linux-kernel - Re: [PATCH 2/7] ARM: LTO: avoid THUMB2

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Date:   Wed, 7 Mar 2018 13:52:03 -0500 (EST)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Matthias Kaehlcke <mka@...omium.org>
cc:     Arnd Bergmann <arnd@...db.de>, Andi Kleen <ak@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] ARM: LTO: avoid THUMB2_KERNEL+LTO

On Wed, 7 Mar 2018, Matthias Kaehlcke wrote:

> El Tue, Feb 20, 2018 at 10:59:49PM +0100 Arnd Bergmann ha dit:
> 
> > Trying to build an LTO-Enabled kernel with Thumb2 instructions failed
> > horribly for me, with an endless output of things like
> > 
> > ccVnNycO.s:2665: Error: thumb conditional instruction should be in IT block -- `bxne lr'
> > ccVnNycO.s:7128: Error: thumb conditional instruction should be in IT block -- `strexeq r5,r2,[r3]'
> > ccVnNycO.s:7258: Error: thumb conditional instruction should be in IT block -- `strexeq lr,r0,[r3]'
> > ccVnNycO.s:17380: Error: thumb conditional instruction should be in IT block -- `strexeq r1,r2,[r6]'
> > ccVnNycO.s:19163: Error: thumb conditional instruction should be in IT block -- `strexeq r8,r6,[r3]'
> > ccVnNycO.s:22722: Error: thumb conditional instruction should be in IT block -- `strexeq r7,r1,[r0]'
> > ccVnNycO.s:24105: conditional infixes are deprecated in unified syntax
> > ccVnNycO.s:24105: Error: thumb conditional instruction should be in IT block -- `sbcccs r1,r1,r3'
> > ccVnNycO.s:24105: Error: thumb conditional instruction should be in IT block -- `movcc r3,#0'
> > ccVnNycO.s:24210: conditional infixes are deprecated in unified syntax
> > ccVnNycO.s:24210: Error: thumb conditional instruction should be in IT block -- `sbcccs r2,r2,r3'
> > ccVnNycO.s:24210: Error: thumb conditional instruction should be in IT block -- `movcc r3,#0'
> 
> For the record:
> 
> The errors about sbcccs and movcc probably stem from here:
> 
> /* We use 33-bit arithmetic here... */
> #define __range_ok(addr, size) ({ \
>         unsigned long flag, roksum; \
>         __chk_user_ptr(addr);   \
>         __asm__("adds %1, %2, %3; sbcccs %1, %1, %0; movcc %0, #0" \
>                 : "=&r" (flag), "=&r" (roksum) \
>                 : "r" (addr), "Ir" (size), "0" (current_thread_info()->addr_limit) \
>                 : "cc"); \
>         flag; })
> 
> arch/arm/include/asm/uaccess.h
> 
> I stumbled across this when trying to build a 32-bit ARM kernel with
> clang.

You have to tell clang to pass -mno-warn-deprecated to gas. That's what 
the gcc build currently does.

In this particular case with LTO, the same trick as done for 
-mimplicit-it=always would do it to work around 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78353.


Nicolas