Remove writes to globals that are never written to (#2741)

Since the global is never read, we know that any write operation
will be unobservable.

17 files changed, 80 insertions, 130 deletions

diff --git a/src/passes/SimplifyGlobals.cpp b/src/passes/SimplifyGlobals.cpp
index 5fa1686fc..71aafa491 100644
--- a/src/passes/SimplifyGlobals.cpp
+++ b/src/passes/SimplifyGlobals.cpp
@@ -51,6 +51,7 @@ struct GlobalInfo {
   bool imported = false;
   bool exported = false;
   std::atomic<bool> written;
+  std::atomic<bool> read;
 };
 
 using GlobalInfoMap = std::map<Name, GlobalInfo>;
@@ -64,6 +65,8 @@ struct GlobalUseScanner : public WalkerPass<PostWalker<GlobalUseScanner>> {
 
   void visitGlobalSet(GlobalSet* curr) { (*infos)[curr->name].written = true; }
 
+  void visitGlobalGet(GlobalGet* curr) { (*infos)[curr->name].read = true; }
+
 private:
   GlobalInfoMap* infos;
 };
@@ -163,6 +166,23 @@ private:
   std::map<Name, Literals> currConstantGlobals;
 };
 
+struct GlobalSetRemover : public WalkerPass<PostWalker<GlobalSetRemover>> {
+  GlobalSetRemover(NameSet* toRemove) : toRemove(toRemove) {}
+
+  bool isFunctionParallel() override { return true; }
+
+  GlobalSetRemover* create() override { return new GlobalSetRemover(toRemove); }
+
+  void visitGlobalSet(GlobalSet* curr) {
+    if (toRemove->count(curr->name) != 0) {
+      ExpressionManipulator::nop(curr);
+    }
+  }
+
+private:
+  NameSet* toRemove;
+};
+
 } // anonymous namespace
 
 struct SimplifyGlobals : public Pass {
@@ -180,6 +200,8 @@ struct SimplifyGlobals : public Pass {
 
     analyze();
 
+    removeWritesToUnreadGlobals();
+
     preferEarlierImports();
 
     propagateConstantsToGlobals();
@@ -211,6 +233,23 @@ struct SimplifyGlobals : public Pass {
     }
   }
 
