Re: AW: basic asm and memory clobbers - Proposed solution

[David Wohlferd <dw at LimeGreenSocks dot com>]

On 12/17/2015 11:30 AM, Bernd Edlinger wrote:
> On Thu, 17 Dec 2015 15:13:07, Bernd Schmidt wrote:
> > >    What's your take on making -Wonly-top-basic-asm a default (either now or
> > >    v7)?  Is making it a non-default a waste of time because no one will
> > >    ever see it?  Or is making it a default too aggressive? What about
> > >    adding it to -Wall?
> > Depends if anyone has one in system headers I guess. We could try to add it to -Wall.
> Sorry, but if I have to object.
>
> Adding this warning to -Wall is too quickly and will bring the ia64, tilegx and mep ports into trouble.

It doesn't look to me like adding the warnings will affect gcc itself.  
But I do see how it could have an impact on people using gcc on those 
platforms, if the warning causes them to convert to extended asm.

> Each of them invokes some special semantics for basic asm:

I'm collecting these for my "How to convert basic asm to extended asm" 
document.  This may need to go in the gcc wiki instead of the user 
guide, since people may find important conversion tips like these 
asynchronous to gcc's releases.

> mep: mep_interrupt_saved_reg looks for ASM_INPUT in the body, and saves different registers if found.

I'm trying to follow this code.  A real challenge since I know nothing 
about mep.  But what I see is:

- This routine only applies to functions marked as 
__attribute__((interrupt)).
- To correctly generate entry/exit, it walks thru each register (up to 
FIRST_PSEUDO_REGISTER) to see if it is used by the routine.  If there is 
any basic asm within the routine (regardless of its contents), the 
register is considered 'in use.'

The net result is that every register gets saved/restored by the 
entry/exit of this routine if it contains basic asm.  The reason this is 
important is that if someone just adds a colon, it would suddenly *not* 
save/restore all the registers.  Depending on what the asm does, this 
could be bad.

Does that sound about right?

This is certainly worth mentioning in the 'convert' doc.

I wonder how often this 'auto-clobber' feature is used, though.  I don't 
see it mentioned in the 'interrupt' attribute docs for mep, and it's not 
in the basic asm docs either.  If your interrupt doesn't need many 
registers, it seems like you'd want to know this and possibly use 
extended.  And you'd really want to know if you are doing a (redundant) 
push/pop in your interrupt.

> tilegx:  They never wrap {} around inline asm. But extended asm, are handled differently, probably automatically surrounded by braces.

I know nothing about tilegx either.  I've tried to read the code, and it 
seems like basic asm does not get 'bundled' while extended might be.

Bundling for tilegx (as I understand it) is when you explicitly fill 
multiple pipelines by doing something like this:

   { add r3,r4,r5 ; add r7,r8,r9 ; lw r10,r11 }

So if you have a basic asm statement, you wouldn't want it automatically 
bundled by the compiler, since your asm could be more than 3 statements 
(the max?).  Or your asm may do its own bundling. So it makes sense to 
never output braces when outputting basic asm.

I know I'm guessing about what this means, but doesn't it seem like 
those same limitations would apply to extended?  I wonder if this is a 
bug.  I don't see any discussion of bundling (of this sort) in the docs.

> ia64:  ASM_INPUT emits stop-bits.  extended asm does not the same.  That was already mentioned by Segher.

I already had this one from Segher's email.

--------------------
Given all this, I'm more convinced than ever of the value of 
-Wonly-top-basic-asm.  Basic asm is quirky, has undocumented bits and 
its behavior is incompatible with other compilers, even though it uses 
the same syntax.  If I had any of this in my projects, I'd sure want to 
find it and look it over.

But maybe Bernd is right and it's best to leave the warning disabled in 
v6, even by -Wall.  I may ask this question again in the next phase 1...

With that in mind, how do you feel about the basic plan:

- Update the basic asm docs to describe basic asm's current (and 
historical) semantics (ie clobber nothing).
- Emphasize how that might be different from users' expectations or the 
behavior of other compilers.
- Warn that this could change in future versions of gcc.  To avoid 
impacts from this change, use extended.
- Reference the "How to convert from basic asm to extended asm" guide 
(where ever it ends up living).
- Mention -Wonly-top-basic-asm as a way to locate affected statements.
- -Wonly-top-basic-asm is disabled by default and not enabled by -Wall 
or -Wextra.

Does this seem like a safe and useful change for v6?

dw