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Date: Fri, 14 Aug 2020 11:23:07 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Senthil Kumar Selvaraj <senthil.thecoder@gmail.com>
Cc: gcc@gcc.gnu.org, ebotcazou@adacore.com
Subject: Re: Clobber REG_CC only for some constraint alternatives?
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Hi!

On Fri, Aug 14, 2020 at 04:46:59PM +0530, Senthil Kumar Selvaraj via Gcc wrote:
>   I'm working on porting AVR to MODE_CC,

I'm very happy to see people work on that.

> (define_insn "*mov<mode>_insn_noclobber_flags"
>   [(set (match_operand:ALL1 0 "nonimmediate_operand" "=r,d   ,q,r")
>         (match_operand:ALL1 1 "nox_general_operand"   "r,n Ynn,r,q"))
>    (clobber (const_int 0))]

This is not correct, clobbers like that are not defined RTL, and are
actually used internally (by combine at least), so this will confuse
that.

If you want to say some alternative does not clobber anything, just use
the constraint "X" for that alternative.

$ grep -r "clobber.*scratch.*X" gcc/config/

HtH,


Segher