+  void removeWritesToUnreadGlobals() {
+    // Globals that are not exports and not read from can be eliminated
+    // (even if they are written to).
+    NameSet unreadGlobals;
+    for (auto& global : module->globals) {
+      auto& info = map[global->name];
+      if (!info.imported && !info.exported && !info.read) {
+        unreadGlobals.insert(global->name);
+        // We can now mark this global as immutable, and un-written, since we
+        // are about to remove all the `.set` operations on it.
+        global->mutable_ = false;
+        info.written = false;
+      }
+    }
+    GlobalSetRemover(&unreadGlobals).run(runner, module);
+  }
+
   void preferEarlierImports() {
     // Optimize uses of immutable globals, prefer the earlier import when
     // there is a copy.
diff --git a/src/passes/pass.cpp b/src/passes/pass.cpp
index 733b61467..4da870ae9 100644
--- a/src/passes/pass.cpp
+++ b/src/passes/pass.cpp
@@ -483,6 +483,7 @@ void PassRunner::addDefaultGlobalOptimizationPostPasses() {
   } else {
     add("simplify-globals");
   }
+  add("vacuum"); // simplify-globals can generate extra nops
   add("remove-unused-module-elements");
   // may allow more inlining/dae/etc., need --converge for that
   add("directize");
diff --git a/test/emcc_O2_hello_world.fromasm b/test/emcc_O2_hello_world.fromasm
index ddcb90a43..bd9af676b 100644
--- a/test/emcc_O2_hello_world.fromasm
+++ b/test/emcc_O2_hello_world.fromasm
@@ -13,7 +13,6 @@
  (elem (global.get $__table_base) $b0 $___stdio_close $b1 $b1 $___stdout_write $___stdio_seek $b1 $___stdio_write $b1 $b1 $b2 $b2 $b2 $b2 $_cleanup_418 $b2 $b2 $b2)
  (import "env" "__table_base" (global $__table_base i32))
  (import "env" "STACKTOP" (global $STACKTOP$asm2wasm$import i32))
- (import "env" "STACK_MAX" (global $STACK_MAX$asm2wasm$import i32))
  (import "env" "abort" (func $abort (param i32)))
  (import "env" "_pthread_cleanup_pop" (func $_pthread_cleanup_pop (param i32)))
  (import "env" "_pthread_self" (func $_pthread_self (result i32)))
@@ -30,9 +29,7 @@
  (import "env" "___syscall140" (func $___syscall140 (param i32 i32) (result i32)))
  (import "env" "___syscall146" (func $___syscall146 (param i32 i32) (result i32)))
  (global $STACKTOP (mut i32) (global.get $STACKTOP$asm2wasm$import))
- (global $STACK_MAX (mut i32) (global.get $STACK_MAX$asm2wasm$import))
  (global $__THREW__ (mut i32) (i32.const 0))
- (global $threwValue (mut i32) (i32.const 0))
  (global $tempRet0 (mut i32) (i32.const 0))
  (export "_free" (func $_free))
  (export "_main" (func $_main))
@@ -8972,13 +8969,8 @@
    (i32.eqz
     (global.get $__THREW__)
    )
-   (block
-    (global.set $__THREW__
-     (local.get $0)
-    )
-    (global.set $threwValue
-     (local.get $1)
-    )
+   (global.set $__THREW__
+    (local.get $0)
    )
   )
  )
@@ -9002,9 +8994,6 @@
   (global.set $STACKTOP
    (local.get $0)
   )
-  (global.set $STACK_MAX
-   (local.get $1)
-  )
  )
  (func $dynCall_vi (; has Stack IR ;) (param $0 i32) (param $1 i32)
   (call_indirect (type $i32_=>_none)
diff --git a/test/emcc_O2_hello_world.fromasm.clamp b/test/emcc_O2_hello_world.fromasm.clamp
index ddcb90a43..bd9af676b 100644
--- a/test/emcc_O2_hello_world.fromasm.clamp
+++ b/test/emcc_O2_hello_world.fromasm.clamp
@@ -13,7 +13,6 @@
  (elem (global.get $__table_base) $b0 $___stdio_close $b1 $b1 $___stdout_write $___stdio_seek $b1 $___stdio_write $b1 $b1 $b2 $b2 $b2 $b2 $_cleanup_418 $b2 $b2 $b2)
  (import "env" "__table_base" (global $__table_base i32))
