Add workaround for Math.pow constant folding (only for JS builds) (#2346)

Author: MaxGraey

src/compiler.ts

@@ -199,7 +199,8 @@ import {
   isPowerOf2,
   v128_zero,
   readI32,
-  isIdentifier
+  isIdentifier,
+  accuratePow64
 } from "./util";
 
 import {
@@ -5322,7 +5323,7 @@ export class Compiler extends DiagnosticEmitter {
             let leftValue = getConstValueF32(leftExpr);
             let rightValue = getConstValueF32(rightExpr);
             this.currentType = type;
-            return module.f32(f32(Math.pow(leftValue, rightValue)));
+            return module.f32(f32(accuratePow64(leftValue, rightValue)));
           }
         }
         let instance = this.f32PowInstance;
@@ -5364,7 +5365,7 @@ export class Compiler extends DiagnosticEmitter {
             let leftValue = getConstValueF64(leftExpr);
             let rightValue = getConstValueF64(rightExpr);
             this.currentType = type;
-            return module.f64(Math.pow(leftValue, rightValue));
+            return module.f64(accuratePow64(leftValue, rightValue));
           }
         }
         let instance = this.f64PowInstance;

src/util/math.ts

@@ -7,3 +7,16 @@
 export function isPowerOf2(x: i32): bool {
   return x != 0 && (x & (x - 1)) == 0;
 }
+
+export function accuratePow64(x: f64, y: f64): f64 {
+  if (!ASC_TARGET) { // ASC_TARGET == JS
+    // Engines like V8, WebKit and SpiderMonkey uses powi fast path if exponent is integer
+    // This speculative optimization leads to loose precisions like 10 ** 208 != 1e208
+    // or/and 10 ** -5 != 1e-5 anymore. For avoid this behaviour we are forcing exponent
+    // to fractional form and compensate this afterwards.
+    if (isFinite(y) && Math.abs(y) >= 2 && Math.trunc(y) == y) {
+      return Math.pow(x, y - 0.5) * Math.pow(x, 0.5);
+    }
+  }
+  return Math.pow(x, y);
+}

tests/compiler/std/math.debug.wat

@@ -59299,6 +59299,76 @@
    call $~lib/builtins/abort
    unreachable
   end
+  f64.const 10
+  f64.const 308
+  call $~lib/math/NativeMath.pow
+  f64.const 1.e+308
+  f64.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 32
+   i32.const 4136
+   i32.const 1
+   call $~lib/builtins/abort
+   unreachable
+  end
+  f64.const 10
+  f64.const 208
+  call $~lib/math/NativeMath.pow
+  f64.const 1.e+208
+  f64.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 32
+   i32.const 4137
+   i32.const 1
+   call $~lib/builtins/abort
+   unreachable
+  end
+  f64.const 10
+  f64.const -5
+  call $~lib/math/NativeMath.pow
+  f64.const 1e-05
+  f64.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 32
+   i32.const 4138
+   i32.const 1
+   call $~lib/builtins/abort
+   unreachable
+  end
+  f32.const 10
+  f32.const 38
+  call $~lib/math/NativeMathf.pow
+  f32.const 9999999680285692465065626e13
+  f32.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 32
+   i32.const 4139
+   i32.const 1
+   call $~lib/builtins/abort
+   unreachable
+  end
+  f32.const 10
+  f32.const -5
+  call $~lib/math/NativeMathf.pow
+  f32.const 9.999999747378752e-06
+  f32.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 32
+   i32.const 4140
+   i32.const 1
+   call $~lib/builtins/abort
+   unreachable
+  end
  )
  (func $~start
   call $start:std/math

tests/compiler/std/math.ts

@@ -4131,3 +4131,10 @@ assert(0 ** 0.5 == 0.0);
 assert(0 ** -1.0 == Infinity);
 assert(0.0 ** 0 == 1.0);
 assert(1.0 ** 1 == 1.0);
+
+// Special cases for test constant fold correctness
+assert(10.0 ** 308 == 1e308);
+assert(10.0 ** 208 == 1e208);
+assert(10.0 ** -5 == 1e-5);
+assert(f32(10) ** 38 == f32(1e38));
+assert(f32(10) ** -5 == f32(1e-5));