Remove the need for allowscalar

perf/arraydiff.jl

@@ -68,10 +68,10 @@ trial; the timed block is just `MOI.eval_objective_gradient` plus a
 `CUDA.synchronize` when `gpu=true`.
 """
 function neural(
-    ::Type{T},
-    h::Int,
-    d::Int,
-    n::Int;
+    ::Type{T} = Float32,
+    h::Int = 4096,
+    d::Int = 13,
+    n::Int = 178;
     gpu::Bool = false,
 ) where {T<:Real}
     state = _build(T, h, d, n, gpu)
@@ -90,13 +90,7 @@ function neural(
     return @benchmark(
         begin
             if $gpu
-                # `@allowscalar` covers residual scalar leaves the BLOCK
-                # rewrite can't fold; the hot path is bulk.
-                CUDA.@allowscalar MOI.eval_objective_gradient(
-                    $state.evaluator,
-                    $g,
-                    $x,
-                )
+                MOI.eval_objective_gradient($state.evaluator, $g, $x)
                 CUDA.synchronize()
             else
                 MOI.eval_objective_gradient($state.evaluator, $g, $x)
@@ -111,13 +105,9 @@ function profile_gpu(; T = Float32, h = 4096, d = 13, n = 178)
     x = CUDA.CuVector{T}(vec(state.W1))
     g = CUDA.zeros(T, h * d)
     fill!(state.evaluator.backend.last_x, NaN)
-    CUDA.@sync CUDA.@allowscalar MOI.eval_objective_gradient(
-        state.evaluator,
-        g,
-        x,
-    )
+    CUDA.@sync MOI.eval_objective_gradient(state.evaluator, g, x)
     fill!(state.evaluator.backend.last_x, NaN)
-    return CUDA.@profile CUDA.@sync CUDA.@allowscalar MOI.eval_objective_gradient(
+    return CUDA.@profile CUDA.@sync MOI.eval_objective_gradient(
         state.evaluator,
         g,
         x,

src/mathoptinterface_api.jl

@@ -256,8 +256,10 @@ end
 # Forward-evaluate subexpressions and the residual at `x`.
 function _forward_pass_residual!(d::NLPEvaluator, x)
     for k in d.subexpression_order
+        _forward_eval(d.subexpressions[k], d, x)
+        # FIXME this assumes scalar output
         d.subexpression_forward_values[k] =
-            _forward_eval(d.subexpressions[k], d, x)
+            d.subexpressions[k].forward_storage[1]
     end
     _forward_eval(something(d.residual).expr, d, x)
     return

src/reverse_mode.jl

@@ -92,8 +92,10 @@ function _reverse_mode(d::NLPEvaluator, x)
     end
     # Phase I
     for k in d.subexpression_order
+        _forward_eval(d.subexpressions[k], d, x)
+        # FIXME this assumes scalar output
         d.subexpression_forward_values[k] =
-            _forward_eval(d.subexpressions[k], d, x)
+            d.subexpressions[k].forward_storage[1]
     end
     if d.objective !== nothing
         _forward_eval(something(d.objective).expr, d, x)
@@ -158,7 +160,7 @@ function _forward_eval(
     f::_SubexpressionStorage,
     d::NLPEvaluator,
     x::AbstractVector{T},
-)::T where {T}
+) where {T}
     @assert length(f.forward_storage) >= length(f.nodes)
     @assert length(f.partials_storage) >= length(f.nodes)
     operators = d.data.operators
@@ -633,10 +635,7 @@ function _forward_eval(
             f.partials_storage[rhs] = zero(T)
         end
     end
-    # Caller is responsible for reading the right range of `f.forward_storage`
-    # for vector-valued roots (use `_storage_range(f.sizes, 1)`); the scalar
-    # return is only meaningful when the root is scalar.
-    return f.forward_storage[1]
+    return
 end
 
 """
@@ -823,9 +822,9 @@ Reverse-mode evaluation of an expression tree given in `f`.
  * This function assumes that `f.reverse_storage` has been initialized with 0.0.
 """
 function _reverse_eval(
-    f::_SubexpressionStorage,
-    seed::Union{Nothing,AbstractVector{Float64}} = nothing,
-)
+    f::_SubexpressionStorage{T},
+    seed::Union{Nothing,AbstractVector{T}} = nothing,
+) where {T}
     @assert length(f.reverse_storage) >= _length(f.sizes)
     @assert length(f.partials_storage) >= _length(f.sizes)
     # f.nodes is already in order such that parents always appear before
@@ -834,14 +833,10 @@ function _reverse_eval(
     children_arr = SparseArrays.rowvals(f.adj)
     root_range = _storage_range(f.sizes, 1)
     if seed === nothing
-        for i in root_range
-            f.reverse_storage[i] = one(Float64)
-        end
+        f.reverse_storage[root_range] .= one(T)
     else
         @assert length(seed) == length(root_range)
-        for (j, i) in enumerate(root_range)
-            f.reverse_storage[i] = seed[j]
-        end
+        f.reverse_storage[root_range] .= seed
     end
     for k in 1:length(f.nodes)
         node = f.nodes[k]
@@ -1108,17 +1103,20 @@ function _reverse_eval(
                     elseif op == :^
                         # We start with just .^2 to simplify
                         @assert f.sizes.ndims[rhs] == 0 "Broadcasted ^ requires scalar exponent"
-                        exp = _getscalar(f.forward_storage, f.sizes, rhs)
-                        # To simplify, so we don't need to compute its derivative
+                        # If it is a constant, we can just read it from the `const_values` and avoid a GPU->CPU communication
+                        # We also don't need to compute its derivative
+                        @assert f.nodes[rhs].type == NODE_VALUE
+                        exponent = f.const_values[f.nodes[rhs].index]
+
                         @assert f.nodes[rhs].type == NODE_VALUE
                         rev_parent = _view_linear(f.reverse_storage, f.sizes, k)
                         rev_child =
                             _view_linear(f.reverse_storage, f.sizes, lhs)
-                        if exp == 2
+                        if exponent == 2
                             child =
                                 _view_linear(f.forward_storage, f.sizes, lhs)
                             rev_child .= 2 .* child .* rev_parent
-                        elseif exp == 1
+                        elseif exponent == 1
                             rev_child .= rev_parent
                         else
                             partial =

test/Optimisers_GPU.jl

@@ -50,11 +50,7 @@ function test_neural_optimisers_gpu()
     @objective(model, Min, loss)
     set_attribute(model, "max_iter", 20_000)
     set_attribute(model, "tol", 1e-6)
-    # The variable-load and gradient-extract paths still do scalar reads/writes
-    # against the GPU-resident tape (forward_storage, reverse_storage). Those
-    # are what `@allowscalar` permits. They will be batched in a follow-up;
-    # for now this test is a correctness check, not a performance benchmark.
-    CUDA.@allowscalar optimize!(model)
+    optimize!(model)
     @test objective_value(model) < 1e-3
     return
 end