Re: [RFC PATCH]: Reciprocal sqrt (rsqrt) conversion pass

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

> Also note, that rsqrt is a target builtin (available in all
> frontends).

I think it should be target-independent the same way as cexpi (putting 
it in all the front-ends :-( unfortunately).  Furthermore, place it in 
an optab so that target-independent code in expand can use it to expand 
sqrt(a) to a*sqrt(a) and a/sqrt(b) to a*rsqrt(b).

For the latter, if there is a single use of sqrt(b), TER will build the 
expression for you so that you can magically see it in expand.  For more 
complex cases, the cse-recip pass can do a/sqrt(b)=>a*rsqrt(b), by 
modifying the creation of the reciprocal value (testing if the RHS is a 
sqrt).

I think you should also do the sqrt(a/b)=>rsqrt(b/a) transformation at 
the beginning of the cse-recip pass, possibly with a separate loop.  If 
possible, make sure that it is further converted to rsqrt(b*(1/a)) by 
the same pass if profitable.  (This however is thin ice).  Of course, 
tree-ssa-math-opts.c can also look at the optab to avoid doing a 
fruitless transformation.

> a/b is already mangled by NR-enhanced reciprocal
> transformation. 

NR-enhanced reciprocals should also be created at expansion time only, 
so that the recip pass can see what they do.

Paolo