[Bug c/28831] New: Aggregate copy not elided when using a return value as a pass-by-value parameter

[Bug c/28831] New: Aggregate copy not elided when using a return value as a pass-by-value parameter

[guillaume dot melquiond at ens-lyon dot fr]

Bug 23372 was a missed optimization with respect to GCC 3.4. It is now fixed
when the parameter is a reference. But there is still a regression when the
parameter is the return value of another function. Testcase: (-Wall -O3
--march=i386)

struct A { int a[1000]; };
struct A f();
void g(struct A);
void h() { g(f()); }

GCC 3.3 and 3.4 first allocate the stack frame of g and then require f to
directly store its return value in the parameter location. GCC 4.0, 4.1, and
4.2 (as of 2006-08-23) use another stack location for the return value of f,
then allocate the stack frame of g, and finally copy the value to this new
frame (possibly using a byte-by-byte copy, see bug 27055). The code generated
by GCC 3.x is optimal, the one by GCC 4.x is not.

GCC 3.4:
        movl    %esp, %eax
        subl    $12, %esp
        pushl   %eax
        call    f
        addl    $12, %esp
        call    g

GCC 4.2:
        leal    -4004(%ebp), %ebx
        pushl   %ebx
        call    f
        subl    $3988, %esp
        movl    %esp, %eax
        pushl   %edx
        pushl   $4000
        pushl   %ebx
        pushl   %eax
        call    memcpy
        addl    $16, %esp
        call    g

$ LANG=C /opt/gcc/bin/gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc/configure --enable-languages=c,c++ --prefix=/opt/gcc
Thread model: posix
gcc version 4.2.0 20060823 (experimental)


-- 
           Summary: Aggregate copy not elided when using a return value as a
                    pass-by-value parameter
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: guillaume dot melquiond at ens-lyon dot fr
GCC target triplet: i386-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug c/28831] [4.0/4.1/4.2 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2006-08-24 06:51 -------
Confirmed.  As the other one was a C++ frontend problem this one may be solved
in the C frontend then.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2006-08-24 06:51:23
               date|                            |
            Summary|Aggregate copy not elided   |[4.0/4.1/4.2 Regression]
                   |when using a return value as|Aggregate copy not elided
                   |a pass-by-value parameter   |when using a return value as
                   |                            |a pass-by-value parameter
   Target Milestone|---                         |4.0.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.0/4.1/4.2 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-08-24 11:12 -------
This is going to be hard to fix really, unless we make explicate the target's
behavior for passing structs (via value or by reference).
Right now we get:
  D.1992 = f () [return slot optimization];
  g (D.1992) [tail call];

Which looks ok if we are passing via value but since we need to pass by
reference, the middle-end thinks we need a new stack space for it because it
does not know that D.1992 is not used after the call to g.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |middle-end
 GCC target triplet|i386-linux-gnu              |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.0/4.1/4.2 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[guillaume dot melquiond at ens-lyon dot fr]

------- Comment #3 from guillaume dot melquiond at ens-lyon dot fr  2006-08-25 08:34 -------
> Which looks ok if we are passing via value but since we need to pass by
> reference, the middle-end thinks we need a new stack space for it because it
> does not know that D.1992 is not used after the call to g.

Isn't it the other way around? There is no problem if we are passing by
reference, as the code then looks like "g (&D.1992);". The middle-end will not
do any copy. But, because we are passing by value, D.1992 is copied into the
stack frame instead. I understand your point about the middle-end having to
ensure that D.1992 is not used after g in order to store it directly into the
stack frame of g. But isn't this information already available to the
middle-end?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.0/4.1/4.2 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[mmitchel at gcc dot gnu dot org]

-- 

mmitchel at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.0/4.1/4.2/4.3 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[gdr at gcc dot gnu dot org]

------- Comment #4 from gdr at gcc dot gnu dot org  2007-02-03 19:35 -------
won't fix in GCC-4.0.x.  Adjusting milestone.


-- 

gdr at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.4                       |4.1.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.0/4.1/4.2/4.3 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.1.3                       |4.1.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.0/4.1/4.2/4.3 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[mmitchel at gcc dot gnu dot org]

-- 

mmitchel at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.1.2                       |4.1.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.1/4.2/4.3/4.4 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[jason at gcc dot gnu dot org]

------- Comment #5 from jason at gcc dot gnu dot org  2008-06-25 21:49 -------
Here's another example:

struct A { int i[100]; };
void f(struct A);
int main()
{
  f((struct A){1});
}

Here we build up the compound literal on the stack and then copy it into the
argument slot.

This seems to be a problem with GIMPLE, as there's no way to represent that we
want a particular temporary object to live in the argument slot.

This is both more and less of a problem for C++, as it has many more temporary
struct objects, but also has pass-by-reference (and the ABI does transparent
pass-by-reference for non-POD structs).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.2/4.3/4.4 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[jsm28 at gcc dot gnu dot org]

------- Comment #6 from jsm28 at gcc dot gnu dot org  2008-07-04 21:28 -------
Closing 4.1 branch.


-- 

jsm28 at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[4.1/4.2/4.3/4.4 Regression]|[4.2/4.3/4.4 Regression]
                   |Aggregate copy not elided   |Aggregate copy not elided
                   |when using a return value as|when using a return value as
                   |a pass-by-value parameter   |a pass-by-value parameter
   Target Milestone|4.1.3                       |4.2.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.3/4.4/4.5 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[jsm28 at gcc dot gnu dot org]

------- Comment #7 from jsm28 at gcc dot gnu dot org  2009-03-31 19:40 -------
Closing 4.2 branch.


-- 

jsm28 at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[4.2/4.3/4.4/4.5 Regression]|[4.3/4.4/4.5 Regression]
                   |Aggregate copy not elided   |Aggregate copy not elided
                   |when using a return value as|when using a return value as
                   |a pass-by-value parameter   |a pass-by-value parameter
   Target Milestone|4.2.5                       |4.3.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.3/4.4/4.5 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[rguenth at gcc dot gnu dot org]

------- Comment #8 from rguenth at gcc dot gnu dot org  2009-08-04 12:27 -------
GCC 4.3.4 is being released, adjusting target milestone.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.3.4                       |4.3.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.3/4.4/4.5 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[rguenth at gcc dot gnu dot org]

------- Comment #9 from rguenth at gcc dot gnu dot org  2010-01-17 13:10 -------
Re-confirmed with 4.5.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2007-08-06 14:44:49         |2010-01-17 13:10:45
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831

[Bug middle-end/28831] [4.3/4.4/4.5/4.6 Regression] Aggregate copy not elided when using a return value as a pass-by-value parameter

[rguenth at gcc dot gnu dot org]

------- Comment #10 from rguenth at gcc dot gnu dot org  2010-05-22 18:11 -------
GCC 4.3.5 is being released, adjusting target milestone.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.3.5                       |4.3.6


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28831