feat: improve precision for nx constants (#1574)

Author: polvalente

nx/lib/nx/constants.ex

@@ -3,6 +3,10 @@ defmodule Nx.Constants do
   Common constants used in computations.
 
   This module can be used in `defn`.
+
+  The functions `e/0`, `pi/0` and `i/0` will follow the same rules as
+  literal constants when used inside `defn`. This means that they will
+  use the surrounding precision instead of defaulting to f32.
   """
 
   import Nx.Shared

nx/lib/nx/defn/compiler.ex

@@ -592,6 +592,18 @@ defmodule Nx.Defn.Compiler do
     {{{:., dot_meta, [Complex, :new]}, meta, args}, state}
   end
 
+  defp normalize({{:., dot_meta, [Nx.Constants, :i]}, meta, []}, state) do
+    {{{:., dot_meta, [Complex, :new]}, meta, [0, 1]}, state}
+  end
+
+  defp normalize({{:., dot_meta, [Nx.Constants, :e]}, meta, []}, state) do
+    {{{:., dot_meta, [:math, :exp]}, meta, [1]}, state}
+  end
+
+  defp normalize({{:., dot_meta, [Nx.Constants, :pi]}, meta, []}, state) do
+    {{{:., dot_meta, [:math, :pi]}, meta, []}, state}
+  end
+
   defp normalize({{:., dot_meta, [mod, name]}, meta, args}, state) when mod in @allowed_modules do
     {args, state} = normalize_list(args, state)
     {{{:., dot_meta, [mod, name]}, meta, args}, state}

nx/test/nx/defn_test.exs

@@ -44,6 +44,71 @@ defmodule Nx.DefnTest do
       assert %T{data: %Expr{op: :constant, args: [%Complex{} = c]}} = complex_constant()
       assert c == Complex.new(1, :infinity)
     end
+
+    defn real_constants_as_arbitrary_precision(x) do
+      cond do
+        x == 0 -> Nx.Constants.e()
+        x == 1 -> Nx.Constants.pi()
+        true -> x
+      end
+    end
+
+    defn imaginary_constant_as_arbitrary_precision(x) do
+      x * Nx.Constants.i()
+    end
+
+    test "Defines real constants as numerical when used with arity 0 inside defn" do
+      for input_type <- [f: 8, f: 16, f: 32, f: 64] do
+        assert %T{
+                 type: ^input_type,
+                 data: %Expr{op: :cond, args: [[clause1, clause2], _last]}
+               } =
+                 real_constants_as_arbitrary_precision(Nx.tensor(10.0, type: input_type))
+
+        e = :math.exp(1)
+        pi = :math.pi()
+        assert {_, %T{type: ^input_type, data: %Expr{op: :constant, args: [^e]}}} = clause1
+        assert {_, %T{type: ^input_type, data: %Expr{op: :constant, args: [^pi]}}} = clause2
+      end
+
+      for input_type <- [c: 64, c: 128] do
+        assert %T{
+                 type: ^input_type,
+                 data: %Expr{op: :cond, args: [[clause1, clause2], _last]}
+               } =
+                 real_constants_as_arbitrary_precision(Nx.tensor(10.0, type: input_type))
+
+        e = Complex.new(:math.exp(1))
+        pi = Complex.new(:math.pi())
+        assert {_, %T{type: ^input_type, data: %Expr{op: :constant, args: [^e]}}} = clause1
+        assert {_, %T{type: ^input_type, data: %Expr{op: :constant, args: [^pi]}}} = clause2
+      end
+    end
+
+    test "Defines imaginary constant as Complex.new(0, 1) when used with arity 0 inside defn" do
+      for input_type <- [f: 8, f: 16, f: 32, f: 64, c: 64, c: 128] do
+        type =
+          case input_type do
+            {:f, 64} -> {:c, 128}
+            {:c, 128} -> {:c, 128}
+            _ -> {:c, 64}
+          end
+
+        # note: we expect the number to be shifted to the left
+        # because after AST normalization, but before Expr parsing,
+        # Nx.Constants.i() is converted to Complex.new(0, 1), which is
+        # a literal number.
+        assert %T{
+                 type: ^type,
+                 data: %Expr{op: :multiply, args: [arg0, arg1]}
+               } =
+                 imaginary_constant_as_arbitrary_precision(Nx.tensor(10.0, type: input_type))
+
+        i = Complex.new(0, 1)
+        %T{data: %Expr{op: :constant, args: [^i]}} = arg0
+        %T{data: %Expr{op: :parameter, args: [0]}} = arg1
+      end
+    end
   end
 
   describe "Nx.tensor" do