llvm dev - Mar 2010 - [LLVMdev] folding x * 0 = 0

On Wednesday 03 March 2010 15:11:48 Török Edwin wrote:
> > We need to figure out exactly what "strict" means.  Is it
like "restrict"
> > in C where it only has meaning with respect to other things marked
> > "restrict" or is it more general?  Can I reassociate a
strict with a
> > relaxed?  Is it possible that "strict" will mean different
things for
> > different operations?
> Lets note associative adds with a+, and strict ones with s+.
>
> For associativity for example you need 2 operations to be associative:
> (X a+ Y) a+ Z -> you can transform this to X a+ (Y a+ Z)
>
> (X s+ Y) a+ Z -> you can't transform this
>
> ((X s+ Y) a+ X) a+ Y, you can partially transform it,
>  1) lets note X s+ Y = Z
>  2) (Z a+ X) a+ Y -> Z a+ (X a+ Y) -> (X s+ Y) a+ (X a+ Y)
>  3) you can always choose to restrict a relaxed op (a+ -> s+)
>     (X s+ Y) a+ (X s+ Y) -> 2 a* (X s+ Y)
That's sort of along the lines of what I was thinking.  I was just trying to
list the questions that immediately came to mind, not looking for solutions
just yet.  :)
> > We may need something akin to a fence over which no code motion can
> > occur.
>
> Maybe for signaling NaN, I don't know how those things work.
> I tend to avoid using any FP code in my code ;)
Signalling NaN is one case.  I'm sure there are others.
> > Some compilers use special operations to represent parentheses for
> > Fortran codes.  I don't think that will work very well for LLVM,
however.
>
> With strict/relaxed flags for each operation (not value!) I think it can
> work, IF the frontend sets the flags appropriately.
I agree.

                                                          -Dave

On Mar 3, 2010, at 1:21 PM, David Greene wrote:
>>> 
>>> We may need something akin to a fence over which no code motion can
>>> occur.
>> 
>> Maybe for signaling NaN, I don't know how those things work.
>> I tend to avoid using any FP code in my code ;)
> 
> Signalling NaN is one case.  I'm sure there are others.
The only other thing I could imagine that it is useful for is for rounding mode
control.  IMO rounding mode should be explicitly marked on the instruction as
well.

-Chris

On Wednesday 03 March 2010 15:38:06 Chris Lattner wrote:
> > Signalling NaN is one case.  I'm sure there are others.
>
> The only other thing I could imagine that it is useful for is for rounding
> mode control. 
Yep.
> IMO rounding mode should be explicitly marked on the
> instruction as well.
That would also be useful for some GPUs where each instruction can specify
its own rounding mode.

                                                -Dave

On Wednesday 03 March 2010 21:38:06 Chris Lattner wrote:
> The only other thing I could imagine that it is useful for is for rounding
> mode control.  IMO rounding mode should be explicitly marked on the
> instruction as well.
Ooh, that would be very cool. :-)

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e