[Bug c++/19661] New: many redundant atexit calls emitted into the executable for static objects with empty destructors

[Bug c++/19661] New: many redundant atexit calls emitted into the executable for static objects with empty destructors

[yuri at tsoft dot com]

When some object having empty destructor is declared as static in the local
context of some function g++ still inserts <atexit> call with the empty handler
like <__tcf_0> below.

It may seem like minor issue but redundant instructions increase chances for
I-cache misses and worthen processor pipeline.

Yuri


08048800 <__tcf_0>:
 8048800:       55                      push   %ebp
 8048801:       89 e5                   mov    %esp,%ebp
 8048803:       5d                      pop    %ebp
 8048804:       c3                      ret


--testcase--
begin 644 case-empty-atexit.tgz
M'XL(`.1G^4$``^V6VVJ#0!"&O=ZG&-*;))!D/:Q>)!1"GV31:2.83=`U&$KZ
M[!U-BI)#<U%)"YT/85?\]Z#C/[.Q+G""ZZW=3[3%*K4SIW>D#&2D%+5*JLBG
M5DHW4$U[PG&E&X5>Z(>^YTC7#X/(`=7_5BXI"ZMS`&=?YNEWN@1WF&VVC]C2
M(XDOXE]-7WI>0[I2AD%P,_X4^:_XUW\`Z0//"QV0/>_C*O\\_D(\I2;.R@1A
M4$U7`R&PLI@;V&W2!,H"AZ.Y$/2-;!K#$C3=B+5.S7`$[P(@WI06%@L8H*%1
M@[J+)LGF].@TMJ/)4.^PHSF(WWY[YIK_5SVO<<__7NM_GZ2-_U7$_G\$'?\O
MTDUA<]3K9U$6J7D#H]=8;'6,4-BDR0)Y&5O*`K7SEW4&:+V]I*XY"C!I/0X'
MDC:IY-6<#Z`$<:[\.,XZ&W=D"5Y..YZ1_##G_/%CKOA_0H'I]0QPM_Y+U=;_
M*"*]HHO]_PANE?]CJ6\/`8WGN\<``#VM/<UEG&$8AF$8AF$8AF$8AF$8AF$8
-AF'^#)_;GM5=`"@`````
`
end

-- 
           Summary: many redundant atexit calls emitted into the executable
                    for static objects with empty destructors
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: yuri at tsoft dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: n/a
GCC target triplet: n/a


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug c++/19661] unnecessary atexit calls emitted for static objects with empty destructors

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-01-28 01:08 -------
Confirmed, although I consider this to be a rather minor point since 
the code is actually run only once. Here's a small test: 
-------------------- 
struct A { 
    A(); 
    ~A() {} 
}; 
 
void foo () { 
  static A a; 
} 
------------------------- 
 
g/x> /home/bangerth/bin/gcc-4.0-pre/bin/c++ -S -O3 x.cc 
g/x> cat x.s | c++filt > x.ss 
 
.... 
foo(): 
.LFB5: 
	pushl	%ebp 
.LCFI2: 
	movl	%esp, %ebp 
.LCFI3: 
	pushl	%ebx 
.LCFI4: 
	subl	$20, %esp 
.LCFI5: 
	cmpb	$0, guard variable for foo()::a 
	je	.L12 
.L9: 
	addl	$20, %esp 
	popl	%ebx 
	popl	%ebp 
	ret 
	.p2align 4,,7 
.L12: 
	movl	guard variable for foo()::a, (%esp) 
	call	__cxa_guard_acquire 
	testl	%eax, %eax 
	je	.L9 
	movl	foo()::a, (%esp) 
.LEHB0: 
	call	A::A() 
.LEHE0: 
	movl	guard variable for foo()::a, (%esp) 
	call	__cxa_guard_release 
	movl	$__tcf_0, (%esp) 
	call	atexit 
	addl	$20, %esp 
	popl	%ebx 
	popl	%ebp 
	ret 
... 
 
W. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-01-28 01:08:27
               date|                            |
            Summary|many redundant atexit calls |unnecessary atexit calls
                   |emitted into the executable |emitted for static objects
                   |for static objects with     |with empty destructors
                   |empty destructors           |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug middle-end/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-28 01:22 -------
This is related to PR 17736 which is for empty global variables with empty constructors.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug c++/19661] unnecessary atexit calls emitted for static objects with empty destructors

[yuri at tsoft dot com]

------- Additional Comments From yuri at tsoft dot com  2005-01-28 01:26 -------
(In reply to comment #1)
> Confirmed, although I consider this to be a rather minor point since 
> the code is actually run only once. Here's a small test: 

I agree, but it bloats the code, therefore reducing potential for inlining and
pipeline trashing.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|enhancement                 |normal
          Component|middle-end                  |c++


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug middle-end/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-28 01:45 -------
But it is still an enhancement because any missed optimization that does not happen before is.
Also this is a middle-end bug since the middle-end and not the front-end figure out that the function 
is empty and remove the call to atexit.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
          Component|c++                         |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug c++/19661] unnecessary atexit calls emitted for static objects with empty destructors

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-01-28 17:01 -------
atexit only takes a pointer to a function to be run on exit of the 
program. The fact that this is an empty function is unbeknownst to 
it, and probably the code in the middle-end that has to deal with 
that. I therefore believe that the front-end has to take care of the 
fact that we have may run an empty function; it would also logically 
make sense to guard the call to atexit with a check whether the 
destructor is empty or not. 
 
I'll move this bug back to the C++ component. 
 
W. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |c++


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug middle-end/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-28 18:53 -------
(In reply to comment #5)
> I'll move this bug back to the C++ component. 
Consider the following C++ code:
struct Foo { ~Foo() {} int i; };
struct A { Foo foo[2]; };
void foo () { 
  static A a; 
} 

We don't know the deconstructor for A is empty until we remove the empry loop and do optimizations 
on it.  So this is again a job for the midde-end/tree-optimizations.  We can then mark the function as 
empty and then have a pass (or really fold or something like that) remove calls to atexit (and similar 
functions) which just references empty functions.  This is the correct way of doing this.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end
  GCC build triplet|n/a                         |
 GCC target triplet|n/a                         |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug middle-end/19661] unnecessary atexit calls emitted for static objects with empty destructors

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-01-28 19:11 -------
Why do you think the front-end doesn't know that the destructor is empty? 
It sees its definition, and it knows about potential destructors of 
member objects and base classes, so it should have all the information. 
That being said, I don't consider this to be such an important case as 
to pour a lot of passion into this discussion :-) 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-09-24 17:42 -------
Hmm, the easy way to to have a pass which finds all the atexit calls and sees if the function which is 
passed is a const function and if it is, then just remove the atexit call.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |tree-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661