  (import "env" "STACKTOP" (global $STACKTOP$asm2wasm$import i32))
- (import "env" "STACK_MAX" (global $STACK_MAX$asm2wasm$import i32))
  (import "env" "abort" (func $abort (param i32)))
  (import "env" "_pthread_cleanup_pop" (func $_pthread_cleanup_pop (param i32)))
  (import "env" "_pthread_self" (func $_pthread_self (result i32)))
@@ -30,9 +29,7 @@
  (import "env" "___syscall140" (func $___syscall140 (param i32 i32) (result i32)))
  (import "env" "___syscall146" (func $___syscall146 (param i32 i32) (result i32)))
  (global $STACKTOP (mut i32) (global.get $STACKTOP$asm2wasm$import))
- (global $STACK_MAX (mut i32) (global.get $STACK_MAX$asm2wasm$import))
  (global $__THREW__ (mut i32) (i32.const 0))
- (global $threwValue (mut i32) (i32.const 0))
  (global $tempRet0 (mut i32) (i32.const 0))
  (export "_free" (func $_free))
  (export "_main" (func $_main))
@@ -8972,13 +8969,8 @@
    (i32.eqz
     (global.get $__THREW__)
    )
-   (block
-    (global.set $__THREW__
-     (local.get $0)
-    )
-    (global.set $threwValue
-     (local.get $1)
-    )
+   (global.set $__THREW__
+    (local.get $0)
    )
   )
  )
@@ -9002,9 +8994,6 @@
   (global.set $STACKTOP
    (local.get $0)
   )
-  (global.set $STACK_MAX
-   (local.get $1)
-  )
  )
  (func $dynCall_vi (; has Stack IR ;) (param $0 i32) (param $1 i32)
   (call_indirect (type $i32_=>_none)
diff --git a/test/emcc_O2_hello_world.fromasm.imprecise b/test/emcc_O2_hello_world.fromasm.imprecise
index f08e59565..82edc64b9 100644
--- a/test/emcc_O2_hello_world.fromasm.imprecise
+++ b/test/emcc_O2_hello_world.fromasm.imprecise
@@ -12,7 +12,6 @@
  (elem (global.get $__table_base) $b0 $___stdio_close $b1 $b1 $___stdout_write $___stdio_seek $b1 $___stdio_write $b1 $b1 $b2 $b2 $b2 $b2 $_cleanup_418 $b2 $b2 $b2)
  (import "env" "__table_base" (global $__table_base i32))
  (import "env" "STACKTOP" (global $STACKTOP$asm2wasm$import i32))
- (import "env" "STACK_MAX" (global $STACK_MAX$asm2wasm$import i32))
  (import "env" "abort" (func $abort (param i32)))
  (import "env" "_pthread_cleanup_pop" (func $_pthread_cleanup_pop (param i32)))
  (import "env" "_pthread_self" (func $_pthread_self (result i32)))
@@ -29,9 +28,7 @@
  (import "env" "___syscall140" (func $___syscall140 (param i32 i32) (result i32)))
  (import "env" "___syscall146" (func $___syscall146 (param i32 i32) (result i32)))
  (global $STACKTOP (mut i32) (global.get $STACKTOP$asm2wasm$import))
- (global $STACK_MAX (mut i32) (global.get $STACK_MAX$asm2wasm$import))
  (global $__THREW__ (mut i32) (i32.const 0))
- (global $threwValue (mut i32) (i32.const 0))
  (global $tempRet0 (mut i32) (i32.const 0))
  (export "_free" (func $_free))
  (export "_main" (func $_main))
@@ -8950,13 +8947,8 @@
    (i32.eqz
     (global.get $__THREW__)
    )
-   (block
-    (global.set $__THREW__
-     (local.get $0)
-    )
-    (global.set $threwValue
-     (local.get $1)
-    )
+   (global.set $__THREW__
+    (local.get $0)
    )
   )
  )
@@ -8976,9 +8968,6 @@
   (global.set $STACKTOP
    (local.get $0)
   )
-  (global.set $STACK_MAX
-   (local.get $1)
-  )
  )
  (func $dynCall_vi (; has Stack IR ;) (param $0 i32) (param $1 i32)
   (call_indirect (type $i32_=>_none)
diff --git a/test/emcc_hello_world.fromasm b/test/emcc_hello_world.fromasm
index c4462eb4e..774740dc1 100644
--- a/test/emcc_hello_world.fromasm
+++ b/test/emcc_hello_world.fromasm
@@ -42,7 +42,6 @@
  (global $STACKTOP (mut i32) (global.get $STACKTOP$asm2wasm$import))
  (global $STACK_MAX (mut i32) (global.get $STACK_MAX$asm2wasm$import))
