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Re: Emit more REG_EQUIV notes for function args (PR42235)
- From: Bernd Schmidt <bernds at codesourcery dot com>
- To: Jeff Law <law at redhat dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 14 Jul 2010 23:30:10 +0200
- Subject: Re: Emit more REG_EQUIV notes for function args (PR42235)
- References: <4C3D9C06.email@example.com> <4C3E07EF.firstname.lastname@example.org>
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.