Re: [PATCH] Make basic asm implicitly clobber memory, pr24414
On 12/09/2015 03:18 AM, Bernd Edlinger wrote:
Furthermore there is a documented use for asm(""): The empty assembler string
is used to make a function
volatile, thus calls can not be optimized away. But I think it is not
necessary to make this clobber anything,
nor should it be an instruction scheduling barrier, as it used to be in the
past.
Making that change seems risky; best not to make assumptions about how
these are used. In any case,
... or should I wait for the next stage1?
I think that would be better.
Bernd
Re: [PATCH] Make basic asm implicitly clobber memory, pr24414
Hi,
On 09.12.2015 12:06 Bernd Schmidt wrote:
> On 12/09/2015 03:18 AM, Bernd Edlinger wrote:
>> Furthermore there is a documented use for asm(""): The empty
>> assembler string is used to make a function
>> volatile, thus calls can not be optimized away. But I think it is
>> not necessary to make this clobber anything,
>> nor should it be an instruction scheduling barrier, as it used to be
>> in the past.
>
> Making that change seems risky; best not to make assumptions about how
> these are used. In any case,
>
So would you agree on the general direction of the patch,
if I drop the hunk in sched-deps.c ?
Thanks
Bernd.
Re: [PATCH] Make basic asm implicitly clobber memory, pr24414
Hi,
On 09.12.2015 16:48 Bernd Schmidt wrote:
> On 12/09/2015 04:09 PM, Bernd Edlinger wrote:
>
>> So would you agree on the general direction of the patch,
>> if I drop the hunk in sched-deps.c ?
>
> I'm not sure there was any consensus in that other thread, but I think
> assuming that basic asms clobber memory and CC, can be justified. That
> certainly seems like a safer default. Ideally though I think it would
> be best if we could deprecate basic asms in functions, or at least
> warn about them in -Wall.
>
>
Well no, we did not get to a consensus on the warning issue.
My personal gut feeling on that warning is a bit mixed...
If we have a -Wall-enabled warning on asm("..."), people who know next
to nothing
about assembler will be encouraged to "fix" this warning in a part of
the code which
they probably do not understand at all. This frightens me a bit.
Because I know they will soon find out, that adding a few colons fixes
the warning, but
asm("...":::) is not any better IMHO.
For me, it is just very very unlikely that any piece of assembler really
clobbers nothing
and has no inputs and no outputs at the same time, even it it looks so
at first sight...
It is much more likely that someone forgot to fill in the clobber section.
So for me it would also be good to warn on asm("...":::) and require
that, if they want
to fix this warning, they must at least write something in the clobber
section, like asm ("...":::"nothing"); that would be a new clobber name
which can only
stand alone and, which can get stripped after the warning processing
took place in the FE.
So I think a warning should warn on something that is so unusual that it
is likely a bug.
Bernd.
Re: [PATCH] Make basic asm implicitly clobber memory, pr24414
On 12/09/2015 04:09 PM, Bernd Edlinger wrote: So would you agree on the general direction of the patch, if I drop the hunk in sched-deps.c ? I'm not sure there was any consensus in that other thread, but I think assuming that basic asms clobber memory and CC, can be justified. That certainly seems like a safer default. Ideally though I think it would be best if we could deprecate basic asms in functions, or at least warn about them in -Wall. Bernd
