[Bug tree-optimization/65282] New: wrong code at -Os and above on x86_64-linux-gnu

[Bug tree-optimization/65282] New: wrong code at -Os and above on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65282

            Bug ID: 65282
           Summary: wrong code at -Os and above on x86_64-linux-gnu
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk miscompiles the following code on x86_64-linux at -Os and
above in both 32-bit and 64-bit modes. 

This is a regression from 4.9.x. 

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/5.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure --prefix=/usr/local/gcc-trunk
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
gcc version 5.0.0 20150302 (experimental) [trunk revision 221099] (GCC) 
$ 
$ gcc-trunk -O1 small.c; a.out
$ gcc-4.9.2 -Os small.c; a.out
$ 
$ gcc-trunk -Os small.c
$ ./a.out
Aborted (core dumped)
$ 


-----------------------------------


int a[2];
static int *b = &a[0], *c = &a[1];

int
main ()
{
  *c = 1;
  *b = 0;

  if (a[1] != 1)
    __builtin_abort ();

  return 0;
}

[Bug ipa/65282] [5 Regression] wrong code at -Os and above on x86_64-linux-gnu

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65282

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |ipa

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Another ICF issue...

[Bug ipa/65282] [5 Regression] wrong code at -Os and above on x86_64-linux-gnu

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65282

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Patch has been just sent to ML:
https://gcc.gnu.org/ml/gcc-patches/2015-03/msg00130.html

Thanks,
Martin
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide296

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P4
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2015-03-03
           Assignee|unassigned at gcc dot gnu.org      |gjl at gcc dot gnu.org
     Ever confirmed|0                           |1

[Bug ipa/65282] [5 Regression] wrong code at -Os and above on x86_64-linux-gnu

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65282

--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
Author: marxin
Date: Tue Mar  3 12:09:13 2015
New Revision: 221146

URL: https://gcc.gnu.org/viewcvs?rev=221146&root=gcc&view=rev
Log:
Fix PR ipa/65282.

    * gcc.dg/ipa/PR65282.c: New test.
    PR ipa/65282
    * ipa-icf.c (sem_variable::equals): Fix wrong condition.


Added:
    trunk/gcc/testsuite/gcc.dg/ipa/PR65282.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/ipa-icf.c
    trunk/gcc/testsuite/ChangeLog
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Subject: [Bug ipa/65282] [5 Regression] wrong code at -Os and above on x86_64-linux-gnu
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65282

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
Fixed in 5.0.
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From: "law at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/64317] [5 Regression] Ineffective allocation of PIC base register
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd317

--- Comment #17 from Jeffrey A. Law <law at redhat dot com> ---
Thanks Vlad, that patch helped.  Prior to your patch we had 15 reloads of the
PIC register from memory, after your patch we have just 9.   However, several
of the remaining 9 seem to be redundant.

LRA never considers a block starting with a label as participating in an EBB.
That's overly conservative.  A block can participate in an EBB if all of its
predecessors are part of the same EBB.

That's particularly useful in CFGs like

    A
   /|
  / |
 B  |
  \ |
   \|
    C

[ Flow downward of course. ]


If we assume that B is the fallthru path, then LRA will try to make AB into an
EBB.  But it will not consider C because C will start with a label.  That
ultimately causes missed inheritances in this example.

As an example, we have this in the .reload dump:

[ BB 7 ]

(insn 848 94 582 7 (set (reg:SI 6 bp [107])
        (mem/c:SI (plus:SI (reg/f:SI 7 sp)
                (const_int 28 [0x1c])) [4 %sfp+-4 S4 A32])) k.c:22 90
{*movsi_internal}
     (nil))

[ ... ]

(jump_insn 99 98 100 7 (set (pc)
        (if_then_else (geu (reg:CC 17 flags)
                (const_int 0 [0]))
            (label_ref 103)
            (pc))) k.c:18 613 {*jcc_1}
     (int_list:REG_BR_PROB 5000 (nil))

[ BB 8 ]

(insn 584 100 101 8 (set (reg:SI 3 bx [107])
        (reg:SI 6 bp [107])) k.c:19 90 {*movsi_internal}
     (nil))

[ Note we inherited the value in %ebp, this is good. ]

[ ... ]

[ BB 9 ]

(code_label 103 102 104 9 5 "" [1 uses])

[ ... ]

(insn 850 105 585 9 (set (reg:SI 6 bp [107])
        (mem/c:SI (plus:SI (reg/f:SI 7 sp)
                (const_int 28 [0x1c])) [4 %sfp+-4 S4 A32])) k.c:22 90
{*movsi_internal}
     (nil))

Note insn 850 which reloads the value from memory again.  We correctly formed
an EBB with BB7 and BB8, but we really should have extended that to BB9.

It appears that fixing this would improve things further.  It's also the case
that the post-reload passes aren't doing as good of a job at cleaning things up
as they should, so  I'll look at that in parallel if you can investigate the
LRA side of things.