size(A,2),
-    plot=false, log::Bool=false, kwargs...
-    )
+             tol::Real=size(A,2)*eps(), maxiter::Integer=size(A,2),
+             plot=false, log::Bool=false, kwargs...
+             )
     (plot & !log) && error("Can't plot when log keyword is false")
     K = KrylovSubspace(A, length(b), 1, Vector{Adivtype(A,b)}[])
     init!(K, x)
     history = ConvergenceHistory(partial=!log)
     history[:tol] = tol
+    if all(el->el==0, b)
+        fill!(x, zero(eltype(x)))
+        return log ? (x, history) : x
+    end
     reserve!(history,:resnorm, maxiter)
     cg_method!(history, x, K, b; tol=tol, maxiter=maxiter, kwargs...)
     (plot || log) && shrink!(history)

test/cg.jl

@@ -28,7 +28,11 @@ context("Small full system") do
     F = cholfact(A)
     x2,ch2 = cg(A, rhs; Pl=F, log=true)
     @fact niters(ch2) --> less_than_or_equal(2)
-    @fact nprods(ch2) --> less_than_or_equal(2) 
+    @fact nprods(ch2) --> less_than_or_equal(2)
+
+    # All-zeros rhs should give all-zeros lhs
+    x0 = cg(A, zeros(N))
+    @fact x0 --> zeros(N)
 end
 
 context("Sparse Laplacian") do