basic asm and memory clobbers - Proposed solution

[David Wohlferd]

On 11/24/2015 6:50 PM, David Wohlferd wrote:
> I have solved the problem with my previous patch. Here's the update 
> (feedback welcome): http://www.LimeGreenSocks.com/gcc/24414g.zip
>
> Based on my understanding from the previous thread, this patch now 
> does what it needs to do (code-wise) to resolve this "basic asm and 
> memory clobbers" issue.  As mentioned previously, this patch 
> introduces a new warning (-Wonly-top-basic-asm), which is disabled by 
> default.  When enabled, it triggers a warning for any basic asm inside 
> a function, unless the function has the "naked" attribute.
>
> An argument can be made that the default for this warning should be 
> 'enabled.'  Yes, this will break builds that use basic asm and 
> -Werror, but it can easily be disabled with -Wno-only-top-basic-asm.  
> And if we don't enable it, no one is going to know the check is even 
> available.  Then hidden problems like the one Paul was just describing 
> still won't be found, and optimizations will continue to have 
> unexpected side effects. OTOH, I can also see changing this to 
> 'enabled' as more appropriate in the next phase 1.
>
> Now that I'm done with the code fix, I'm working on an update to the 
> docs.  Obviously they should be checked in as part of the code fix. 
> I'm planning to actually use the word "deprecated" when describing the 
> use of basic asm within functions.  Seems like a big step.
>
> But there's no point in my proceeding any further until someone in 
> authority agrees that this is the desired solution.  I'm not actually 
> sure who that is, but further work is a waste of time if no one is 
> prepared to approve it.
>
> If you are that person, the questions to be answered are:
>
> 1) Is the idea of changing basic asm to "clobber everything" dead?
> 2) Is creating a warning for the use of "basic asm inside a function" 
> the solution for this issue?
> 3) Should the warning be enabled by default in v6?
> 4) Should the warning be enabled by Wall or Wextra?
> 5) Should the v6 docs explicitly describe using "basic asm inside a 
> function" as deprecated?
>
> If you're looking for my recommendations, I would say: Yes, Yes, 
> (reluctantly) No, No and Yes.
>
> With this information in hand, I'll take a shot at finishing this off.
>
> For something that started out as a simple 3 sentence doc patch, this 
> sure has turned into a project...

I have incorporated the feedback from Hans-Peter Nilsson, who pointed 
out that the 'noinline' function attribute explicitly documents behavior 
related to using asm("").  The patch now includes an exception for 
this.  Given how often this bit of code is used in various projects, 
allowing this exception will surely ease the transition.

I'm told that some people won't review patches unless they are included 
as attachments, so this time the patch is attached.

The patch includes first cut docs for the warning message, but I'm still 
hoping to hear from someone before trying to update the basic asm docs.

dw
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