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

[Bernd Schmidt <bernds at codesourcery dot com>]

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?

If necessary I can try to test for a single insn with single_set and
push it to the sequence otherwise.


Bernd