[match.pd] PR83750 - CSE erf/erfc pair

[Richard Biener]

On Mon, 18 Oct 2021, Prathamesh Kulkarni wrote:

> Hi Richard,
> As suggested in PR, I have attached WIP patch that adds two patterns
> to match.pd:
> erfc(x) --> 1 - erf(x) if canonicalize_math_p() and,
> 1 - erf(x) --> erfc(x) if !canonicalize_math_p().
> 
> This works to remove call to erfc for the following test:
> double f(double x)
> {
>   double g(double, double);
> 
>   double t1 = __builtin_erf (x);
>   double t2 = __builtin_erfc (x);
>   return g(t1, t2);
> }
> 
> with .optimized dump shows:
>   t1_2 = __builtin_erf (x_1(D));
>   t2_3 = 1.0e+0 - t1_2;
> 
> However, for the following test:
> double f(double x)
> {
>   double g(double, double);
> 
>   double t1 = __builtin_erfc (x);
>   return t1;
> }
> 
> It canonicalizes erfc(x) to 1 - erf(x), but does not transform 1 -
> erf(x) to erfc(x) again
> post canonicalization.
> -fdump-tree-folding shows that 1 - erf(x) --> erfc(x) gets applied,
> but then it tries to
> resimplify erfc(x), which fails post canonicalization. So we end up
> with erfc(x) transformed to
> 1 - erf(x) in .optimized dump, which I suppose isn't ideal.
> Could you suggest how to proceed ?

I applied your patch manually and it does the intended
simplifications so I wonder what I am missing?

Richard.

> Thanks,
> Prathamesh
> 

-- 
Richard Biener <rguenther@suse.de>
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