Minor optimization of the LOBPCG solver

src/eigen/lobpcg_hyper_impl.jl

@@ -76,29 +76,37 @@
     (n, m)
 end
 
-Base.Array(A::LazyHcat) = stack(A.blocks)
-
+Base.Array(A::LazyHcat)   = stack(A.blocks)
 Base.adjoint(A::LazyHcat) = Adjoint(A)
 
-@views function Base.:*(Aadj::Adjoint{T,<:LazyHcat}, B::LazyHcat) where {T}
-    A = Aadj.parent
-    rows = size(A)[2]
-    cols = size(B)[2]
-    ret = similar(A.blocks[1], rows, cols)
-
-    orow = 0  # row offset
-    for blA in A.blocks
-        ocol = 0  # column offset
-        for blB in B.blocks
+# Computes A*B matrix product for LazyHcat type. Special case if product is assumed to be Hermitian
+@views function _mul(A::Adjoint{T,<:LazyHcat}, B::LazyHcat; hermitian=Val(false)) where {T}
+    Ap = A.parent
+    rows = size(Ap, 2)
+    cols = size(B, 2)
+    ret = similar(B.blocks[1], rows, cols)
+
+    # Only popuplate the upper block diagonal in Hermitian case
+    ocol = 0  # column offset
+    for (ib, blB) in enumerate(B.blocks)
+        orow = 0  # row offset
+        for (ia, blA) in enumerate(Ap.blocks)
+            (hermitian isa Val{true} && ib < ia) && continue
             ret[orow .+ (1:size(blA, 2)), ocol .+ (1:size(blB, 2))] .= blA' * blB
-            ocol += size(blB, 2)
+            orow += size(blA, 2)
         end
-        orow += size(blA, 2)
+        ocol += size(blB, 2)
+    end
+
+    if hermitian isa Val{true}
+        Hermitian(ret)
+    else
+        ret
     end
-    ret
 end
 
-Base.:*(Aadj::Adjoint{T,<:LazyHcat}, B::AbstractMatrix) where {T} = Aadj * LazyHcat(B)
+Base.:*(A::Adjoint{T,<:LazyHcat}, B::LazyHcat) where {T}       = _mul(A, B)
+Base.:*(A::Adjoint{T,<:LazyHcat}, B::AbstractMatrix) where {T} = A * LazyHcat(B)
 
 @views function *(Ablock::LazyHcat, B::AbstractMatrix)
     res = Ablock.blocks[1] * B[1:size(Ablock.blocks[1], 2), :]  # First multiplication
@@ -115,11 +123,16 @@
     mul!(res, Ablock*B, I, α, β)
 end
 
+mul_hermi(A, B) = Hermitian(A * B)
+function mul_hermi(A::Adjoint{T,<:LazyHcat}, B::LazyHcat) where {T}
+    _mul(A, B; hermitian=Val(true))
+end
+
 # Perform a Rayleigh-Ritz for the N first eigenvectors.
 @timing function rayleigh_ritz(X, AX, N)
-    XAX = X' * AX
-    @assert !any(isnan, XAX)
-    rayleigh_ritz(Hermitian(XAX), N)
+    XAX = mul_hermi(X', AX)
+    @assert !any(isnan, UpperTriangular(parent(XAX)))
+    rayleigh_ritz(XAX, N)
 end
 @views function rayleigh_ritz(XAX::Hermitian, N)
     # Fallback: Use whatever is the default dense eigensolver.
@@ -149,7 +162,7 @@
 # (which implies that X'BX is relatively well-conditioned, and
 # therefore that it is safe to cholesky it and reuse the B apply)
 function B_ortho!(X, BX)
-    O = Hermitian(X'*BX)
+    O = mul_hermi(X', BX)
     U = cholesky(O).U
     @assert !any(isnan, U)
     rdiv!(X, U)
@@ -173,7 +186,7 @@
     success = false
     nchol = 0
     while true
-        O = Hermitian(X'X)
+        O = mul_hermi(X', X)
         try
             R = cholesky(O).U
             nchol += 1

test/lobpcg.jl

@@ -124,6 +124,7 @@ end
 @testitem "LOBPCG Internal data structures" setup=[TestCases] begin
     using DFTK
     using LinearAlgebra
+    import DFTK: mul_hermi
 
     a1 = rand(10, 5)
     a2 = rand(10, 2)
@@ -142,4 +143,6 @@ end
 
     D = rand(10, 4)
     @test mul!(D,Ablock, C, 1, 0) ≈ A*C
+
+    @test mul_hermi(Ablock', Ablock) ≈ A' * A
 end