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Re: RFC/RFA: Fix bug with REE optimization corrupting extended registers

From: Jeff Law <law at redhat dot com>
To: Bernd Schmidt <bschmidt at redhat dot com>, Nick Clifton <nickc at redhat dot com>, gcc-patches at gcc dot gnu dot org
Date: Wed, 18 Nov 2015 18:56:20 -0700
Subject: Re: RFC/RFA: Fix bug with REE optimization corrupting extended registers
Authentication-results: sourceware.org; auth=none
References: <87d1v7v1g9 dot fsf at redhat dot com> <564D1D3B dot 9070609 at redhat dot com>

On 11/18/2015 05:52 PM, Bernd Schmidt wrote:

   (This is on the RL78 target where HImode values occupy two hard
   registers and QImode values only one.  The bug however is generic, not
   RL78 specific).

   The REE pass transforms this into:

     (insn  44 (set (reg:QI r11) (mem:QI (reg:HI r20)))
     (insn  45 (set (reg:HI r10) (zero_extend:HI (mem:QI (reg:HI r18))))
     [...]
     (insn  71 (set (reg:HI r14) (zero_extend:HI (reg:QI r11)))
     [...]
     (insn  88 deleted)

   Note how the new set at insn 45 clobbers the value loaded by insn 44
   into r11.


I had a look around. There's code testing HARD_REGNO_NREGS in
ree.c:combine_set_extension. It's inside #if 0, and labelled
"temporarily disabled". See if enabling that helps you? (Jeff, that #if
0 was added by you).

I was going to dig into this discussion tomorrow since I was the lastone in that code. Just had to get that threading regression resolvedfirst :-)


jeff

References:
- RFC/RFA: Fix bug with REE optimization corrupting extended registers
  - From: Nick Clifton
- Re: RFC/RFA: Fix bug with REE optimization corrupting extended registers
  - From: Bernd Schmidt

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