On 02/03/20 18:13, Ryan Libby wrote:
> On Mon, Mar 2, 2020 at 12:45 AM Alexander V. Chernikov <melif...@ipfw.ru>
> wrote:
>>
>> 28.02.2020, 18:32, "Ryan Libby" <rli...@freebsd.org>:
>>> Author: rlibby
>>> Date: Fri Feb 28 18:32:36 2020
>>> New Revision: 358439
>>> URL: https://svnweb.freebsd.org/changeset/base/358439
>>>
>>> Log:
>>> amd64 atomic.h: minor codegen optimization in flag access
>>>
>>> Previously the pattern to extract status flags from inline assembly
>>> blocks was to use setcc in the block to write the flag to a register.
>>> This was suboptimal in a few ways:
>>> - It would lead to code like: sete %cl; test %cl; jne, i.e. a flag
>>> would just be loaded into a register and then reloaded to a flag.
>>> - The setcc would force the block to use an additional register.
>>> - If the client code didn't care for the flag value then the setcc
>>> would be entirely pointless but could not be eliminated by the
>>> optimizer.
>>>
>>> A more modern inline asm construct (since gcc 6 and clang 9) allows for
>> This effectively restricts kernel builds by all older compilers.
>> Is there any chance of making it conditional depending on the compiler
>> version/features?
>
> Yes, it is possible to test for __GCC_ASM_FLAG_OUTPUTS__. It is more
> maintenance effort going forward. If building current with an old cross
> compiler is an important scenario, we can either revert this and the
> following revision or work up a patch to make it conditional. I'll see
> what that might look like.
>
Actually this causes emulators/virtualbox-ose port to fail to build:
In file included from /usr/src/sys/sys/systm.h:44:
/usr/include/machine/atomic.h:230:1: error: invalid output constraint
'=@cce' in asm
ATOMIC_CMPSET(char);
^
/usr/include/machine/atomic.h:205:4: note: expanded from macro
'ATOMIC_CMPSET'
: "=@cce" (res), /* 0 */ \
^
/usr/include/machine/atomic.h:230:1: error: invalid output constraint
'=@cce' in asm
(and so on)
the virtualbox-ose port is forced to use an older clang version due to
crashes when compiled with newer ones.
Not sure whose responsibility is to fix this.
Should I file a bug report on bugzilla?
--
Guido Falsi <madpi...@freebsd.org>
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