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

[pinskia@gmail.com]

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

Thanks,
Andrew

> 
> 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>