[PR65768] Check rtx_cost when propagating constant

[Jeff Law]

On 05/29/2015 12:32 AM, Kugan wrote:
>>>>
>>>>      PR target/65768
>>>>      * cprop.c (try_replace_reg): Check cost of constants before
>>>> propagating.
>> I should have also noted, fresh bootstrap & regression test is needed too.
>
> Thanks Jeff for the comments. I did a fresh bootstrap and regression
> testing on x86_64-linux-gnu with no new regression. I will wait for you ACK.
Can you address the 3 issues in my prior message?  I'll include them 
here for clarity:

--

The "const_p" variable is poorly named, though I can kindof see how you 
settled on it.  Maybe "check_rtx_costs" or something along those lines 
would be better.

The comment for the second hunk would probably be better as:

/* If TO is a constant, check the cost of the set after propagation
    to the cost of the set before the propagation.  If the cost is
    higher, then do not replace FROM with TO.  */


You should try to produce a testcase where this change shows a code 
generation improvement.    Given we're checking target costs, that test 
will naturally be target specific.  But please do try.

So with the two nits fixed and a testcase, I think this can go forward.
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