  (global $__THREW__ (mut i32) (i32.const 0))
- (global $threwValue (mut i32) (i32.const 0))
  (global $tempRet0 (mut i32) (i32.const 0))
  (export "_i64Subtract" (func $_i64Subtract))
  (export "_free" (func $_free))
@@ -117,13 +116,8 @@
    (i32.eqz
     (global.get $__THREW__)
    )
-   (block
-    (global.set $__THREW__
-     (local.get $0)
-    )
-    (global.set $threwValue
-     (local.get $1)
-    )
+   (global.set $__THREW__
+    (local.get $0)
    )
   )
  )
diff --git a/test/emcc_hello_world.fromasm.clamp b/test/emcc_hello_world.fromasm.clamp
index e524ddd07..fecd5a564 100644
--- a/test/emcc_hello_world.fromasm.clamp
+++ b/test/emcc_hello_world.fromasm.clamp
@@ -41,7 +41,6 @@
  (global $STACKTOP (mut i32) (global.get $STACKTOP$asm2wasm$import))
  (global $STACK_MAX (mut i32) (global.get $STACK_MAX$asm2wasm$import))
  (global $__THREW__ (mut i32) (i32.const 0))
- (global $threwValue (mut i32) (i32.const 0))
  (global $tempRet0 (mut i32) (i32.const 0))
  (export "_i64Subtract" (func $_i64Subtract))
  (export "_free" (func $_free))
@@ -116,13 +115,8 @@
    (i32.eqz
     (global.get $__THREW__)
    )
-   (block
-    (global.set $__THREW__
-     (local.get $0)
-    )
-    (global.set $threwValue
-     (local.get $1)
-    )
+   (global.set $__THREW__
+    (local.get $0)
    )
   )
  )
diff --git a/test/emcc_hello_world.fromasm.imprecise b/test/emcc_hello_world.fromasm.imprecise
index 89edb7f44..4c5d7afd2 100644
--- a/test/emcc_hello_world.fromasm.imprecise
+++ b/test/emcc_hello_world.fromasm.imprecise
@@ -39,7 +39,6 @@
  (global $STACKTOP (mut i32) (global.get $STACKTOP$asm2wasm$import))
  (global $STACK_MAX (mut i32) (global.get $STACK_MAX$asm2wasm$import))
  (global $__THREW__ (mut i32) (i32.const 0))
- (global $threwValue (mut i32) (i32.const 0))
  (global $tempRet0 (mut i32) (i32.const 0))
  (export "_i64Subtract" (func $_i64Subtract))
  (export "_free" (func $_free))
@@ -114,13 +113,8 @@
    (i32.eqz
     (global.get $__THREW__)
    )
-   (block
-    (global.set $__THREW__
-     (local.get $0)
-    )
-    (global.set $threwValue
-     (local.get $1)
-    )
+   (global.set $__THREW__
+    (local.get $0)
    )
   )
  )
diff --git a/test/memorygrowth.fromasm b/test/memorygrowth.fromasm
index 220bba783..9c909afcf 100644
--- a/test/memorygrowth.fromasm
+++ b/test/memorygrowth.fromasm
@@ -14,7 +14,6 @@
  (import "env" "__memory_base" (global $__memory_base i32))
  (import "env" "__table_base" (global $__table_base i32))
  (import "env" "STACKTOP" (global $r$asm2wasm$import i32))
- (import "env" "STACK_MAX" (global $s$asm2wasm$import i32))
  (import "env" "abort" (func $ja (param i32)))
  (import "env" "_pthread_cleanup_pop" (func $oa (param i32)))
  (import "env" "___lock" (func $pa (param i32)))
@@ -28,9 +27,7 @@
  (import "env" "___unlock" (func $xa (param i32)))
  (import "env" "___syscall146" (func $ya (param i32 i32) (result i32)))
  (global $r (mut i32) (global.get $r$asm2wasm$import))
- (global $s (mut i32) (global.get $s$asm2wasm$import))
  (global $v (mut i32) (i32.const 0))
- (global $w (mut i32) (i32.const 0))
  (global $K (mut i32) (i32.const 0))
  (export "__growWasmMemory" (func $__growWasmMemory))
  (export "_free" (func $fb))
@@ -9012,13 +9009,8 @@
    (i32.eqz
     (global.get $v)
    )
-   (block
-    (global.set $v
-     (local.get $0)
-    )
-    (global.set $w
-     (local.get $1)
-    )
+   (global.set $v
+    (local.get $0)
    )
   )
  )
@@ -9054,9 +9046,6 @@
   (global.set $r
