[Bug c++/16405] New: Non optimized code when using default copy constructor

[Bug c++/16405] New: Non optimized code when using default copy constructor

[guillaume dot melquiond at ens-lyon dot fr]

I have been compiling the following snippet with g++ version 3.3.4, 3.4.0,
3.5.0. They are Debian packages, so the 3.4.0 version is a bit old (and a
prerelease), sorry. So maybe the bug has already been fixed in g++ 3.4. However
it is still present in a recent snapshot of the trunk. In the following, the
function goes from using only one temporary in g++ 3.3 to using four temporaries
in g++ 3.5.

--------------------------------------------------------------------
struct T {
  int a[128];
  // T(T const &v);
  T &operator+=(T const &v);
  T operator+(T const &v) const { T t = *this; t += v; return t; }
};

extern T a, b, c;
void f() { a = b + c; }
--------------------------------------------------------------------

So the code just describes a big plain structure and its operator+ is defined
with its operator+=. The function f computes an addition and operator+ is
inlined into it.

When compiling with g++ 3.3, the generated assembly is optimal: there is only
one temporary, b is copied into this temporary, c is added to this temporary,
the result is then assigned to a.

When compiling with g++ 3.4, it is no more optimal: there are two temporaries, b
is copied into one, c is added to this temporary holding b, the result is copied
into the other temporary, and the temporary is then assigned to a. Maybe the
NRVO did not kick in?

When compiling with g++ 3.5, it is even worse: there are four temporaries, b is
copied into one, c is added to this temporary holding b, the result then goes
into two successive temporaries before being assigned to a. So there is one copy
that does not mean anything with respect to the code? And please note there is
one temporary that just lies on the stack and is never used (since only three
temporaries are used, not four).

If a copy constructor is added (just comment out the line), the three compilers
generate a similar code: only one temporary is ever used. It is the expected and
optimal code generation. So it seems g++ has some kind of problem with the
default copy constructor starting with 3.4.

As a side note, even with a non-default copy constructor, g++ 3.5 still
allocates a temporary that is never used. Maybe it is an unrelated bug?

The snippet is each time compiled with
$ g++-... -Wall -O3 -S test.cpp

Here are the exact versions used.

$ LANG=C g++-3.3 -v
Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.4/specs
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib
--enable-nls --without-included-gettext --enable-__cxa_atexit
--enable-clocale=gnu --enable-debug --enable-java-gc=boehm
--enable-java-awt=xlib --enable-objc-gc i486-linux
Thread model: posix
gcc version 3.3.4 (Debian 1:3.3.4-2)

$ LANG=C g++-3.4 -v
Reading specs from /usr/lib/gcc/i486-linux/3.4.0/specs
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr
--libexecdir=/usr/lib --with-gxx-include-dir=/usr/include/c++/3.4
--enable-shared --with-system-zlib --enable-nls --enable-threads=posix
--without-included-gettext --program-suffix=-3.4 --enable-__cxa_atexit
--enable-clocale=gnu --enable-libstdcxx-debug --enable-java-gc=boehm
--enable-java-awt=gtk --disable-werror i486-linux
Thread model: posix
gcc version 3.4.0 20040403 (prerelease) (Debian)

$ LANG=C g++-snapshot -v
Reading specs from /usr/lib/gcc-snapshot/lib/gcc/i486-linux-gnu/3.5.0/specs
Configured with: ../src/configure -v --enable-languages=c,c++,java,f95,objc
--prefix=/usr/lib/gcc-snapshot --enable-shared --with-system-zlib --enable-nls
--enable-threads=posix --without-included-gettext --disable-werror
--enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug
--enable-java-gc=boehm --enable-java-awt=gtk i486-linux-gnu
Thread model: posix
gcc version 3.5.0 20040704 (experimental)

-- 
           Summary: Non optimized code when using default copy constructor
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: guillaume dot melquiond at ens-lyon dot fr
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] Non optimized code when using default copy constructor

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] Non optimized code when using default copy constructor

[guillaume dot melquiond at ens-lyon dot fr]

------- Additional Comments From guillaume dot melquiond at ens-lyon dot fr  2004-07-19 09:19 -------
As a follow-up, I also tested with g++ 3.4.1 and 3.5-20040717.

The problem still exists with 3.4.1. There are still one more temporary and one
more copy than with 3.3.4.

The situation is a bit better with 3.5 than it was with the previous tested
snapshot. It is almost as good as with 3.4.1. There are only two more
temporaries and one more copy than with 3.3.4.

So both gcc 3.4 and 3.5 produce one more copy than gcc 3.3. I timed the cost of
this extra copy on the code below (-O3 compilation as before). It incurs a ~25%
slowdown of the whole program on an Intel P4 processor.

-------------------------------------------------------
struct T {
  int a[500];
  T &operator+=(T const &v) { for(int i = 0; i < 500; ++i) a[i] += v.a[i];
return *this; }
  T operator+(T const &v) const { T t = *this; t += v; return t; }
};

T a, b, c;

int main() {
  for(int i = 0; i < 10000000; ++i) a = b + c;
  return 0;
}

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4/4.0 Regression] Temporary aggregate copy not elided

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-07 11:51 -------
Confirmed. This won't probably be fixed in the 3.4 branch, but it should in 4.0 
at least. 

Testcase (same of original description):
------------------------------------
struct T {
  int a[128];
  T &operator+=(T const &v);
  T operator+(T const &v) const { T t = *this; t += v; return t; }
};

extern T a, b, c;
void f() { a = b + c; }
------------------------------------


The optimized dump (without the copy constructor) from mainline is:

------------------------------------
void f() ()
{
  struct T t;
  struct T * const this;
  struct T & v;
  struct T * D.1598;
  struct T D.1594;
  struct T t;

<bb 0>:
  t = b;
  operator+= (&t, &c);
  D.1594 = t;
  a = D.1594;
  return;

}
------------------------------------

So there is one additional temporary which is not removed.

