Optimize float formatting fast paths and validate scalar/SIMD output

Author: JairusSW

NOTICE

@@ -95,7 +95,7 @@ the following terms:
 * xjb-as: https://github.com/JairusSW/xjb-as
 
   The ECMAScript-compatible f64/f32 -> string (dtoa/ftoa) implementation in
-  std/assembly/util/xjb.ts is vendored from xjb-as.
+  std/assembly/util/dtoa.ts is vendored from xjb-as.
 
   Copyright (c) Jairus Tanaka
   Apache License, Version 2.0 (https://opensource.org/licenses/Apache-2.0)

package.json

@@ -79,9 +79,10 @@
     "build": "node scripts/build",
     "watch": "node scripts/build --watch",
     "coverage": "npx c8 -- npm test",
-    "test": "npm run test:parser && npm run test:compiler -- --parallel && npm run test:browser && npm run test:asconfig && npm run test:transform && npm run test:cli",
+    "test": "npm run test:parser && npm run test:compiler -- --parallel && npm run test:dtoa && npm run test:browser && npm run test:asconfig && npm run test:transform && npm run test:cli",
     "test:parser": "node --enable-source-maps tests/parser",
     "test:compiler": "node --enable-source-maps --no-warnings tests/compiler",
+    "test:dtoa": "node tests/dtoa",
     "test:browser": "node --enable-source-maps tests/browser",
     "test:asconfig": "cd tests/asconfig && npm run test",
     "test:transform": "npm run test:transform:esm && npm run test:transform:cjs",

std/assembly/util/dtoa.ts

@@ -983,6 +983,20 @@ export function dtoa_buffered(buffer: usize, value: f64): u32 {
       store<u16>(buf, CharCode.MINUS);
       buf += 2;
     }
+    let q = binExp - FLOAT_EXP_OFFSET;
+    if (q >= -FLOAT_SIGNIFICAND_SIZE && q < 24) {
+      let c = binSig | FLOAT_HIDDEN_BIT;
+      let intValue: u64 = 0;
+      if (q < 0) {
+        let shift = -q;
+        let mask = ((<u64>1) << shift) - 1;
+        if ((c & mask) == 0) intValue = c >> shift;
+      } else {
+        intValue = c << q;
+        if (intValue > 16777216) intValue = 0;
+      }
+      if (intValue != 0) return finishInteger(writeUInt16(buf, intValue), dotZero);
+    }
     toDecimalFloat(binSig | FLOAT_HIDDEN_BIT, binExp, binSig != 0);
   }
 

std/assembly/util/number.ts

@@ -413,6 +413,11 @@ export function itoa64(value: i64, radix: i32): String {
 @lazy @inline const dtoa_buf = memory.data(128);
 
 export function dtoa<T extends number>(value: T): String {
+  if (value == 0) return "0.0";
+  if (!isFinite(value)) {
+    if (isNaN(value)) return "NaN";
+    return select<String>("-Infinity", "Infinity", value < 0);
+  }
   let len: u32;
   if (isFloat<T>() && sizeof<T>() == 4) {
     // @ts-ignore: type