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Re: Request for discussion: Rewrite of inline assembler docs


On Fri, Mar 28, 2014 at 01:15:39PM +0000, Andrew Haley wrote:
> On 03/28/2014 10:46 AM, Hannes Frederic Sowa wrote:
> > On Fri, Mar 28, 2014 at 09:41:41AM +0000, Andrew Haley wrote:
> >> On 03/28/2014 09:30 AM, Hannes Frederic Sowa wrote:
> >>> On Fri, Mar 28, 2014 at 09:10:11AM +0000, Andrew Haley wrote:
> >>>> On 03/28/2014 06:20 AM, dw wrote:
> >>>>> Using this clobber causes the compiler to flush all (modified) registers 
> >>>>> being used to store memory-based values to memory before executing the 
> >>>>> @code{asm} statement.
> >>>>
> >>>> I don't know what a "memory-based" value is.  This phrase doesn't help
> >>>> at all.  In addition, it isn't true.  Modified registers are not
> >>>> flushed to memory unless they are clobbered or their address is taken.
> >>>> This sentence should be deleted.
> >>>
> >>> i and sum form your example are not allocated in memory. If you make
> >>> one of them static you should be able to see the effect.
> >>
> >> I know.  That's not the point.  "memory-based" is not a well-defined
> >> term.  In order to write clear documentation it is necessary either to
> >> define terms or to use terms that are already well-defined.
> > 
> > Ok, I see the problem. Maybe something like this by avoiding the term?
> > 
> > Using this clobber causes the compiler to flush all (modified) registers
> > being used to store values which gcc decided to originally allocate in
> > memory before executing the @code{asm} statement.
> 
> Not really, no: that's not true.

Hmm, ok?

> >> What is true here is that all registers used to cache variables that
> >> are reachable from pointers in the program are flushed.  Anything that
> >> is statically allocated is reachable, as is anything dynamically
> >> allocated by malloc; auto variables are not reachable unless their
> >> address is taken.
> > 
> > One would have to go into detail of various optimizations which could
> > remove the address taking
> 
> One would not: any such optimization would be incorrect.  Any memory
> reachable from an inline asm statement with a memory clobber must
> be flushed to memory.

If I extend your example with an additional pointer and take the address of s
it still doesn't allocate s in memory and as such cannot flush out the
register content, which is perfectly legal. The pointer is completley
eliminated (this is an extreme example).

int sum(int a[], int l) {
  int i;
  int s = 0;
  int *sp = &s;
  for (i = 0; i < l; i++) {
    asm volatile("nop # Nothing at all":::"memory");
    *sp += a[i];
  }
  return s;
}

> > e.g. IMHO the last sentence of the paragraph already
> > deals with this.
> 
> Which last sentence of what paragraph?

This one:

| For performance reasons, some variables only exist in registers and never
| get written to memory.  The "memory" clobber does not force these values
| to get written to memory.

Btw. very nice work!

Thanks,

  Hannes


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