Re: [DOC Patch] Add sample for @cc constraint

[Bernd Schmidt <bschmidt at redhat dot com>]

In principle we probably should have an example, but once again I have 
some problems with the style of the added documentation. I prefer 
concise writing without unnecessary repetition. Any other reviewers can 
of course override me, but the following is my opinion on these changes.

More problematic than a lack of documentation is that I haven't been 
able to find an executable testcase. If you could adapt your example for 
use in gcc.target/i386, that would be even more important.

On 03/13/2016 05:00 AM, David Wohlferd wrote:
> Index: extend.texi
> ===================================================================
> --- extend.texi	(revision 234163)
> +++ extend.texi	(working copy)
> @@ -8047,6 +8047,7 @@
>
>   Because of the special nature of the flag output operands, the constraint
>   may not include alternatives.
> +Do not clobber flags if they are being used as outputs.

I don't think the manual should point out the obvious. I'd be surprised 
if this wasn't documented or at least strongly implied elsewhere for 
normal operands.

> +For builds that don't support flag output operands,

This feels strange, we should just be documenting the capabilities of 
this feature. Other parts of the docs already show what to do without 
it. Hence, reduce the example to this (plus the surrounding setup stuff):

> +/* Avoid the redundant setc/testb and use the carry flag directly.  */
> +asm ("bt $0, %1"
> +  : "=@@ccc" (a)
> +  : "r" (b));
> +
> +#endif

> +Note: On the x86 platform, flags are normally considered clobbered by
> +extended asm whether the @code{"cc"} clobber is specified or not.

Is it really necessary or helpful to mention that here? Not only is it 
not strictly correct (an output operand is not also considered 
clobbered), but to me it breaks the flow because you're left wondering 
how that sentence relates to the example (it doesn't).

>   @anchor{InputOperands}
> @@ -8260,6 +8298,8 @@
>   On other machines, condition code handling is different,
>   and specifying @code{"cc"} has no effect. But
>   it is valid no matter what the target.
> +For platform-specific uses of flags, see also
> +@ref{FlagOutputOperands,Flag Output Operands}.

Is this likely to be helpful? Someone who's looking at how to use flag 
outputs probably isn't looking in the "Clobbers" section?


Bernd