On 09/03/2015 12:53 PM, Wilco Dijkstra wrote:
Segher Boessenkool wrote:
On Thu, Sep 03, 2015 at 10:09:36AM -0600, Jeff Law wrote:
You will end up with a *lot* of target hooks like this. It will also
make testing harder (less coverage). I am not sure that is a good idea.
We certainly need a lot more target hooks in general so GCC can do the
right thing
(rather than using costs inconsistently all over the place). But that's a
different
discussion...
Let's be very careful here, target hooks aren't always the solution.
I'd rather see the costing models fixed and use those across the board.
But frankly, I don't know how to fix the costing models.
Combine doesn't currently use costs to decide how to simplify and
canonicalise things. Simplifications are what is simpler RTL; combine's
job is to make fewer RTL instructions (which is not the same thing as
fewer machine instructions, or cheaper instructions). Changing what is
canonical based on target hooks would be, uh, interesting.
Would it be reasonable to query the rtx_cost of a compare+and and if the cost
is the same as an AND assume that that instruction does not need to be
"improved"
into the canonical form? That way it will use the compare+and pattern if it
exists
and still try the zero_extract/shift+and forms for targets that don't have a
compare+and instruction.
Perhaps -- but you also have to make sure that you don't regress cases
where canonicalization in turn exposes simplifications due to related
insns in the chain.
Jeff
