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From: Eric Botcazou <ebotcazou@adacore.com>
To: Richard Sandiford <richard.sandiford@arm.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] lra: Tighten check for reloading paradoxical subregs
 [PR94052]
Date: Fri, 13 Mar 2020 11:44:10 +0100
Message-ID: <3276106.PqY0JBGKGl@polaris>
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> I think there are two problems with this:
> 
> (1) It never actually checks whether the hard register is valid for the
>     outer mode (in the hard_regno_mode_ok sense).  If it isn't, any attempt
>     to reload in the outer mode is likely to cycle, because the implied
>     regno/mode combination will be just as invalid next time
>     curr_insn_transform sees the subreg.
> 
> (2) The check is valid for little-endian only.  For big-endian we need
>     to move hard_regno backwards.
> 
> Using simplify_subreg_regno should avoid both problems.

We have apparently a cycle in LRA on the SPARC, see PR rtl-opt/92303.

-- 
Eric Botcazou