    (local.get $0)
   )
-  (global.set $s
-   (local.get $1)
-  )
  )
  (func $nb (; has Stack IR ;) (param $0 i32) (result i32)
   (call $ja
diff --git a/test/memorygrowth.fromasm.clamp b/test/memorygrowth.fromasm.clamp
index 220bba783..9c909afcf 100644
--- a/test/memorygrowth.fromasm.clamp
+++ b/test/memorygrowth.fromasm.clamp
@@ -14,7 +14,6 @@
  (import "env" "__memory_base" (global $__memory_base i32))
  (import "env" "__table_base" (global $__table_base i32))
  (import "env" "STACKTOP" (global $r$asm2wasm$import i32))
- (import "env" "STACK_MAX" (global $s$asm2wasm$import i32))
  (import "env" "abort" (func $ja (param i32)))
  (import "env" "_pthread_cleanup_pop" (func $oa (param i32)))
  (import "env" "___lock" (func $pa (param i32)))
@@ -28,9 +27,7 @@
  (import "env" "___unlock" (func $xa (param i32)))
  (import "env" "___syscall146" (func $ya (param i32 i32) (result i32)))
  (global $r (mut i32) (global.get $r$asm2wasm$import))
- (global $s (mut i32) (global.get $s$asm2wasm$import))
  (global $v (mut i32) (i32.const 0))
- (global $w (mut i32) (i32.const 0))
  (global $K (mut i32) (i32.const 0))
  (export "__growWasmMemory" (func $__growWasmMemory))
  (export "_free" (func $fb))
@@ -9012,13 +9009,8 @@
    (i32.eqz
     (global.get $v)
    )
-   (block
-    (global.set $v
-     (local.get $0)
-    )
-    (global.set $w
-     (local.get $1)
-    )
+   (global.set $v
+    (local.get $0)
    )
   )
  )
@@ -9054,9 +9046,6 @@
   (global.set $r
    (local.get $0)
   )
-  (global.set $s
-   (local.get $1)
-  )
  )
  (func $nb (; has Stack IR ;) (param $0 i32) (result i32)
   (call $ja
diff --git a/test/memorygrowth.fromasm.imprecise b/test/memorygrowth.fromasm.imprecise
index cec88c90f..b90edfcd6 100644
--- a/test/memorygrowth.fromasm.imprecise
+++ b/test/memorygrowth.fromasm.imprecise
@@ -12,7 +12,6 @@
  (elem (global.get $__table_base) $nb $Oa $ob $Va $Ua $Ra $pb $Sa)
  (import "env" "__table_base" (global $__table_base i32))
  (import "env" "STACKTOP" (global $r$asm2wasm$import i32))
- (import "env" "STACK_MAX" (global $s$asm2wasm$import i32))
  (import "env" "abort" (func $ja (param i32)))
  (import "env" "_pthread_cleanup_pop" (func $oa (param i32)))
  (import "env" "___lock" (func $pa (param i32)))
@@ -26,9 +25,7 @@
  (import "env" "___unlock" (func $xa (param i32)))
  (import "env" "___syscall146" (func $ya (param i32 i32) (result i32)))
  (global $r (mut i32) (global.get $r$asm2wasm$import))
- (global $s (mut i32) (global.get $s$asm2wasm$import))
  (global $v (mut i32) (i32.const 0))
- (global $w (mut i32) (i32.const 0))
  (global $K (mut i32) (i32.const 0))
  (export "__growWasmMemory" (func $__growWasmMemory))
  (export "_free" (func $fb))
@@ -8986,13 +8983,8 @@
    (i32.eqz
     (global.get $v)
    )
-   (block
-    (global.set $v
-     (local.get $0)
-    )
-    (global.set $w
-     (local.get $1)
-    )
+   (global.set $v
+    (local.get $0)
    )
   )
  )
@@ -9024,9 +9016,6 @@
   (global.set $r
    (local.get $0)
   )
-  (global.set $s
-   (local.get $1)
-  )
  )
  (func $nb (; has Stack IR ;) (param $0 i32) (result i32)
   (call $ja
diff --git a/test/passes/O1.txt b/test/passes/O1.txt
index f8bad3bb6..11352bab2 100644
--- a/test/passes/O1.txt
+++ b/test/passes/O1.txt
@@ -1,12 +1,8 @@
 (module
  (type $none_=>_i32 (func (result i32)))
  (memory $0 1 1)
- (global $global$0 (mut i32) (i32.const 10))
  (export "foo" (func $0))
  (func $0 (result i32)
-  (global.set $global$0
-   (i32.const 0)
-  )
   (i32.load align=1
    (i32.const 4)
