Re: Emit more REG_EQUIV notes for function args (PR42235)

[Jeff Law <law@redhat.com>]

On 07/14/10 15:30, Bernd Schmidt wrote:
> On 07/14/2010 08:54 PM, Jeff Law wrote:
>    
>> On 07/14/10 05:14, Bernd Schmidt wrote:
>>      
>>> When moving arguments into pseudos, we are being very careful not to
>>> emit any instructions that could possibly clobber other argument
>>> registers.  Currently, we generate unnecessarily complicated sequences
>>> of code for simple zero or sign extensions.
>>>
>>> With this patch, we check can_extend_p and the necessary predicates to
>>> see if an extend insn is available for the conversion we have to do.
>>>        
>> Right, but what guarantee do we have that the conversion insn doesn't
>> clobber a function argument register?   ISTM that to be safe you
>> actually have to scan the insns created by gen_extend_insn to ensure
>> they don't clobber something important.
>>
>> I'm not an expert on what ports do these days, but I did work on a port
>> (mn10200) where conversion "insns" where implemented as special function
>> calls under the hood.  I don't recall if we allowed those special
>> function calls to have visible side effects, but if they did, they'd
>> show up as clobbers/uses attached to the normal conversion insn.    Of
>> course the mn102 is dead, but I think it's method for implementing
>> conversions was valid and if another port were to do something similar
>> it would likely not interact well with your change.
>>      
> Hmm, ok.  That's awful, but I kind of expected someone would say that.
> Did this really happen for integer zero/sign extend, or only for
> floating point stuff?
>    
It was all the PSImode crap.

> If necessary I can try to test for a single insn with single_set and
> push it to the sequence otherwise.
>    
For the mn103, the conversions were single insns...

Ultimately,  I think you have to peek at the insn(s) and see what 
registers they set/clobber.

jeff