[Bug rtl-optimization/41574] New: Distribute floating point expressions causes bad code.

[Bug rtl-optimization/41574] New: Distribute floating point expressions causes bad code.

[dougkwan at google dot com]

We found this problem on ARM but I believe the problem affect other platforms
as well.  In the function distribute_and_simplify() of combine.c, there is not
check for floating point expressions.  Sometimes it incorrectly optimizes a
floating point RTL.

-----bug.c----
static const double one=1.0;

double
f(double x)
{
  /* This is incorrectly transformed to x + x*x */
  return x*(one+x);
}
---------

$ arm-eabi-gcc -O2 -S -march=armv7-a -mfloat-abi=hard -mfpu=neon
-fno-unsafe-math-optimizations -g0 bug.c
$ cat bug.s
        .arch armv7-a
        .eabi_attribute 27, 3
        .eabi_attribute 28, 1
        .fpu neon
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 1
        .eabi_attribute 30, 2
        .eabi_attribute 18, 4
        .file   "bug.c"
        .text
        .align  2
        .global f
        .type   f, %function
f:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        fmacd   d0, d0, d0
        bx      lr
        .size   f, .-f
        .ident  "GCC: (GNU) 4.5.0 20091005 (experimental)"

The expression x*(1.0 + x) above is optmized into x + x * x using the fmacd
instruction, which multiplies and accumlates.  The following is part combine
pass dump, note how instruction 13 is modified.:

---

;; Function f (f)

starting the processing of deferred insns
ending the processing of deferred insns
df_analyze called
insn_cost 2: 4
insn_cost 6: 4
insn_cost 7: 4
insn_cost 8: 4
insn_cost 13: 4
insn_cost 16: 0
deferring deletion of insn with uid = 2.
modifying insn i2     7 r138:DF=s0:DF+r139:DF
      REG_DEAD: r139:DF
deferring rescan insn with uid = 7.
modifying insn i3     8 r137:DF=r138:DF*s0:DF
      REG_DEAD: r138:DF
deferring rescan insn with uid = 8.
deferring deletion of insn with uid = 7.
deferring deletion of insn with uid = 6.
modifying insn i3     8 r137:DF=s0:DF*s0:DF+s0:DF
deferring rescan insn with uid = 8.
deferring deletion of insn with uid = 8.
modifying insn i3    13 s0:DF=s0:DF*s0:DF+s0:DF
deferring rescan insn with uid = 13.
(note 4 0 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)

(note 2 4 3 2 NOTE_INSN_DELETED)

(note 3 2 6 2 NOTE_INSN_FUNCTION_BEG)

(note 6 3 7 2 NOTE_INSN_DELETED)

(note 7 6 8 2 NOTE_INSN_DELETED)

(note 8 7 13 2 NOTE_INSN_DELETED)

(insn 13 8 16 2 bug.c:8 (set (reg/i:DF 63 s0)
        (plus:DF (mult:DF (reg:DF 63 s0 [ x ])
                (reg:DF 63 s0 [ x ]))
            (reg:DF 63 s0 [ x ]))) 610 {*muldf3adddf_vfp} (nil))

(insn 16 13 0 2 bug.c:8 (use (reg/i:DF 63 s0)) -1 (nil))
starting the processing of deferred insns
deleting insn with uid = 2.
deleting insn with uid = 6.
deleting insn with uid = 7.
deleting insn with uid = 8.
rescanning insn with uid = 13.
deleting insn with uid = 13.
ending the processing of deferred insns

;; Combiner totals: 10 attempts, 10 substitutions (3 requiring new space),
;; 3 successes.

---

The problem happens in this part of distribute_and_simplify_rtx ():


  tmp = apply_distributive_law (simplify_gen_binary (inner_code, mode,
                                                     new_op0, new_op1));
  if (GET_CODE (tmp) != outer_code
      && rtx_cost (tmp, SET, optimize_this_for_speed_p)
         < rtx_cost (x, SET, optimize_this_for_speed_p))
    return tmp;

It synthesizes a new expression by distributing one of the sub-expressions and
then call apply_distribute_law.  In the test-case above, apply_distribute_law
detects a floating point RTL expression and returns immediately but the
simplified expression generated has a lower RTL-cost than the original
expression.  Hence distribute_and_simplify_rtx returns the simplified
expression, eventhough it should not unless -funsafe-math-optimizations is
given.

