FixedDec <=> FixedDec conversion performance

Author: NHDaly

src/FixedPointDecimals.jl

@@ -302,10 +302,7 @@ function Base.convert(::Type{FD{T, f}}, x::Integer) where {T, f}
     xT = convert(T, x)  # This won't throw, since we already checked, above.
     # Perform x * C, and check for overflow. This is cheaper than a widemul, especially for
     # 128-bit T, since those widen into a BigInt.
-    v, overflow = Base.mul_with_overflow(xT, C)
-    if overflow
-        throw_inexact()
-    end
+    v = _mul_checked_overflow(throw_inexact, xT, C)
     reinterpret(FD{T, f}, v)
 end
 function Base.convert(::Type{FD{BigInt, f}}, x::Integer) where {f}
@@ -321,6 +318,15 @@ function Base.convert(::Type{FD{BigInt, f}}, x::Integer) where {f}
     reinterpret(FD{BigInt, f}, v)
 end
 
+# x * y - if overflow, report an InexactError(FDT, )
+function _mul_checked_overflow(overflow_callback, x, y)
+    v, overflow = Base.mul_with_overflow(x, y)
+    if overflow
+        overflow_callback()
+    end
+    return v
+end
+
 Base.convert(::Type{T}, x::AbstractFloat) where {T <: FD} = round(T, x)
 
 function Base.convert(::Type{FD{T, f}}, x::Rational) where {T, f}
@@ -332,7 +338,10 @@ function Base.convert(::Type{FD{T, f}}, x::FD{U, g}) where {T, f, U, g}
     if f ≥ g
         # Compute `10^(f - g)` without overflow
         powt = div(coefficient(FD{T, f}), coefficient(FD{U, g}))
-        reinterpret(FD{T, f}, T(widemul(x.i, powt)))
+        v = _mul_checked_overflow(promote(x.i, powt)...) do
+            throw(InexactError(:convert, FD{T, f}, x))
+        end
+        reinterpret(FD{T, f}, T(v))
     else
         # Compute `10^(g - f)` without overflow
         powt = div(coefficient(FD{U, g}), coefficient(FD{T, f}))

test/FixedDecimal.jl

@@ -336,6 +336,7 @@ const SFD4 = FixedDecimal{Int16, 4}
 const F64D2 = FixedDecimal{Int64, 2}
 const UF64D2 = FixedDecimal{UInt64, 2}
 const WFD2 = FixedDecimal{Int128, 2}
+const WFD3 = FixedDecimal{Int128, 3}
 const UFD2 = FixedDecimal{UInt, 2}
 const UWFD2 = FixedDecimal{UInt128, 2}
 
@@ -369,6 +370,19 @@ const UWFD2 = FixedDecimal{UInt128, 2}
     @test @allocated(convert(UF64D2, UInt128(1))) == 0
     @test @allocated(convert(F64D2, Int128(-1))) == 0
 end
+
+@testset "128-bit FD to FD conversion performance" begin
+    @test @allocated(convert(WFD3, WFD2(1))) == 0
+    @test @allocated(convert(SFD4, SFD2(1))) == 0
+    @test @allocated(convert(SFD4, WFD2(1))) == 0
+end
+
+@testset "128-bit FD to FD conversion performance" begin
+    @test @allocated(convert(WFD3, WFD2(1))) == 0
+    @test @allocated(convert(SFD4, SFD2(1))) == 0
+    @test @allocated(convert(SFD4, WFD2(1))) == 0
+end
+
 end  # if @static Int === Int64
 
 @testset "BigInt conversion performance" begin