[Bug target/21508] New: non-optimal code with __builtin_signbit

[Bug target/21508] New: non-optimal code with __builtin_signbit

[uros at kss-loka dot si]

This testcase:

int test (double a) {
        return __builtin_signbit (a + 1.0);
}

produces quite non-optimal code when compiled with '-O2' (-fomit-frame-pointer):

test:
        subl    $12, %esp
!       xorl    %eax, %eax
!       movl    $1072693248, %edx
!       movl    %eax, (%esp)
!       movl    %edx, 4(%esp)
!       fldl    (%esp)
        faddl   16(%esp)
        fstpl   (%esp)
?       movl    4(%esp), %edx
        addl    $12, %esp
?       movl    %edx, %eax
        andl    $-2147483648, %eax
        ret

Instructions marked with (!) could be replaced with a fld1.
Instructions marked with (?) could be replaced with a "movl 4(%esp), %eax".

Perhaps fxam could be used in this case? Then the code would look something like:

fldl   4(%esp)
fxam
fnstsw %ax
fstp   %st(0)
andl   0x02, %eax
ret

-- 
           Summary: non-optimal code with __builtin_signbit
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: uros at kss-loka dot si
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21508

[Bug target/21508] non-optimal code with __builtin_signbit

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-11 11:25 -------
Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |missed-optimization
      Known to fail|                            |4.0.0 4.1.0
   Last reconfirmed|0000-00-00 00:00:00         |2005-05-11 11:25:47
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21508

[Bug target/21508] non-optimal code with __builtin_signbit

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2005-06-03 08:13 -------
It looks that REG_EQUAL notes gets lost between regmove and lreg pass. A 
relevant part from compiling an example in description with '-O2' shows:

pr21508.c.19.regmove:

(note:HI 7 6 12 0 NOTE_INSN_FUNCTION_BEG)

(insn:HI 12 7 13 0 (set (reg:DF 62)
        (mem/u/i:DF (symbol_ref/u:SI ("*.LC1") [flags 0x2]) [2 S8 A64])) 65 
{*movdf_nointeger} (nil)
    (expr_list:REG_EQUAL (const_double:DF -2147483648 [0x80000000] 1.0e+0 
[0x0.8p+1])
        (nil)))

in pr21508.c.22.lreg:
(note:HI 7 6 12 0 NOTE_INSN_FUNCTION_BEG)

(insn:HI 12 7 13 0 (set (reg:DF 62)
        (mem/u/i:DF (symbol_ref/u:SI ("*.LC1") [flags 0x2]) [2 S8 A64])) 65 
{*movdf_nointeger} (nil)
    (nil))


However, compiling a simple program, such as:

double test (double a) {
   return a + 1.0;
}

results in fld1 insn being used. REG_EQUAL notes are still present in lreg dump 
file:

(note:HI 7 6 11 0 NOTE_INSN_FUNCTION_BEG)

(insn:HI 11 7 12 0 (set (reg:DF 61)
        (mem/u/i:DF (symbol_ref/u:SI ("*.LC1") [flags 0x2]) [2 S8 A64])) 65 
{*movdf_nointeger} (nil)
    (expr_list:REG_EQUAL (const_double:DF -2147483648 [0x80000000] 1.0e+0 
[0x0.8p+1])
        (nil)))


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21508

[Bug target/21508] non-optimal code with __builtin_signbit

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2006-10-22 21:01 -------
On the mainline we get:
        subl    $12, %esp
        fld1
        faddl   16(%esp)
        fstpl   (%esp)
        movl    4(%esp), %eax
        addl    $12, %esp
        andl    $-2147483648, %eax
        ret

Which is optimial as there is no extra movl and there is fld1.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.3.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21508