wasm2js: Switch optimizations (#2141)

This pattern-matches towers of blocks + a br_table into a JS switch. This is much smaller in code size and also avoids heavy nesting that can exceed the recursion limits of JS parsers.

This is not enough yet, because it pattern-matches very specifically. In reality, switches can look slightly different. Followup PRs will extend this. For now, this passes the test suite (what passed before - not including the massive-switch tests) + fuzzing so it's a good start.

1 files changed, 37 insertions, 52 deletions

diff --git a/test/wasm2js/labels.2asm.js b/test/wasm2js/labels.2asm.js
index c4a93491c..fa0bd0a22 100644
--- a/test/wasm2js/labels.2asm.js
+++ b/test/wasm2js/labels.2asm.js
@@ -167,54 +167,39 @@ function asmFunc(global, env, buffer) {
  
  function $8($0_1) {
   $0_1 = $0_1 | 0;
-  var $2_2 = 0, $3_2 = 0;
+  var $2_1 = 0, $3_1 = 0;
   ret : {
    exit : {
     $0_2 : {
-     default_ : {
-      $3_1 : {
-       $2_1 : {
-        $1_1 : {
-         switch ($0_1 | 0) {
-         case 0:
-          break $0_2;
-         case 1:
-          break $1_1;
-         case 2:
-          break $2_1;
-         case 3:
-          break $3_1;
-         default:
-          break default_;
-         };
-        }
-       }
-       $2_2 = 2;
-       break exit;
-      }
-      $3_2 = 3;
+     switch ($0_1 | 0) {
+     case 1:
+     case 2:
+      $2_1 = 2;
+      break exit;
+     case 3:
+      $3_1 = 3;
       break ret;
-     }
+     default:
+     case 0:
+      break $0_2;
+     };
     }
-    $2_2 = 5;
+    $2_1 = 5;
    }
-   $3_2 = Math_imul(10, $2_2);
+   $3_1 = Math_imul(10, $2_1);
   }
-  return $3_2 | 0;
+  return $3_1 | 0;
  }
  
  function $9($0_1) {
   $0_1 = $0_1 | 0;
   $1_1 : {
-   $0_2 : {
-    switch ($0_1 | 0) {
-    case 0:
-     break $0_2;
-    default:
-     break $1_1;
-    };
-   }
-   return 0 | 0;
+   switch ($0_1 | 0) {
+   case 0:
+    return 0 | 0;
+   default:
+    break $1_1;
+   };
   }
   return 2 | 0;
  }
@@ -251,41 +236,41 @@ function asmFunc(global, env, buffer) {
  }
  
  function $11() {
-  var $2_2 = 0, $0_1 = 0;
+  var $2_1 = 0, $0_1 = 0;
   l0 : {
    l1 : {
     $0_1 = 1;
     break l1;
    }
-   $2_2 = $0_1;
+   $2_1 = $0_1;
    if (1) {
     break l0
    }
-   $2_2 = 1;
+   $2_1 = 1;
   }
-  return $2_2 | 0;
+  return $2_1 | 0;
  }
  
  function $12() {
-  var $2_2 = 0, $0_1 = 0;
+  var $2_1 = 0, $0_1 = 0;
   l0 : {
    l1 : {
     $0_1 = 1;
     break l1;
    }
-   $2_2 = $0_1;
+   $2_1 = $0_1;
    break l0;
   }
-  return $2_2 | 0;
+  return $2_1 | 0;
  }
  
  function $13() {
-  var i1 = 0, $7_1 = 0, $3_2 = 0;
+  var i1 = 0, $7_1 = 0, $3_1 = 0;
   l0 : {
    i1 = 1;
-   $3_2 = i1;
+   $3_1 = i1;
    i1 = 2;
-   $7_1 = $3_2;
+   $7_1 = $3_1;
    if (i1) {
     break l0
    }
@@ -295,16 +280,16 @@ function asmFunc(global, env, buffer) {
  }
  
  function $14() {
-  var $2_2 = 0, $0_1 = 0, $3_2 = 0;
+  var $2_1 = 0, $0_1 = 0, $3_1 = 0;
   l0 : {
    l1 : {
     $0_1 = 1;
     break l1;
    }
-   $2_2 = $0_1;
+   $2_1 = $0_1;
    break l0;
   }
-  return $2_2 | 0;
+  return $2_1 | 0;
  }
  
  function $15() {
@@ -317,15 +302,15 @@ function asmFunc(global, env, buffer) {
  }
  
  function $16() {
-  var $1_2 = 0, $2_2 = 0;
+  var $1_2 = 0, $2_1 = 0;
   l1 : {
    $1_2 = 2;
    l113 : {
-    $2_2 = 3;
+    $2_1 = 3;
     break l113;
    }
   }
-  return $1_2 + $2_2 | 0 | 0;
+  return $1_2 + $2_1 | 0 | 0;
  }
  
  var FUNCTION_TABLE = [];