Re: [PATCH 1/2, combine] Try REG_EQUAL for nonzero_bits

[Jeff Law <law@redhat.com>]

On 02/09/15 19:19, Thomas Preud'homme wrote:
>> From: Andrew Pinski [mailto:pinskia@gmail.com]
>> Sent: Tuesday, February 10, 2015 9:57 AM
>
>>> +#ifdef SHORT_IMMEDIATES_SIGN_EXTEND
>>> +/* If MODE has a precision lower than PREC and SRC is a non-negative
>> constant
>>> +   that would appear negative in MODE, sign-extend SRC for use in
>> nonzero_bits
>>> +   because some machines (maybe most) will actually do the sign-
>> extension and
>>> +   this is the conservative approach.
>>> +
>>> +   ??? For 2.5, try to tighten up the MD files in this regard instead of
>> this
>>> +   kludge.  */
>>
>> I don't know if this has been mentioned and even though you are just
>> copying a comment from below but would it make sense to look fixing
>> what the comment says we should look at after GCC 2.5 (which was over
>> 20 years ago)? Or maybe just remove the comment if it no longer
>> applies.
>
> Actually this bit seems unnecessary as there is already some logic in
> nonzero_bits1 for the CONST_INT case. So I guess the code can be
> removed and the comment be moved there at the very least but
> I'd prefer people from one of the affected target to test it.
>
> Looking for backend that define SHORT_IMMEDIATES_SIGN_EXTEND, that
> would be someone interested in alpha, frv, lm32, m32r, mep, mips, rs6000,
> rx, sh, tilegx or tilepro.
FWIW, I went back into the old gcc development archives for 1993 and 
couldn't find any reference/justification for this patch.  So no help in 
understanding the precise issue from the old archives.

Given the rs6000 is affected, one could do before/after tests natively 
in the gcc farm to ensure that removing that code doesn't change the 
generated code across a bootstrap.

That's probably how I'd approach gathering some data about whether or 
not the comment/code is still appropriate/needed.

jeff