[Bug lto/41589] New: lto does not eliminate unused variables

[Bug lto/41589] New: lto does not eliminate unused variables

[andi-gcc at firstfloor dot org]

Using built-in specs.
COLLECT_GCC=gcc45
COLLECT_LTO_WRAPPER=/pkg/gcc-4.5-091004/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.5.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc/configure --prefix=/pkg/gcc-4.5-091004
--enable-checking=release --enable-languages=c,c++ --disable-nls --enable-lto
Thread model: posix
gcc version 4.5.0 20091004 (experimental) (GCC) 


With the attached simple lto test case I would have expected f1 and f2 and y to
be optimized away when built with -fwhole-program. But that was not the case.
nm ./tlto
...

0000000000601020 b completed.5856
0000000000601010 W data_start
0000000000601028 b dtor_idx.5858
0000000000400560 T f1
0000000000400550 T f2
00000000004004f0 t frame_dummy
0000000000400520 T main
                 U printf@@GLIBC_2.2.5
0000000000601030 B y


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           Summary: lto does not eliminate unused variables
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: andi-gcc at firstfloor dot org
  GCC host triplet: x86_64-linux
GCC target triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug lto/41589] lto does not eliminate unused variables

[andi-gcc at firstfloor dot org]

------- Comment #1 from andi-gcc at firstfloor dot org  2009-10-05 14:04 -------
Created an attachment (id=18710)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18710&action=view)
tlto1.c


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug lto/41589] lto does not eliminate unused variables

[andi-gcc at firstfloor dot org]

------- Comment #2 from andi-gcc at firstfloor dot org  2009-10-05 14:04 -------
Created an attachment (id=18711)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18711&action=view)
tlto2.c


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug lto/41589] lto does not eliminate unused variables

[andi-gcc at firstfloor dot org]

------- Comment #3 from andi-gcc at firstfloor dot org  2009-10-05 14:04 -------
Created an attachment (id=18712)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18712&action=view)
Makefile.lto


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug tree-optimization/41589] lto does not eliminate unused variables

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2009-10-05 14:20 -------
Confirmed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu dot
                   |                            |org
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|lto                         |tree-optimization
     Ever Confirmed|0                           |1
           Keywords|                            |lto, missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2009-10-05 14:20:35
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug tree-optimization/41589] lto does not eliminate unused variables

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2009-10-05 15:16 -------
I think this optimization is up to the linker as the variables are marked as
hidden and not marked as local to the Translational unit.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug tree-optimization/41589] lto does not eliminate unused variables

[andi-gcc at firstfloor dot org]

------- Comment #6 from andi-gcc at firstfloor dot org  2009-10-05 15:42 -------
I use binutils 2.19 (from opensuse 11.1).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug tree-optimization/41589] lto does not eliminate unused variables

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2009-10-05 17:07 -------
(In reply to comment #6)
> I use binutils 2.19 (from opensuse 11.1).

What I meant was that -ffunction-sections -fdata-sections and then
-Wl,--gc-sections to remove the unused variables/functions.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589

[Bug tree-optimization/41589] lto does not eliminate unused variables

[hubicka at gcc dot gnu dot org]

------- Comment #8 from hubicka at gcc dot gnu dot org  2009-11-12 13:23 -------
There are some bugs on LTO and unused vars elimination, but in this testcase we
end up with:
main:
.LFB2:  
        subq    $8, %rsp
.LCFI0: 
        movl    y(%rip), %esi
        movl    $.LC0, %edi
        xorl    %eax, %eax
        addl    $5, %esi
        movl    %esi, y(%rip)
        call    printf
        xorl    %eax, %eax
        addq    $8, %rsp
.LCFI1: 
        ret
.LFE2:  

While GCC might be able to recognize that main is not reentrant and thus
computing the static variable is useless, this is bit tricky to do.

Honza


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41589