Compiler: micro-optimizations in merge_if_vars (#12432)

* Don't create `MetaVar` unless needed

* Make check earlier to avoid extra Hash lookups

* Even less extra Hash lookups

* Compiler: avoid creating Set in merge_if_vars (#12433)

src/compiler/crystal/semantic/main_visitor.cr

@@ -1887,85 +1887,88 @@ module Crystal
     #   - Don't use the type of a branch that is unreachable (ends with return,
     #     break or with a call that is NoReturn)
     def merge_if_vars(node, cond_vars, then_vars, else_vars, before_then_vars, before_else_vars, then_unreachable, else_unreachable)
-      all_vars_names = Set(String).new
-      then_vars.each_key do |name|
-        all_vars_names << name
-      end
-      else_vars.each_key do |name|
-        all_vars_names << name
+      then_vars.each do |name, then_var|
+        else_var = else_vars[name]?
+
+        merge_if_var(name, node, cond_vars, then_var, else_var, before_then_vars, before_else_vars, then_unreachable, else_unreachable)
       end
 
-      all_vars_names.each do |name|
-        cond_var = cond_vars[name]?
-        then_var = then_vars[name]?
-        before_then_var = before_then_vars[name]?
-        else_var = else_vars[name]?
-        before_else_var = before_else_vars[name]?
+      else_vars.each do |name, else_var|
+        next if then_vars.has_key?(name)
 
-        # Check whether the var didn't change at all
-        next if then_var.same?(else_var)
+        merge_if_var(name, node, cond_vars, nil, else_var, before_then_vars, before_else_vars, then_unreachable, else_unreachable)
+      end
+    end
 
-        if_var = MetaVar.new(name)
+    def merge_if_var(name, node, cond_vars, then_var, else_var, before_then_vars, before_else_vars, then_unreachable, else_unreachable)
+      # Check whether the var didn't change at all
+      return if then_var.same?(else_var)
 
-        # Only copy `nil_if_read` from each branch if it's not unreachable
-        then_var_nil_if_read = !then_unreachable && then_var.try(&.nil_if_read?)
-        else_var_nil_if_read = !else_unreachable && else_var.try(&.nil_if_read?)
+      cond_var = cond_vars[name]?
 
-        if_var.nil_if_read = !!(then_var_nil_if_read || else_var_nil_if_read)
+      # Only copy `nil_if_read` from each branch if it's not unreachable
+      then_var_nil_if_read = !then_unreachable && then_var.try(&.nil_if_read?)
+      else_var_nil_if_read = !else_unreachable && else_var.try(&.nil_if_read?)
+      if_var_nil_if_read = !!(then_var_nil_if_read || else_var_nil_if_read)
 
-        # Check if no types were changes in either then 'then' and 'else' branches
-        if cond_var && then_var.same?(before_then_var) && else_var.same?(before_else_var) && !then_unreachable && !else_unreachable
-          cond_var.nil_if_read = if_var.nil_if_read?
+      # Check if no types were changes in either then 'then' and 'else' branches
+      if cond_var && !then_unreachable && !else_unreachable
+        if then_var.same?(before_then_vars[name]?) &&
+           else_var.same?(before_else_vars[name]?)
+          cond_var.nil_if_read = if_var_nil_if_read
           @vars[name] = cond_var
-          next
+          return
         end
+      end
 
-        if then_var && else_var
-          if then_unreachable
-            if_var.bind_to conditional_no_return(node.then, then_var)
-          else
-            if_var.bind_to then_var
-          end
+      if_var = MetaVar.new(name)
+      if_var.nil_if_read = if_var_nil_if_read
 
-          if else_unreachable
-            if_var.bind_to conditional_no_return(node.else, else_var)
-          else
-            if_var.bind_to else_var
-          end
-        elsif then_var
-          if then_unreachable
-            if_var.bind_to conditional_no_return(node.then, then_var)
-          else
-            if_var.bind_to then_var
-          end
+      if then_var && else_var
+        if then_unreachable
+          if_var.bind_to conditional_no_return(node.then, then_var)
+        else
+          if_var.bind_to then_var
+        end
 
-          if cond_var
-            if_var.bind_to cond_var
-          elsif !else_unreachable
-            if_var.bind_to program.nil_var
-            if_var.nil_if_read = true
-          else
-            if_var.bind_to conditional_no_return(node.else, @program.nil_var)
-          end
-        elsif else_var
-          if else_unreachable
-            if_var.bind_to conditional_no_return(node.else, else_var)
-          else
-            if_var.bind_to else_var
-          end
+        if else_unreachable
+          if_var.bind_to conditional_no_return(node.else, else_var)
+        else
+          if_var.bind_to else_var
+        end
+      elsif then_var
+        if then_unreachable
+          if_var.bind_to conditional_no_return(node.then, then_var)
+        else
+          if_var.bind_to then_var
+        end
 
-          if cond_var
-            if_var.bind_to cond_var
-          elsif !then_unreachable
-            if_var.bind_to program.nil_var
-            if_var.nil_if_read = true
-          else
-            if_var.bind_to conditional_no_return(node.then, @program.nil_var)
-          end
+        if cond_var
+          if_var.bind_to cond_var
+        elsif !else_unreachable
+          if_var.bind_to program.nil_var
+          if_var.nil_if_read = true
+        else
+          if_var.bind_to conditional_no_return(node.else, @program.nil_var)
+        end
+      elsif else_var
+        if else_unreachable
+          if_var.bind_to conditional_no_return(node.else, else_var)
+        else
+          if_var.bind_to else_var
         end
 
-        @vars[name] = if_var
+        if cond_var
+          if_var.bind_to cond_var
+        elsif !then_unreachable
+          if_var.bind_to program.nil_var
+          if_var.nil_if_read = true
+        else
+          if_var.bind_to conditional_no_return(node.then, @program.nil_var)
+        end
       end
+
+      @vars[name] = if_var
     end
 
     def conditional_no_return(node, var)