Use empty blocks instead of nops for empty scopes in IRBuilder (#7080)

When IRBuilder builds an empty non-block scope such as a function body,
an if arm, a try block, etc, it needs to produce some expression to
represent the empty contents. Previously it produced a nop, but change
it to produce an empty block instead. The binary writer and printer have
special logic to elide empty blocks, so this produces smaller output.

Update J2CLOpts to recognize functions containing empty blocks as
trivial to avoid regressing one of its tests.

src/passes/J2CLOpts.cpp

@@ -43,6 +43,7 @@ Expression* getTrivialFunctionBody(Function* func) {
   // Only consider trivial the following instructions which can be safely
   // inlined and note that their size is at most 2.
   if (body->is<Nop>() || body->is<GlobalGet>() || body->is<Const>() ||
+      (body->is<Block>() && body->cast<Block>()->list.empty()) ||
       // Call with no arguments.
       (body->is<Call>() && body->dynCast<Call>()->operands.size() == 0) ||
       // Simple global.set with a constant.

src/wasm/wasm-ir-builder.cpp

@@ -157,7 +157,7 @@ void IRBuilder::push(Expression* expr) {
 
 Result<Expression*> IRBuilder::build() {
   if (scopeStack.empty()) {
-    return builder.makeNop();
+    return builder.makeBlock();
   }
   if (scopeStack.size() > 1 || !scopeStack.back().isNone()) {
     return Err{"unfinished block context"};
@@ -792,12 +792,13 @@ Result<Expression*> IRBuilder::finishScope(Block* block) {
   Expression* ret = nullptr;
   if (scope.exprStack.size() == 0) {
     // No expressions for this scope, but we need something. If we were given a
-    // block, we can empty it out and return it, but otherwise we need a nop.
+    // block, we can empty it out and return it, but otherwise create a new
+    // empty block.
     if (block) {
       block->list.clear();
       ret = block;
     } else {
-      ret = builder.makeNop();
+      ret = builder.makeBlock();
     }
   } else if (scope.exprStack.size() == 1) {
     // We can put our single expression directly into the surrounding scope.

test/binaryen.js/debug-info.js.txt

@@ -5,7 +5,6 @@ debugInfo=false
  (memory $0 0)
  (export "test" (func $0))
  (func $0
-  (nop)
  )
 )
 
@@ -16,7 +15,6 @@ debugInfo=true
  (memory $0 0)
  (export "test" (func $test))
  (func $test
-  (nop)
  )
 )
 
@@ -27,7 +25,6 @@ debugInfo=false
  (memory $0 0)
  (export "test" (func $0))
  (func $0
-  (nop)
  )
 )
 

test/binaryen.js/debug-names.js.txt

@@ -16,7 +16,6 @@
  (table $wor 0 0 funcref)
  (func $of (param $wasm i32)
   (local $!#$%&'*+-./:<=>?@\^_`|~ f64)
-  (nop)
  )
 )
 
@@ -28,7 +27,6 @@
  (table $wor 0 0 funcref)
  (func $of (param $js i32)
   (local $!#$%&'*+-./:<=>?@\5c^_`|~ f64)
-  (nop)
  )
 )
 
@@ -40,7 +38,6 @@
  (table $wor 0 0 funcref)
  (func $of (param $js i32)
   (local $!#$%&'*+-./:<=>?@\5c^_`|~ f64)
-  (nop)
  )
 )
 

test/example/module-splitting.txt

@@ -585,7 +585,6 @@ Before:
  (elem $0 (global.get $base) $null $foo)
  (export "foo" (func $foo))
  (func $null (type $0)
-  (nop)
  )
  (func $foo (type $1) (param $0 i32) (result i32)
   (local.get $0)
@@ -607,7 +606,6 @@ After:
  (export "%table_2" (table $0))
  (export "%global" (global $base))
  (func $null (type $0)
-  (nop)
  )
  (func $foo (type $1) (param $0 i32) (result i32)
   (call_indirect $0 (type $1)
@@ -1144,7 +1142,6 @@ Before:
  (export "foo1" (func $foo))
  (export "foo2" (func $foo))
  (func $foo (type $0)
-  (nop)
  )
 )
 Keeping: <none>
@@ -1169,7 +1166,6 @@ Secondary:
  (import "primary" "%table" (table $0 1 funcref))
  (elem $0 (i32.const 0) $foo)
  (func $foo (type $0)
-  (nop)
  )
 )
 

test/gtest/cfg.cpp

@@ -149,7 +149,7 @@ TEST_F(CFGTest, Empty) {
 ;; entry
 ;; exit
 0:
-  0: nop
+  0: block
 )cfg";
 
   Module wasm;

test/lit/basic/complexTextNames.wast

@@ -13,12 +13,10 @@
   ;; CHECK-TEXT:      (type $0 (func))
 
   ;; CHECK-TEXT:      (func $foo\20\28.bar\29 (type $0)
-  ;; CHECK-TEXT-NEXT:  (nop)
   ;; CHECK-TEXT-NEXT: )
   ;; CHECK-BIN:      (type $0 (func))
 
   ;; CHECK-BIN:      (func $foo\20\28.bar\29 (type $0)
-  ;; CHECK-BIN-NEXT:  (nop)
   ;; CHECK-BIN-NEXT: )
   (func $foo\20\28.bar\29)
 
@@ -34,7 +32,6 @@
 ;; CHECK-BIN-NODEBUG:      (type $0 (func))
 
 ;; CHECK-BIN-NODEBUG:      (func $0 (type $0)
-;; CHECK-BIN-NODEBUG-NEXT:  (nop)
 ;; CHECK-BIN-NODEBUG-NEXT: )
 
 ;; CHECK-BIN-NODEBUG:      (func $1 (type $0)

test/lit/basic/duplicate_types.wast

@@ -25,12 +25,10 @@
  ;; CHECK-TEXT:      (type $1 (func (param i32) (result i32)))
 
  ;; CHECK-TEXT:      (func $f0 (type $0) (param $0 i32)
- ;; CHECK-TEXT-NEXT:  (nop)
  ;; CHECK-TEXT-NEXT: )
  ;; CHECK-BIN:      (type $1 (func (param i32) (result i32)))
 
  ;; CHECK-BIN:      (func $f0 (type $0) (param $0 i32)
- ;; CHECK-BIN-NEXT:  (nop)
  ;; CHECK-BIN-NEXT: )
  (func $f0 (param i32))
 
@@ -47,7 +45,6 @@
 ;; CHECK-BIN-NODEBUG:      (type $1 (func (param i32) (result i32)))
 
 ;; CHECK-BIN-NODEBUG:      (func $0 (type $0) (param $0 i32)
-;; CHECK-BIN-NODEBUG-NEXT:  (nop)
 ;; CHECK-BIN-NODEBUG-NEXT: )
 
 ;; CHECK-BIN-NODEBUG:      (func $1 (type $1) (param $0 i32) (result i32)