On 11/22/13 18:43, Kai Tietz wrote:
----- Original Message -----
That is actual wished. We shouldn't come to patterns, which have more
type-casts by this patch.
What we see here is the normalization of shift-left/right operations. This
patch takes care that we prefer in general (Type) (X shift-left/right y) over
((Type) X) shift-left/right y notation.
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....
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.
Unfortunately, within a pass (or even a phase of a pass) we can have
both cases. Let's look at the code in tree-ssa-forwprop.c you're
proposing we change.
The code in question loops over every statement and tries to combine the
current statement with the statements which define its operands, then it
moves on to the next statement in execution order.
So your patch seems to make perfect sense when STMT is a shift. It
moves the typecast out of the way of combining STMT with the statements
which define its operands.
However, once we move to the next statement in the IL, the shift is now
defining operands for later statements and the other form would seem to
be preferred.
So it almost seems to me you want to apply the transformation on STMT,
try to combine into the STMT, then apply the reverse transformation on
STMT if it's safe to do so.
That kindof implies there really isn't a perferred global form for
shifts with typecasts, but instead there is a form that is preferred
based on the optimizations we're trying to perform at any given time.
It may also argue that type promotion/demotion in general shouldn't be
separate passes. I'm still pondering that.
Thoughts?
Jeff
