On Mon, 5 Dec 2016, Jeff Law wrote: > On 12/02/2016 01:33 AM, Richard Biener wrote: > > > The LHS on the assignment makes it easier to identify when a tail call is > > > possible. It's not needed for correctness. Not having the LHS on the > > > assignment just means we won't get an optimized tail call. > > > > > > Under what circumstances would the LHS possibly be removed? We know the > > > return statement references the LHS, so it's not going to be something > > > that > > > DCE will do. > > > > Well, I thought Prathamesh added the optimization to copy-propagate > > the lhs from the returned argument. So we'd have both transforms here. > That seems like a mistake -- the fact that we can copy propagate the LHS from > the returned argument is interesting, but in practice I've found it to not be > useful to do so. > > The problem is it makes the value look live across a the call and we're then > dependent upon the register allocator to know the trick about the returned > argument value and apply it consistently -- which it does not last I checked. > > I think we're better off leaving the call in the form of LHS = call () if the > return value is used. That's going to be more palatable to tail calling.
Yes, that's something I also raised earlier in the thread. Note that any kind of value-numbering probably wants to know the equivalence for simplifications but in the end wants to disable propagating the copy (in fact it should propagate the return value from the point of the call). I suppose I know how to implement that in FRE/PRE given it has separate value-numbering and elimination phases. Something for GCC 8. > > Of course as always the user could have written the code in this way. > > > > If the LHS is not required for correctness then I don't think we need > > to put it there - Pratamesh verified the call is tail-called already > > if marked by the tailcall pass, even if the LHS is not present. > But if the function returns the value from the tail call, then going through > an LHS is the right thing to do. Using the magic "argX will be the return > value" seems clever, but actually hurts in practice. So we do want the reverse transform (for the case of returning by reference that's going to be tricky if not impossible due to the IL hygiene we enforce). Richard.
