fix: speculative execution in wasm

indutny · web-flow · commit ce16dd9083d5 · 2025-04-23T08:58:28.000-07:00
In #74 we make sure to increment position by a constant whenever in a happy path so that the CPU could proceed through the loop header (`endp - p >= 16`) speculatively instead of blocking and waiting for vector to scalar to fully resolve. This doesn't seem to give an obvious performance boost yet, but potentially might have an effect with the future V8 updates.
diff --git a/src/implementation/c/node/table-lookup.ts b/src/implementation/c/node/table-lookup.ts
@@ -288,7 +288,7 @@ export class TableLookup extends Node<frontend.node.TableLookup> {
     }
 
     out.push('#ifdef __wasm_simd128__');
-    out.push(`if (${ctx.endPosArg()} - ${ctx.posArg()} >= 16) {`);
+    out.push(`while (${ctx.endPosArg()} - ${ctx.posArg()} >= 16) {`);
     out.push('  v128_t input;');
     out.push('  v128_t mask;');
     out.push('  v128_t single;');
@@ -327,21 +327,23 @@ export class TableLookup extends Node<frontend.node.TableLookup> {
     out.push('  match_len = __builtin_ctz(');
     out.push('    ~wasm_i8x16_bitmask(mask)');
     out.push('  );');
-    out.push(`  ${ctx.posArg()} += match_len;`);
     out.push('  if (match_len != 16) {');
+    out.push(`    ${ctx.posArg()} += match_len;`);
     {
       const tmp: string[] = [];
       this.tailTo(tmp, this.ref.otherwise!);
       ctx.indent(out, tmp, '    ');
     }
     out.push('  }');
+    out.push(`  ${ctx.posArg()} += 16;`);
+    out.push('}');
 
-    const tmp: string[] = [];
-    this.tailTo(tmp, {
-      noAdvance: true,
-      node: edge.node,
-    });
-    ctx.indent(out, tmp, '  ');
+    out.push(`if (${ctx.posArg()} == ${ctx.endPosArg()}) {`);
+    {
+      const tmp: string[] = [];
+      this.pause(tmp);
+      this.compilation.indent(out, tmp, '  ');
+    }
     out.push('}');
 
     out.push('#endif  /* __wasm_simd128__ */');