I think the fix is to add this test at the entrance of the function
distribute_and_simplify_rtx

  /* Distributivity is not true for floating point as it can change the
     value.  So we don't do it unless -funsafe-math-optimizations.  */
  if (FLOAT_MODE_P (GET_MODE (x))
      && ! flag_unsafe_math_optimizations)
    return NULL_RTX;


-- 
           Summary: Distribute floating point expressions causes bad code.
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dougkwan at google dot com
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: arm-none-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[dougkwan at gcc dot gnu dot org]

------- Comment #1 from dougkwan at gcc dot gnu dot org  2009-10-05 09:09 -------
Subject: Bug 41574

Author: dougkwan
Date: Mon Oct  5 09:08:46 2009
New Revision: 152443

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=152443
Log:
2009-10-05  Doug Kwan  <dougkwan@google.com>

        PR rtl-optimization/41574
        Index: combine.c (distribute_and_simplify_rtx): Quit if RTX mode is
        floating point and we are not doing unsafe math optimizations.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/combine.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[ramana at gcc dot gnu dot org]

------- Comment #2 from ramana at gcc dot gnu dot org  2009-10-05 09:32 -------
Fixed.


-- 

ramana at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[jakub at gcc dot gnu dot org]

------- Comment #3 from jakub at gcc dot gnu dot org  2009-10-05 09:48 -------
The ChangeLog entry is wrong.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[ebotcazou at gcc dot gnu dot org]

------- Comment #4 from ebotcazou at gcc dot gnu dot org  2009-10-05 10:00 -------
> The ChangeLog entry is wrong.

And folks from Google shouldn't feel entitled to break a freeze imposed by
other folks from Google even if, yes, it is annoyingly long. :-)


-- 

ebotcazou at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[dougkwan at google dot com]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
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------- Comment #5 from dougkwan at google dot com  2009-10-05 15:44 -------
Subject: Re:  Distribute floating point 
        expressions causes bad code.

I am aware of the fact the stage one has ended but this is a bug fix,
not an experimental new feature.  Did I break a code freeze?  If so, I
am sorry and can back out the fix until the tree is reopen.

2009/10/5 ebotcazou at gcc dot gnu dot org <gcc-bugzilla@gcc.gnu.org>:
>
>
> ------- Comment #4 from ebotcazou at gcc dot gnu dot org  2009-10-05 10:00 -------
>> The ChangeLog entry is wrong.
>
> And folks from Google shouldn't feel entitled to break a freeze imposed by
> other folks from Google even if, yes, it is annoyingly long. :-)
>
>
> --
>
> ebotcazou at gcc dot gnu dot org changed:
>
>           What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                 CC|                            |ebotcazou at gcc dot gnu dot
>                   |                            |org
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574
>
> ------- You are receiving this mail because: -------
> You reported the bug, or are watching the reporter.
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[dougkwan at google dot com]

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------- Comment #6 from dougkwan at google dot com  2009-10-05 15:48 -------
Subject: Re:  Distribute floating point 
        expressions causes bad code.

Just saw Diego's e-mail about the me breaking the freeze.  Sorry I
should have checked that before checking thing in.  It was just my
bad.

2009/10/5 Doug Kwan (Ãö®¶¼w) <dougkwan@google.com>:
> I am aware of the fact the stage one has ended but this is a bug fix,
> not an experimental new feature.  Did I break a code freeze?  If so, I
> am sorry and can back out the fix until the tree is reopen.
>
> 2009/10/5 ebotcazou at gcc dot gnu dot org <gcc-bugzilla@gcc.gnu.org>:
>>
>>
>> ------- Comment #4 from ebotcazou at gcc dot gnu dot org  2009-10-05 10:00 -------
>>> The ChangeLog entry is wrong.
>>
>> And folks from Google shouldn't feel entitled to break a freeze imposed by
>> other folks from Google even if, yes, it is annoyingly long. :-)
>>
>>
>> --
>>
>> ebotcazou at gcc dot gnu dot org changed:
>>
>>           What    |Removed                     |Added
>> ----------------------------------------------------------------------------
>>                 CC|                            |ebotcazou at gcc dot gnu dot
>>                   |                            |org
>>
>>
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574
>>
>> ------- You are receiving this mail because: -------
>> You reported the bug, or are watching the reporter.
>>
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[dougkwan at gcc dot gnu dot org]

