[Bug target/35926] Pushing / Poping ebx without using it.

[Bug target/35926] Pushing / Poping ebx without using it.

[tony.poppleton at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926

--- Comment #5 from Tony Poppleton <tony.poppleton at gmail dot com> 2011-01-25 19:33:20 UTC ---
I can confirm this still exists on both GCC 4.5.1 and GCC 4.6.0 (20110115),
when compiling with -O3.

I did some basic investigation into the files produced by the --dump-tree-all
flag, which shows:

add (struct toto_s * a, struct toto_s * b)
{
  int64_t tmp;
  int D.2686;
  struct toto_s * D.2685;
  long long int D.2684;
  int D.2683;
  int b.1;
  long long int D.2681;
  int a.0;

<bb 2>:
  a.0_2 = (int) a_1(d);
  D.2681_3 = (long long int) a.0_2;
  b.1_5 = (int) b_4(d);
  D.2683_6 = b.1_5 & -2;
  D.2684_7 = (long long int) D.2683_6;
  tmp_8 = D.2681_3 + D.2684_7;
  D.2686_9 = (int) tmp_8;
  D.2685_10 = (struct toto_s *) D.2686_9;
  return D.2685_10;
}

What I don't understand here, is the excessive casting;

The addition in tmp_8 is being done as (long long int), yet both terms are
ultimiately derived from (int) variables.  Is this casting to (long long int)
necessary to deal with an overflow on the addition?

If so, then why does the final asm code not appear to be catering for overflow?

Alternatively, could the whole block be simplified down to (int) during this
phase of the compile, thereby fixing the subsequent unnecessary usage of BX
during the RTL phase (as per comment #3)?

As an aside (possibly another bug report?), it appears there is a regression in
4.6.0, which requires the use of an additional movl compared to what is in the
original bug description (4.5.1 does not suffer from this)

         .file   "PR35926.c"
        .text
        .p2align 4,,15
        .globl  add
        .type   add, @function
add:
.LFB0:
        .cfi_startproc
        pushl   %ebx
        .cfi_def_cfa_offset 8
        .cfi_offset 3, -8
        movl    12(%esp), %eax
        movl    8(%esp), %ecx
        popl    %ebx
        .cfi_def_cfa_offset 4
        .cfi_restore 3
        andl    $-2, %eax
        addl    %eax, %ecx       <==== order of regs inverted
        movl    %ecx, %eax       <==== resulting in unnecessary movl
        ret
        .cfi_endproc
.LFE0:
        .size   add, .-add
        .ident  "GCC: (GNU) 4.6.0 20110115 (experimental)"
        .section        .note.GNU-stack,"",@progbits

[Bug target/35926] Pushing / Poping ebx without using it.

[tony.poppleton at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926

--- Comment #6 from Tony Poppleton <tony.poppleton at gmail dot com> 2011-01-27 16:55:26 UTC ---
For the record, the additional movl noticed above in GCC 4.6.0 has been
factored out into PR47477

[Bug target/35926] Pushing / Poping ebx without using it.

[tony.poppleton at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926

Tony Poppleton <tony.poppleton at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2008-12-28 06:57:47         |2011-02-01 13:13
      Known to fail|                            |4.1.2, 4.3.5, 4.4.5, 4.5.2,
                   |                            |4.6.0
           Severity|normal                      |enhancement

--- Comment #7 from Tony Poppleton <tony.poppleton at gmail dot com> 2011-02-01 13:44:28 UTC ---
Set known to fail field, last reconfirmed field, and modified to "enhancement"

[Bug target/35926] Pushing / Poping ebx without using it.

[tony.poppleton at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926

--- Comment #8 from Tony Poppleton <tony.poppleton at gmail dot com> 2013-04-10 01:42:20 UTC ---
This appears to be fixed in GCC 4.8.0, compiling with just -S and -O3, the asm
output is now a much simpler:

        .file   "test.c"
        .text
        .p2align 4,,15
        .globl  add
        .type   add, @function
add:
.LFB0:
        .cfi_startproc
        andq    $-2, %rsi
        leaq    (%rdi,%rsi), %rax
        ret
        .cfi_endproc
.LFE0:
        .size   add, .-add
        .ident  "GCC: (GNU) 4.8.0 20130316 (Red Hat 4.8.0-0.17)"
        .section        .note.GNU-stack,"",@progbits

[Bug target/35926] Pushing / Poping ebx without using it.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
   Target Milestone|---                         |5.0
             Status|NEW                         |RESOLVED
      Known to work|                            |5.1.0

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 5+.

I know comment #8 said it was fixed in GCC 4.8.0 but I could reproduce the
missed optimization in released GCC 4.8.1 through 4.9.4.

[Bug target/35926] Pushing / Poping ebx without using it.

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2008-12-28 06:57 -------
Confirmed, still happens even after IRA.  ebx is still being marked as being
used even though the RA does not use it.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
  GCC build triplet|i686-pc-linux-gnu           |
   GCC host triplet|i686-pc-linux-gnu           |
 GCC target triplet|i686-pc-linux-gnu           |i?86-*-*
           Keywords|                            |missed-optimization, ra
   Last reconfirmed|0000-00-00 00:00:00         |2008-12-28 06:57:47
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926

[Bug target/35926] Pushing / Poping ebx without using it.

[ubizjak at gmail dot com]

------- Comment #3 from ubizjak at gmail dot com  2008-04-14 11:12 -------
(In reply to comment #2)
> This is setting up the frame pointer which is needed for unwinding/debugging
> on some targets.  You can use -fomit-frame-pointer to avoid this.

No, -fomit-frame-pointer uses %ebp.

%ebx is marked as used due to long long value in %ecx/%ebx. For some reason,
%ebx is still marked as live when function prologue and epilogue are
constructed.


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926

[Bug target/35926] Pushing / Poping ebx without using it.

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2008-04-14 09:18 -------
This is setting up the frame pointer which is needed for unwinding/debugging
on some targets.  You can use -fomit-frame-pointer to avoid this.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35926