Re: Question about merging two instructions.

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

> 1. Can you please give me an example of something bad that can happen to 
> the LHS.  Maybe I'm missing something here.

In this case nothing, but if NEW were a subreg, it can change a lot.

> 3. Isn't it reasonable to expect that every instance on old_rtx will be 
> replaced by new_rtx even if it can't be simplified?
> This is what I understand from the function's documentation.
> But actually every expressions that can't be simplified is not replaced.

SET is not an expression, so it is not handled by simplify_replace_rtx.

I agree that it is confusing, that

  x = simplify_replace_rtx (a, b, c);

is different from

  x = simplify_rtx (replace_rtx (a, b, c));

So maybe all you need is changing the final "return x" to "return 
replace_rtx (x, old, new);".  I reckon this change would be much cleaner 
than handling SET, but I would very much want to see what a GWP person 
thinks about this.

Paolo