Re: [patch tree-ssa-forwprop]: Add type raising in shift-operations

[Jeff Law <law at redhat dot com>]

On 11/26/13 02:58, Richard Biener wrote:
> > What is the rationale behind one for over the other?  Or is it arbitrary?  I
> > can easily make a case for either form based on what we're trying to
> > optimize.  In general, it seems to me that....
>
> When (Type) is a widening cast then you have to worry to not introduce
> undefined behavior when y is larger than the number of bits in X.
> When (Type) is a shortening cast then we lose information - because
> then the range we can determine for y from the shift operation will
> be larger.
>
> So I'm not sure I follow the reasoning to move out the casting when
> possible.
Assume that we're not introducing undefined behaviour.  Obviously if 
we're introducing undefined behaviour, then the transformation is wrong, 
plain and simple.

If by moving the cast we can eliminate an ALU operation because the 
shift (in this example) combines with something earlier or later in the 
IL, then we win.  Similarly if by moving the cast we expose redundant 
casting, then we win.

A narrower range is obviously good, but often just having a narrower 
range won't in and of itself provide an optimization opportunity.



> The goal for canonicalization should be to shorten operations where
> possible.  Thus both cases, sinking and hoisting the cast should
> be applied dependent on validity and whether the shift will be performed
> in a smaller type in the end.
I disagree, there are going to be times when sinking or hoisting the 
cast allows the shift to combine with other instructions in the IL. 
There are times when the ability to combine with other instructions in 
the stream should be driving these decisions.

I would buy that as a guiding principle that applies in cases where we 
don't have other optimization opportunities we can expose by 
hoisting/sinking the type casts.  I would even buy that we prefer the 
narrowest range through some point in the compiler (say vrp2), then we 
allow wider ranges as needed to facilitate other optimizations.



> > If we're more interested in combining the shift with statements that define
> > the shift's operands, then the former is going to be a better representation
> > because we're typecasting the result and thus the typecast doesn't interfere
> > with the desired optimization.
> >
> > In contrast if we are more interested in cases where the shift defines
> > operands for some other statement, then the latter form is more beneficial
> > because we're typecasting the inputs and thus the typecast doesn't interfere
> > with the desired optimization.
>
> Always a trade-off which is why combining passes always have to
> consider casts.
Actually, I would say quite the opposite here.  There's a very clear 
line when we want to go from one representation to the other within the 
forwprop passes.  By doing so, we relieve the pattern matching aspects 
of that pass from needing to concern themselves with the typecasting issues.

> I think the goal of "canonicalizing" operations to work on the smallest
> mode possible during early GIMPLE passes makes the most sense
> (even if then a cast is not always outside or inside an operation).
>
> Later (currently during RTL expansion) we can widen operations again
> according to target needs.
But what you end up missing here is optimization opportunities that 
expose themselves when the casts are moved from inputs to outputs or 
vice-versa.


> Shorter operations simply carry more implicit information on value ranges
> (and are usually easier to vectorize for example).
Yes.  There's no argument about that.

jeff