This comment was originally written by ngeoffray@google.com

Fixing the C.JSString_methods.$add here: https://codereview.chromium.org/23496037/.

Added this to the M7 milestone.

Is this fixed?

Removed this from the M7 milestone.
Added this to the Later milestone.

This comment was originally written by ngeoffray@google.com

No, we are not emitting += for s = s + 'foo';

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This comment was originally written by ngeoffray@google.com

Set owner to @floitschG.

Removed this from the Later milestone.
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The SSA output is now:

    main: function() {
      var x, s;
      x = K.returnString(1);
      s = "y" + H.S(x) + H.S(K.returnString(0)) + H.S(x);
      P.print(s);
      s = s + H.S(K.returnString(0)) + H.S(x);
      P.print(s);
      P.print(s);
    }

Although the SSA backend will convert s = s + "foo" to s += "foo", this is not the same as

s = s + a + b  -->  s += a + b

That requires reassociation of (s + a) + b to s + (a + b), which by itself is harmful since it requires introduction of parentheses.

The CPS output is

    main: function() {
      var x = K.returnString(1), s = "y" + H.S(x) + H.S(K.returnString(0)) + H.S(x);
      P.print(s);
      P.print(s = s + H.S(K.returnString(0)) + H.S(x));
      P.print(s);
    }

Generating += in any context is still TODO.

I'm closing this, since the occasional size benefit from reassociation followed by "+=" is small.
(dart2js does do limited reassociation combined with constant-folding,)