Optimize select fallthrough values (#2220)

This became noticeable after #2216 which led to some eqz eqz pairs in the test suite.

Author: kripken

src/passes/OptimizeInstructions.cpp

@@ -939,6 +939,7 @@ struct OptimizeInstructions
 
   // Optimize given that the expression is flowing into a boolean context
   Expression* optimizeBoolean(Expression* boolean) {
+    // TODO use a general getFallthroughs
     if (auto* unary = boolean->dynCast<Unary>()) {
       if (unary && unary->op == EqZInt32) {
         auto* unary2 = unary->value->dynCast<Unary>();
@@ -975,6 +976,9 @@ struct OptimizeInstructions
         iff->ifTrue = optimizeBoolean(iff->ifTrue);
         iff->ifFalse = optimizeBoolean(iff->ifFalse);
       }
+    } else if (auto* select = boolean->dynCast<Select>()) {
+      select->ifTrue = optimizeBoolean(select->ifTrue);
+      select->ifFalse = optimizeBoolean(select->ifFalse);
     }
     // TODO: recurse into br values?
     return boolean;

test/emcc_hello_world.fromasm

@@ -10329,13 +10329,9 @@
             (block
              (br_if $label$break$L279
               (select
-               (i32.eqz
-                (i32.eqz
-                 (local.tee $3
-                  (i32.load
-                   (i32.const 616)
-                  )
-                 )
+               (local.tee $3
+                (i32.load
+                 (i32.const 616)
                 )
                )
                (i32.const 0)

test/emcc_hello_world.fromasm.clamp

@@ -10379,13 +10379,9 @@
             (block
              (br_if $label$break$L279
               (select
-               (i32.eqz
-                (i32.eqz
-                 (local.tee $3
-                  (i32.load
-                   (i32.const 616)
-                  )
-                 )
+               (local.tee $3
+                (i32.load
+                 (i32.const 616)
                 )
                )
                (i32.const 0)

test/emcc_hello_world.fromasm.imprecise

@@ -10225,13 +10225,9 @@
             (block
              (br_if $label$break$L279
               (select
-               (i32.eqz
-                (i32.eqz
-                 (local.tee $3
-                  (i32.load
-                   (i32.const 616)
-                  )
-                 )
+               (local.tee $3
+                (i32.load
+                 (i32.const 616)
                 )
                )
                (i32.const 0)

test/passes/optimize-instructions_enable-threads.txt

@@ -3293,6 +3293,16 @@
    )
   )
  )
+ (func $select-into-arms (; 77 ;) (type $FUNCSIG$vii) (param $x i32) (param $y i32)
+  (if
+   (select
+    (local.get $x)
+    (local.get $y)
+    (local.get $y)
+   )
+   (unreachable)
+  )
+ )
 )
 (module
  (type $FUNCSIG$v (func))

test/passes/optimize-instructions_enable-threads.wast

@@ -3738,6 +3738,16 @@
     ))
     ;; TODO: more stuff here
   )
+  (func $select-into-arms (param $x i32) (param $y i32)
+    (if
+      (select
+        (i32.eqz (i32.eqz (local.get $x)))
+        (i32.eqz (i32.eqz (local.get $y)))
+        (local.get $y)
+      )
+      (unreachable)
+    )
+  )
 )
 (module
   (import "env" "memory" (memory $0 (shared 256 256)))