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Date: Thu, 20 Aug 2015 08:22:00 -0000
Message-ID: <CAFiYyc1TTP9xixrB3Aw3eGYN+kYue+r3MH1dUQUmuWXRsHtRdw@mail.gmail.com>
Subject: Re: Move some flag_unsafe_math_optimizations using simplify and match
From: Richard Biener <richard.guenther@gmail.com>
To: GCC Patches <gcc-patches@gcc.gnu.org>
Cc: "Hurugalawadi, Naveen" <Naveen.Hurugalawadi@caviumnetworks.com>
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On Thu, Aug 20, 2015 at 7:38 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Thu, 20 Aug 2015, Hurugalawadi, Naveen wrote:
>
>> The following testcase does not generate "x" as needed.
>> ====================
>> double t (double x)
>> {
>> x = sqrt (x) * sqrt (x);
>> return x;
>> }
>> ====================
>
>
> With -fno-math-errno, we CSE the calls to sqrt, so I would expect this to
> match:
>
>   (mult (SQRT@1 @0) @1)
>
> Without the flag, I expect that one will apply
>
>  (simplify
>   (mult (SQRT:s @0) (SQRT:s @1))
>   (SQRT (mult @0 @1)))
>
> and then maybe we have something converting sqrt(x*x) to abs(x) or maybe
> not.

ICK.  I'd rather have CSE still CSE the two calls by adding some
tricks regarding
to errno ...

> I wonder if all the unsafe math optimizations are really ok without
> -fno-math-errno...

Well, on GIMPLE they will preserve the original calls because of their
side-effects
setting errno...  on GENERIC probably not.

Richard.

> --
> Marc Glisse