Re: [Patch match.pd] Add a simplify rule for x * copysign (1.0, y);

[James Greenhalgh <james dot greenhalgh at arm dot com>]

On Thu, Oct 01, 2015 at 03:28:22PM +0100, pinskia@gmail.com wrote:
> > 
> > On Oct 1, 2015, at 6:57 AM, James Greenhalgh <james.greenhalgh@arm.com> wrote:
> > 
> > 
> > Hi,
> > 
> > If it is cheap enough to treat a floating-point value as an integer and
> > to do bitwise arithmetic on it (as it is for AArch64) we can rewrite:
> > 
> >  x * copysign (1.0, y)
> > 
> > as:
> > 
> >  x ^ (y & (1 << sign_bit_position))
> 
> Why not just convert it to copysign (x, y) instead and let expand chose
> the better implementation?

Because that transformation is invalid :-)

let x = -1.0, y = -1.0

  x * copysign (1.0, y)
    =  -1.0 * copysign (1.0, -1.0)
    = -1.0 * -1.0
    = 1.0

  copysign (x, y)
    = copysign (-1.0, -1.0)
    = -1.0

Or have I completely lost my maths skills :-)

> Also I think this can only be done for finite and non trapping types. 

That may be well true, I swithered either way and went for no checks, but
I'd happily go back on that and wrap this in something suitable restrictive
if I need to.

Thanks,
James


> > 
> > This patch implements that rewriting rule in match.pd, and a testcase
> > expecting the transform.
> > 
> > This is worth about 6% in 481.wrf for AArch64. I don't don't know enough
> > about the x86 microarchitectures to know how productive this transformation
> > is there. In Spec2006FP I didn't see any interesting results in either
> > direction. Looking at code generation for the testcase I add, I think the
> > x86 code generation looks worse, but I can't understand why it doesn't use
> > a vector-side xor and load the mask vector-side. With that fixed up I think
> > the code generation would look better - though as I say, I'm not an expert
> > here...
> > 
> > Bootstrapped on both aarch64-none-linux-gnu and x86_64 with no issues.
> > 
> > OK for trunk?
> > 
> > Thanks,
> > James
> > 
> > ---
> > gcc/
> > 
> > 2015-10-01  James Greenhalgh  <james.greenhalgh@arm.com>
> > 
> >    * match.pd (mult (COPYSIGN:s real_onep @0) @1): New simplifier.
> > 
> > gcc/testsuite/
> > 
> > 2015-10-01  James Greenhalgh  <james.greenhalgh@arm.com>
> > 
> >    * gcc.dg/tree-ssa/copysign.c: New.
> > 
> > <0001-Patch-match.pd-Add-a-simplify-rule-for-x-copysign-1..patch>
>