Re: How to avoid some built-in expansions in gcc?

[Georg-Johann Lay <avr@gjlay.de>]

Am 31.05.24 um 19:32 schrieb Richard Biener:
> 
> 
>> Am 31.05.2024 um 17:25 schrieb Paul Koning via Gcc <gcc@gcc.gnu.org>:
>>
>> 
>>
>>> On May 31, 2024, at 11:06 AM, Georg-Johann Lay <avr@gjlay.de> wrote:
>>>
>>>
>>>
>>> Am 31.05.24 um 17:00 schrieb Paul Koning:
>>>>>> On May 31, 2024, at 9:52 AM, Georg-Johann Lay <avr@gjlay.de> wrote:
>>>>>>
>>>>>> What's the recommended way to stop built-in expansions in gcc?
>>>>>>
>>>>>> For example, avr-gcc expands isinff() to a bloated version of an isinff() implementation that's written in asm (PR115307).
>>>>>>
>>>>>> Johann
>>>> Isn't that up to the target back end?
>>>>     paul
>>>
>>>
>>> Yes, that's the reason why it's a target PR.
>>>
>>> My question is where/how to do it.
>>>
>>> It's clear that twiddling the options works and is a simple and comprehensible solution, but it seems a bit of a hack to me.
>>>
>>> Johann
>>
>> I haven't dug deep into this, but I would think at least part of the answer is in the target cost functions.  If those assign RTX cost according to size, then the result would be the optimizer would favor smaller code.  Right?
>>
>> Does inline assembly expansion of builtins depend on target code supplying that expansion?  If so, the answer would be not to supply it, or at least not unless asked for by an option.  If it comes from common code, that's a different matter, then perhaps there should be target hooks to let the target disallow or discourage such expansion.  I might want such a thing for pdp11 as well.
> 
> The function in question is folded to a comparison very early if the target does not implement an optab for it.  After that everything is lost.  A workaround is to define an optab but let expansion always FAIL.
> 
> Richard

You have a pointer how to define a target optab? I looked into optabs 
code but found no appropriate hook.  For isinf<mode> is seems is is 
enough to provide a failing expander, but other functions like isnan 
don't have an optab entry, so there is a hook mechanism to extend optabs?

I also looked into patching options, but there is no way to hook in, or 
at least I did not find how to use targetm.handle_option or an 
appropriate place to call disable_builtin_function; it's all baken into 
the C front without any hook opportunity.

Johann