Re: Libgcc divide vectorization question

[Andrew Stubbs <ams@codesourcery.com>]

On 22/03/2023 13:56, Richard Biener wrote:
>> Basically, the -ffast-math instructions will always be the fastest way,
>> but the goal is that the default optimization shouldn't just disable
>> vectorization entirely for any loop that has a divide in it.
> 
> We try to express division as multiplication, but yes, I think there's
> currently no way to tell the vectorizer that vectorized division is
> available as libcall (nor for any other arithmetic operator that is not
> a call in the first place).

I have considered creating a new builtin code, similar to the libm 
functions, that would be enabled by a backend hook, or maybe just if 
TARGET_VECTORIZE_BUILTIN_VECTORIZED_FUNCTION doesn't return NULL. The 
vectorizer would then use that, somehow. To treat it just like any other 
builtin it would have to be set before the vectorizer pass encounters 
it, which is probably not ideal for all the other passes that want to 
handle divide operators. Alternatively, the vectorizable_operation 
function could detect and introduce the builtin where appropriate.

Would this be acceptable, or am I wasting my time planning something 
that would get rejected?

Thanks

Andrew