------- Comment #7 from dougkwan at gcc dot gnu dot org  2009-10-08 22:17 -------
Subject: Bug 41574

Author: dougkwan
Date: Thu Oct  8 22:16:58 2009
New Revision: 152580

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=152580
Log:
2009-10-08  Doug Kwan  <dougkwan@google.com>

        PR rtl-optimization/41574
        * gcc.dg/pr41574.c: New test.

Added:
    trunk/gcc/testsuite/gcc.dg/pr41574.c
Modified:
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code.

[dougkwan at google dot com]

------- Comment #8 from dougkwan at google dot com  2009-10-08 22:20 -------
This is fixed in trunk but at least gcc-4.4.0, where this bug was found, is
still broken.


-- 

dougkwan at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
      Known to fail|                            |4.4.0
         Resolution|FIXED                       |
            Version|4.5.0                       |4.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code [4.4 only]

[ramana at gcc dot gnu dot org]

------- Comment #9 from ramana at gcc dot gnu dot org  2009-10-15 10:44 -------
(In reply to comment #8)
> This is fixed in trunk but at least gcc-4.4.0, where this bug was found, is
> still broken.
> 

I have no approval rights but can you test & ask to backport this to 4.4 branch
after the freeze for the 4.4.2 release is lifted ? 

Ramana


-- 

ramana at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-10-15 10:44:31
               date|                            |
            Summary|Distribute floating point   |Distribute floating point
                   |expressions causes bad code.|expressions causes bad code
                   |                            |[4.4 only]


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code [4.4 only]

[dougkwan at google dot com]

------- Comment #10 from dougkwan at google dot com  2009-10-20 06:22 -------
(In reply to comment #9)
> (In reply to comment #8)
> > This is fixed in trunk but at least gcc-4.4.0, where this bug was found, is
> > still broken.
> > 
> 
> I have no approval rights but can you test & ask to backport this to 4.4 branch
> after the freeze for the 4.4.2 release is lifted ? 

Sorry about the late reply. Yes, I can prepare a fix for 4.4.2

-Doug 


-- 

dougkwan at google dot com changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code [4.4 only]

[ramana at gcc dot gnu dot org]

-- 

ramana at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.4.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code [4.4 only]

[ramana at gcc dot gnu dot org]

------- Comment #11 from ramana at gcc dot gnu dot org  2009-12-11 11:21 -------
Subject: Bug 41574

Author: ramana
Date: Fri Dec 11 11:21:33 2009
New Revision: 155157

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=155157
Log:
Fix PR41574 on 4.4 branch.

2009-12-11  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

        2009-10-05  Doug Kwan  <dougkwan@google.com>

        PR rtl-optimization/41574
        * combine.c (distribute_and_simplify_rtx): Quit if RTX mode is
        floating point and we are not doing unsafe math optimizations.


2009-12-11  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

        2009-10-08  Doug Kwan  <dougkwan@google.com>

        PR rtl-optimization/41574
        * gcc.dg/pr41574.c: New test.



Added:
    branches/gcc-4_4-branch/gcc/testsuite/gcc.dg/pr41574.c
      - copied unchanged from r152580, trunk/gcc/testsuite/gcc.dg/pr41574.c
Modified:
    branches/gcc-4_4-branch/gcc/ChangeLog
    branches/gcc-4_4-branch/gcc/combine.c
    branches/gcc-4_4-branch/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574

[Bug rtl-optimization/41574] Distribute floating point expressions causes bad code [4.4 only]

[ramana at gcc dot gnu dot org]

------- Comment #12 from ramana at gcc dot gnu dot org  2009-12-11 17:46 -------
Fixed .


-- 

ramana at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574