Re: [Libjit-developers] What happens if I branch to a label that has not been defined?

[Paul Brannan]

On Fri, Sep 14, 2007 at 09:37:30AM +0700, Aleksey Demakov wrote:
> If I got correctly what you want perhaps we need
> to add a function that explicitly converts a given
> instruction into a no-op.

Or provide an interface to detach_block and attach_block that provide
more options for moving code around within the function.

> Meanwhile it might work if you insert a label just
> after the branch and in case you need to remove
> the branch you generate an extra code that
> branches back to that label:
> 
>           br label_1
> label_2:
>           ...
>           return
> label_1:
>           br label_2
> 
> These extra branches should be removed by
> the branch optimization pass and will not get
> into the final code.

I'm not sure whether or not that would work for what I'm trying to do.
I'll have to give it some thought.

The reason I asked the question in the first place is that I have DSL on
the Ruby side of ruby-libjit that looks like this:

  class Function
    def if(cond, end_label = Label.new, &block)
      false_label = Label.new
      insn_branch_if_not(cond, false_label)
      block.call
      insn_branch(end_label)
      insn_label(false_label)
      return If.new(self, end_label)
    end

    class If
      def initialize(function, end_label)
        @function = function
        @end_label = end_label
      end

      def else(&block)
        block.call
        return self
      end

      def end
        @function.insn_label(@end_label)
      end
    end
  end

So I can write:

  Function.compile(context, signature) do |f|
    f.if(f.get_param(0) == f.const(Type::INT, 1)) {
      f.debug_print("arg0 == 1")
    } .else {
      f.debug_print("arg0 != 1")
    } .end
    f.insn_return
  end

  f.apply(1) #=> "arg0 == 1"

I was trying to eliminate the need to call #end to finish the
conditional.

Paul