Improvements to numeric exponent rules (#224)

* Only exponentiate once

* Don't complexify if input and output are real

* Simplify checks

* Move test to fastmath.jl

* Test discontinuity for negative x

* Increment version number

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRules"
 uuid = "082447d4-558c-5d27-93f4-14fc19e9eca2"
-version = "0.7.6"
+version = "0.7.7"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/rulesets/Base/fastmath_able.jl

@@ -134,9 +134,14 @@ let
         @scalar_rule x + y (One(), One())
         @scalar_rule x - y (One(), -1)
         @scalar_rule x / y (inv(y), -((x / y) / y))
-        #log(complex(x)) is require so it give correct complex answer for x<0
+        #log(complex(x)) is required so it gives correct complex answer for x<0
         @scalar_rule(x ^ y,
-            (ifelse(iszero(y), zero(Ω), y * x ^ (y - 1)), Ω * log(complex(x))),
+            (ifelse(iszero(x), zero(Ω), y * Ω / x), Ω * log(complex(x))),
+        )
+        # x^y for x < 0 errors when y is not an integer, but then derivative wrt y
+        # is undefined, so we adopt subgradient convention and set derivative to 0.
+        @scalar_rule(x::Real ^ y::Real,
+            (ifelse(iszero(x), zero(Ω), y * Ω / x), Ω * log(ifelse(x ≤ 0, one(x), x))),
         )
         @scalar_rule(
             rem(x, y),

test/rulesets/Base/base.jl

@@ -117,17 +117,6 @@
         rrule_test(mod, Δz, (x, x̄), (y, ȳ))
     end
 
-    @testset "^(x::$T, n::$T)" for T in (Float64, ComplexF64)
-        # for real x and n, x must be >0
-        x = T <: Real ? 15rand() : 15randn(ComplexF64)
-        Δx, x̄ = 10rand(T, 2)
-        y, Δy, ȳ = rand(T, 3)
-        Δz = rand(T)
-
-        frule_test(^, (x, Δx), (y, Δy))
-        rrule_test(^, Δz, (x, x̄), (y, ȳ))
-    end
-
     @testset "identity" for T in (Float64, ComplexF64)
         frule_test(identity, (randn(T), randn(T)))
         frule_test(identity, (randn(T, 4), randn(T, 4)))

test/rulesets/Base/fastmath_able.jl

@@ -139,6 +139,34 @@ const FASTABLE_AST = quote
             frule_test(f, (x, Δx), (y, Δy))
             rrule_test(f, Δz, (x, x̄), (y, ȳ))
         end
+
+        @testset "^(x::$T, n::$T)" for T in (Float64, ComplexF64)
+            # for real x and n, x must be >0
+            x = T <: Real ? 15rand() : 15randn(ComplexF64)
+            Δx, x̄ = 10rand(T, 2)
+            y, Δy, ȳ = rand(T, 3)
+            Δz = rand(T)
+
+            frule_test(^, (x, Δx), (y, Δy))
+            rrule_test(^, Δz, (x, x̄), (y, ȳ))
+
+            T <: Real && @testset "discontinuity for ^(x::Real, n::Int) when x ≤ 0" begin
+                # finite differences doesn't work for x < 0, so we check manually
+                x, y = -10rand(T), 3
+                Δx = randn(T)
+                Δy = randn(T)
+                Δz = randn(T)
+
+                @test frule((Zero(), Δx, Δy), ^, x, y)[2] ≈ Δx * y * x^(y - 1)
+                @test frule((Zero(), Δx, Δy), ^, zero(x), y)[2] ≈ 0
+                _, ∂x, ∂y = rrule(^, x, y)[2](Δz)
+                @test ∂x ≈ Δz * y * x^(y - 1)
+                @test ∂y ≈ 0
+                _, ∂x, ∂y = rrule(^, zero(x), y)[2](Δz)
+                @test ∂x ≈ 0
+                @test ∂y ≈ 0
+            end
+        end
     end
 
     @testset "sign" begin