I cannot see a regression anymore with the copy constructor, so this bug only 
tracks this now. Guillaume, if you still see a regression with the copy 
constructor please open a new bug report. It is wrong to track two different 
testcases (although similar) in the same report.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |giovannibajo at libero dot
                   |                            |it
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
      Known to fail|                            |3.4.3 4.0.0
      Known to work|                            |3.3.4
   Last reconfirmed|0000-00-00 00:00:00         |2004-10-07 11:51:22
               date|                            |
            Summary|Non optimized code when     |[3.4/4.0 Regression]
                   |using default copy          |Temporary aggregate copy not
                   |constructor                 |elided
   Target Milestone|---                         |3.4.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug tree-optimization/16405] [3.4/4.0 Regression] Temporary aggregate copy not elided

[mmitchel at gcc dot gnu dot org]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2004-11-01 00:45 -------
Postponed until GCC 3.4.4.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|3.4.3                       |3.4.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4/4.0 Regression] Temporary aggregate copy not elided

[mmitchel at gcc dot gnu dot org]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2004-12-23 04:45 -------
Working on a fix.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |mark at codesourcery dot com
                   |dot org                     |
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4/4.0 Regression] Temporary aggregate copy not elided

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2004-12-23 08:16 -------
Subject: Bug 16405

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	mmitchel@gcc.gnu.org	2004-12-23 08:14:34

Modified files:
	gcc            : ChangeLog gimplify.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/g++.dg/opt: temp1.C 

Log message:
	PR c++/16405
	* gimplify.c (gimplify_modify_expr_rhs): Handle
	INDIRECT_REF/ADDR_EXPR combinations.
	
	PR c++/16405
	* g++.dg/opt/temp1.C: New test.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.6937&r2=2.6938
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/gimplify.c.diff?cvsroot=gcc&r1=2.96&r2=2.97
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.4801&r2=1.4802
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/opt/temp1.C.diff?cvsroot=gcc&r1=NONE&r2=1.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[mmitchel at gcc dot gnu dot org]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2004-12-23 08:17 -------
Fixed in GCC 4.0.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|3.4.3 4.0.0                 |3.4.3
      Known to work|3.3.4 3.3.2                 |3.3.4 3.3.2 4.0.0
            Summary|[3.4/4.0 Regression]        |[3.4 Regression] Temporary
                   |Temporary aggregate copy not|aggregate copy not elided
                   |elided                      |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2004-12-23 16:27 -------
Subject: Bug 16405

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	mmitchel@gcc.gnu.org	2004-12-23 16:27:12

Modified files:
	gcc/testsuite  : ChangeLog 
	gcc/testsuite/g++.dg/opt: temp1.C 

Log message:
	PR c++/16405
	* gimplify.c (gimplify_modify_expr_rhs): Handle
	INDIRECT_REF/ADDR_EXPR combinations.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.4805&r2=1.4806
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/opt/temp1.C.diff?cvsroot=gcc&r1=1.1&r2=1.2



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-02-13 06:44 -------
Subject: Bug 16405

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	jason@gcc.gnu.org	2005-02-13 06:43:58

Modified files:
	gcc            : ChangeLog fold-const.c gimplify.c tree.h 

Log message:
	PR mudflap/19319
	* gimplify.c (gimplify_modify_expr_rhs) [CALL_EXPR]: Make return
	slot explicit.
	
	PR c++/16405
	* fold-const.c (fold_indirect_ref_1): Split out from...
	(build_fold_indirect_ref): Here.
	(fold_indirect_ref): New fn.
	* tree.h: Declare it.
	* gimplify.c (gimplify_compound_lval): Call fold_indirect_ref.
	(gimplify_modify_expr_rhs): Likewise.
	(gimplify_expr): Likewise.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.7461&r2=2.7462
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/fold-const.c.diff?cvsroot=gcc&r1=1.507&r2=1.508
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/gimplify.c.diff?cvsroot=gcc&r1=2.108&r2=2.109
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree.h.diff?cvsroot=gcc&r1=1.689&r2=1.690



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[mmitchel at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|3.4.4                       |3.4.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[gdr at gcc dot gnu dot org]

------- Comment #14 from gdr at gcc dot gnu dot org  2006-02-28 09:35 -------
Fixed in 4.1.0.  won't fix for 3.4.6.


-- 

gdr at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|3.4.6                       |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[pinskia at gcc dot gnu dot org]

------- Comment #13 from pinskia at gcc dot gnu dot org  2006-02-10 23:08 -------
Fixed now in 4.1.0 also.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[3.4/4.1 Regression]        |[3.4 Regression] Temporary
                   |Temporary aggregate copy not|aggregate copy not elided
                   |elided                      |
   Target Milestone|4.1.0                       |3.4.6


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[jason at gcc dot gnu dot org]

------- Comment #9 from jason at gcc dot gnu dot org  2006-02-03 21:56 -------
Disabling this optimization for now, so this is again a regression in 4.1 and
4.2.


-- 

jason at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|mark at codesourcery dot com|jason at gcc dot gnu dot org
      Known to fail|3.4.3                       |3.4.3 4.1.0 4.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405

[Bug c++/16405] [3.4 Regression] Temporary aggregate copy not elided

[gdr at gcc dot gnu dot org]

-- 

gdr at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|3.4.5                       |3.4.6


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405