This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: basic asm and memory clobbers


> On Nov 23, 2015, at 8:39 PM, David Wohlferd <dw@LimeGreenSocks.com> wrote:
> 
> On 11/23/2015 1:44 PM, Paul_Koning@Dell.com wrote:
>>> On Nov 23, 2015, at 4:36 PM, David Wohlferd <dw@LimeGreenSocks.com> wrote:
>>> 
>>> ...
>>>> The more I think about it, I'm just not keen on forcing all those old-style asms to change.
>>> If you mean you aren't keen to change them to "clobber all," I'm with you.  If you are worried about changing them from basic to extended, what kinds of problems do you foresee?  I've been reading a lot of basic asm lately, and it seems to me that most of it would be fine with a simple colon.  Certainly no worse than the current behavior.
>> I'm not sure.  I have some asm("sync") which I think assume that this means asm("sync"::"memory")
> 
> Another excellent reason to nudge people towards using extended asm.  If you saw asm("sync":::"memory"), you would *know* what it did, without having to read the docs (which don't say anyway).
> 
> I'm pretty confident that asm("") doesn't clobber memory on i386, but maybe that behavior is platform-specific.  Since i386 doesn't have "sync", I assume you are on something else?

Yes, MIPS.
> 
> If you have a chance to experiment, I'd love confirmation from other platforms that asm("blah") is the same as asm("blah":).  Feel free to email me off list to discuss.

I'm really concerned with loosening the meaning of basic asm.  I wish I could find the documentation that says, or implies, that it is a memory clobber.  And/or that it is implicitly volatile.

The problem is that it's clear from existing code that this assumption was made, and that defining it otherwise would break such code.  For example, the code I quoted clearly won't work if stores are moved across the asm("sync").

Given the ever improving optimizers, these things are time bombs -- code that worked for years might suddenly break when the compiler is upgraded.

If such breakage is to be done, it must at least come with a warning (which must default to ON).  But I'd prefer to see the more conservative approach (more clobbers) taken.

	paul


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]