Re: [PATCH]middle-end Add optimized float addsub without needing VEC_PERM_EXPR.

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/17/2022 2:33 PM, Andrew Pinski via Gcc-patches wrote:
> On Thu, Jun 16, 2022 at 3:59 AM Tamar Christina via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>> Hi All,
>>
>> For IEEE 754 floating point formats we can replace a sequence of alternative
>> +/- with fneg of a wider type followed by an fadd.  This eliminated the need for
>> using a permutation.  This patch adds a math.pd rule to recognize and do this
>> rewriting.
> I don't think this is correct. You don't check the format of the
> floating point to make sure this is valid (e.g. REAL_MODE_FORMAT's
> signbit_rw/signbit_ro field).
> Also would just be better if you do the xor in integer mode (using
> signbit_rw field for the correct bit)?
> And then making sure the target optimizes the xor to the neg
> instruction when needed?
Whether or not the xor trick is better or not would be highly target 
dependent.  That seems like it's better left for expansion to figure out 
since we have target costing information at that point.

Jeff