StackIR: Remove nops (#5746)

No nop instruction is necessary in wasm, so in StackIR we can simply
remove them all.

Fixes #5745

1 files changed, 17 insertions, 0 deletions

diff --git a/src/passes/StackIR.cpp b/src/passes/StackIR.cpp
index bbce6bae8..438ed8b55 100644
--- a/src/passes/StackIR.cpp
+++ b/src/passes/StackIR.cpp
@@ -73,6 +73,7 @@ public:
     }
     removeUnneededBlocks();
     dce();
+    vacuum();
   }
 
 private:
@@ -100,6 +101,22 @@ private:
     }
   }
 
+  // Remove obviously-unneeded code.
+  void vacuum() {
+    // In the wasm binary format a nop is never needed. (In Binaryen IR, in
+    // comparison, it is necessary e.g. in a function body or an if arm.)
+    //
+    // It is especially important to remove nops because we add nops when we
+    // read wasm into Binaryen IR. That is, this avoids a potential increase in
+    // code size.
+    for (Index i = 0; i < insts.size(); i++) {
+      auto*& inst = insts[i];
+      if (inst && inst->origin->is<Nop>()) {
+        inst = nullptr;
+      }
+    }
+  }
+
   // If ordered properly, we can avoid a local.set/local.get pair,
   // and use the value directly from the stack, for example
   //    [..produce a value on the stack..]