   )
diff --git a/test/passes/O4_disable-bulk-memory.txt b/test/passes/O4_disable-bulk-memory.txt
index 71fcdf9e8..78c130ffb 100644
--- a/test/passes/O4_disable-bulk-memory.txt
+++ b/test/passes/O4_disable-bulk-memory.txt
@@ -1,11 +1,7 @@
 (module
  (type $none_=>_none (func))
- (global $global$0 (mut i32) (i32.const 10))
  (export "func_59_invoker" (func $0))
  (func $0 (; has Stack IR ;)
-  (global.set $global$0
-   (i32.const 0)
-  )
   (unreachable)
  )
 )
diff --git a/test/passes/simplify-globals-optimizing_enable-mutable-globals.txt b/test/passes/simplify-globals-optimizing_enable-mutable-globals.txt
index e026f01e8..2876b4f37 100644
--- a/test/passes/simplify-globals-optimizing_enable-mutable-globals.txt
+++ b/test/passes/simplify-globals-optimizing_enable-mutable-globals.txt
@@ -39,11 +39,9 @@
 (module
  (type $none_=>_none (func))
  (import "env" "global-1" (global $g1 i32))
- (global $g2 (mut i32) (global.get $g1))
+ (global $g2 i32 (global.get $g1))
  (func $foo
-  (global.set $g2
-   (unreachable)
-  )
+  (nop)
  )
 )
 (module
@@ -134,7 +132,7 @@
 (module
  (type $i32_=>_i32 (func (param i32) (result i32)))
  (global $g1 (mut i32) (i32.const 1))
- (global $g2 (mut i32) (i32.const 1))
+ (global $g2 i32 (i32.const 1))
  (func $no (param $x i32) (result i32)
   (global.set $g1
    (i32.const 100)
@@ -159,9 +157,6 @@
   (global.set $g1
    (i32.const 100)
   )
-  (global.set $g2
-   (local.get $0)
-  )
   (i32.const 100)
  )
 )
diff --git a/test/passes/simplify-globals_all-features.txt b/test/passes/simplify-globals_all-features.txt
index d52c74cc4..c851d67b5 100644
--- a/test/passes/simplify-globals_all-features.txt
+++ b/test/passes/simplify-globals_all-features.txt
@@ -39,11 +39,9 @@
 (module
  (type $none_=>_none (func))
  (import "env" "global-1" (global $g1 i32))
- (global $g2 (mut i32) (global.get $g1))
+ (global $g2 i32 (global.get $g1))
  (func $foo
-  (global.set $g2
-   (unreachable)
-  )
+  (nop)
  )
 )
 (module
@@ -180,7 +178,7 @@
 (module
  (type $i32_=>_i32 (func (param i32) (result i32)))
  (global $g1 (mut i32) (i32.const 1))
- (global $g2 (mut i32) (i32.const 1))
+ (global $g2 i32 (i32.const 1))
  (func $no (param $x i32) (result i32)
   (global.set $g1
    (i32.const 100)
@@ -205,9 +203,7 @@
   (global.set $g1
    (i32.const 100)
   )
-  (global.set $g2
-   (local.get $x)
-  )
+  (nop)
   (i32.const 100)
  )
 )
@@ -230,3 +226,10 @@
   )
  )
 )
+(module
+ (type $none_=>_none (func))
+ (global $write-only i32 (i32.const 1))
+ (func $foo
+  (nop)
+ )
+)
diff --git a/test/passes/simplify-globals_all-features.wast b/test/passes/simplify-globals_all-features.wast
index f7470f959..8c93d1390 100644
--- a/test/passes/simplify-globals_all-features.wast
+++ b/test/passes/simplify-globals_all-features.wast
@@ -139,3 +139,10 @@
     (drop (global.get $g3))
   )
 )
+;; Global is used by `set` but never `get` can be eliminated.
+(module
+  (global $write-only (mut i32) (i32.const 1))
+  (func $foo
+    (global.set $write-only (i32.const 2))
+  )
+)
diff --git a/test/wasm2js/global_i64.2asm.js.opt b/test/wasm2js/global_i64.2asm.js.opt
index f4bbd539f..d57e44121 100644
--- a/test/wasm2js/global_i64.2asm.js.opt
+++ b/test/wasm2js/global_i64.2asm.js.opt
@@ -20,11 +20,8 @@ function asmFunc(global, env, buffer) {
  var abort = env.abort;
  var nan = global.NaN;
  var infinity = global.Infinity;
- var f = -1412567121;
- var f$hi = 305419896;
  function $1() {
-  f = 1432778632;
-  f$hi = 287454020;
+  
  }
  
  var FUNCTION_TABLE = [];