Re: [PATCH 1/2] extend.texi: update Global Register Variables section

[Jeff Law <law at redhat dot com>]

On 05/10/2018 03:47 AM, Alexander Monakov wrote:
> 
> 
> On Mon, 23 Apr 2018, Alexander Monakov wrote:
> 
>> This rewrites global register vars doc to reflect that the register is no longer
>> reserved exclusively, but in fact is available for general allocation, and also
>> adds the requirement to properly inform the compiler where inline asms are
>> accessing the variable.
>>
>> This:
>> -@item The register is not saved and restored by any functions.
>> is reworded to verbosely spell out gotchas related to calls/returns.
>>
>> 	* extend.texi (Global Register Variables): Rewrite the bullet list.
>> 	Note that the register is available for allocation. Note that access
>> 	via inline asm must use constraints. Add note about async-signal
>> 	handlers. Remove paragraph about automagic register selection.
>> ---
>>  gcc/doc/extend.texi | 29 +++++++++++++++++++----------
>>  1 file changed, 19 insertions(+), 10 deletions(-)
> 
> Ping? I believe the substance of new text has been hashed out with Michael.
> I'd appreciate a review for language and style issues.
> 
> Thanks.
> Alexander
> 
>> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
>> index 5571d05d93b..f663741e36c 100644
>> --- a/gcc/doc/extend.texi
>> +++ b/gcc/doc/extend.texi
>> @@ -9549,11 +9549,21 @@ After defining a global register variable, for the current compilation
>>  unit:
>>  
>>  @itemize @bullet
>> -@item The register is reserved entirely for this use, and will not be 
>> -allocated for any other purpose.
>> -@item The register is not saved and restored by any functions.
>> -@item Stores into this register are never deleted even if they appear to be 
>> -dead, but references may be deleted, moved or simplified.
>> +@item If the register is a call-saved register, call ABI is affected:
>> +the register will not be restored in function epilogue sequences after
>> +the variable has been assigned.  Therefore, functions cannot safely
>> +return to callers that assume standard ABI.
Whether or not a global register is saved in the prologue and restored
in the epilogue is actually a function of the target's prologue/epilogue
implementation.

So ISTM this paragraph needs to be refined a bit.  Essentially it may or
may not change the ABI.  It really depends on the target implementation.


The rest of the doc changes look fine.

jeff