Re: [cxx-abi-dev] Revisiting Issue C-18: Result buffers

[Mark Mitchell <mark@xxxxxxxxxxxxxxxx>]

Cary Coutant wrote:
> I was recently asked about Section 3.1.4 (Return Values) in the C++ ABI 
> spec, where it states:
> "... if the return value type has a non-trivial copy constructor or 
> destructor, the caller allocates space for a temporary, and passes a 
> pointer to the temporary as an implicit first parameter preceding both 
> the this parameter and user parameters. The callee constructs the return 
> value into this temporary. On Itanium, the pointer is passed in out0, 
> different from other large class result buffer pointers, passed in r8."
> The HP compiler follows this convention, but g++ on HP-UX does not, 
> causing a binary incompatibility between the two compilers.
> I dug up Issue C-18, which purports to have changed this convention to 
> use r8, just as the base C ABI would, and as g++ does:
Yes, I distinctly remember lobbying to eliminate the special Itanium 
rule, and I remember the change being accepted.  So, I belive the G++ 
behavior to be correct -- although you are of course correct that 
somehow the specification was never updated to reflect that.  The change 
was made before the specification moved to our web site, so I really 
don't know what happened.
I have checked in the attached change.  If anyone objects to that, 
please let me know.
> What does the C++ ABI test suite check for?
Our testsuite doesn't check for particular registers; instead, it checks 
this constraint by calling a function from C++ and implementing it in C, 
with the pointer made explicit, and then checks that the value is really 
there.  So, essentially, it checks for the G++ behavior, not the special 
 Itanium behavior.
--
Mark Mitchell
CodeSourcery, LLC
mark@xxxxxxxxxxxxxxxx
(916) 791-8304

? diffs.txt
Index: abi.html
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RCS file: /home/cvs/Repository/cxx-abi/abi.html,v
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diff -c -5 -p -r1.81 abi.html
*** abi.html	17 Feb 2005 20:36:57 -0000	1.81
--- abi.html	4 May 2005 23:34:31 -0000
*************** or destructor,
*** 2660,2672 ****
  the caller allocates space for a temporary,
  and passes a pointer to the temporary as an implicit
  first parameter
  preceding both the <code>this</code> parameter and user parameters.
  The callee constructs the return value into this temporary.
- On Itanium, the pointer is passed in <code>out0</code>,
- different from other large class result buffer pointers,
- passed in <code>r8</code>.
  
  <p>
  A result of an empty class type will be returned as though it were
  a struct containing a single char,
  i.e. <code>struct S { char c; };</code>.
--- 2660,2669 ----
*************** unwind table location.
*** 4957,4966 ****
--- 4954,4967 ----
  <p> <hr> <p>
  
  <p>This version of this document is $Revision: 1.81 $.  No special
  significance should be attached to the form of the revision number; it
  is simply a identifying number.</p>
+ 
+ <p>
+ <font color=blue>[050405]</font>
+ Remove use of <code>out0</code> for by-value return types on Itanium.
  
  <p>
  <font color=blue>[050211]</font>
  Reverse treatment of ambiguous arguments to __cxa_demangle (3.4).