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Re: Patch for the shift count problem


Marek Michalkiewicz wrote:
On Wed, Jan 19, 2005 at 10:39:46PM +0100, Björn Haase wrote:

Bernardo Innocenti asked me to forward you the following patch concerning the shift count problem. It's a slightly more comprehensive patch that addresses the same problem Bernardo Innocenti has solved.

Sorry Bjorn, it didn't occur to me that testing for values outside the datatype range isn't useful at all in the patterns that my patch fixed:

 ashlqi3_out
 ashrqi3_out
 lshrqi3_out


Your patch instead takes care of additional patterns:


 ashrqi3_out
 ashrsi3_out
 ashrsi3_out
 lshrqi3_out
 lshrhi3_out
 lshrsi3_out

(you're missing ashlqi3_out, btw).


So I think our patches should be combined, using my solution for the first three patterns, and yours for the rest.

After all, PR19293 and PR19329 weren't quite the
same bug, so I regret closing the latter as a
duplicate!


Thanks, but I see a potential problem: while shift count == 0
is clearly a no-op (as done in the original patch), "strange"
shift counts (by more bits than operand width) shouldn't be
treated as such IMHO.  The way it works now (clear all bits,
or copy sign bit to all bits for ashr*) looks correct to me.

This is undefined behavior according to the C standard. Some hardware barrel shifters just wrap the shift count to the word size. Others output 0 as we do.

Not sure what to do with negative shift counts: does this
ever happen?  Should this be handled like count == 0, or
perhaps by shifting in the opposite direction?  If it should
never happen, better report an internal compiler error...

I think the C standard says this is undefined behavior too, and GCC emits a warning if you do it explicitly in source code.

--
 // Bernardo Innocenti - Develer S.r.l., R&D dept.
\X/  http://www.develer.com/


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