[C/C++ PATCH] Don't convert RHS of a shift-expression to int (PR c/63862)

[Richard Biener]

On Wed, 26 Nov 2014, Jakub Jelinek wrote:

> On Wed, Nov 26, 2014 at 10:20:39PM +0100, Richard Biener wrote:
> > Well, if you want to aggressively prune unused bits then you could "back-propagate" the shift count value-range.
> > 
> > And note that all the frontend shorten-optimizations should move to the middle-end.
> > 
> > That said, instead of casting to int we could cast to unsigned char...
> 
> As long as we don't have > 256 bitsize integers, yes, for the purposes of
> GIMPLE.  Though, as with type demotion, then it might be worthwhile to try
> to promote it to wider type if the shift instructions operate on wider shift
> count modes than QImode.

True.  I suppose the promotion pass could promote/demote the shift count
to a mode appropriate for the target.  But that's again orthogonal to
optimizing the computation of the shift count based on its value-range
which does not invoke undefined behavior (thus, shortening of those
computations).  We don't have a pass yet that allows this kind of
"back"-propagation though of course a simple shortening pass wouldn't
be hard to implement (or similar to what the FEs do now we can add
some patterns to match.pd catching those opportunities).

Richard.