[Bug rtl-optimization/47764] The constant load instruction should be hoisted out of loop

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47764

Uros Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|arm-linux-androideabi       |
                 CC|                            |ubizjak at gmail dot com
          Component|target                      |rtl-optimization
      Known to fail|                            |4.7.0, 4.8.0

--- Comment #5 from Uros Bizjak <ubizjak at gmail dot com> 2013-01-24 07:25:04 UTC ---
This is a problem with rtl-optimization, gcse2 pass.

Following testcase also fails on x86_64, with 4.8 [1] that removes (!o,F)
alternative.

Following test, when compiled with -O3 hoists memory load out of the loop:

--cut here--
volatile double y;

void
test ()
{
  int z;

  for (z = 0; z < 1000; z++)
    y = 0.1;
}
--cut here--

_.210r.postreload:

   15: L15:
    8: NOTE_INSN_BASIC_BLOCK 3
   23: xmm0:DF=[`*.LC0']
   10: [`y']=xmm0:DF
      REG_DEAD xmm0:DF
   11: NOTE_INSN_DELETED
   12: {flags:CCZ=cmp(ax:SI-0x1,0);ax:SI=ax:SI-0x1;}
   13: pc={(flags:CCZ!=0)?L15:pc}
      REG_BR_PROB 0x26ab

_.211r.gcse2:

   26: xmm0:DF=[`*.LC0']
   15: L15:
    8: NOTE_INSN_BASIC_BLOCK 3
   10: [`y']=xmm0:DF
      REG_DEAD xmm0:DF
   11: NOTE_INSN_DELETED
   12: {flags:CCZ=cmp(ax:SI-0x1,0);ax:SI=ax:SI-0x1;}
   13: pc={(flags:CCZ!=0)?L15:pc}
      REG_BR_PROB 0x26ab

However, when constant is changed to 0.0 (so, we can load it directly to %xmm
register using xorpd insn):

--cut here--
volatile double y;

void
test ()
{
  int z;

  for (z = 0; z < 1000; z++)
    y = 0.0;
}
--cut here--

gcc -O3:

_.211r.gcse2:

   15: L15:
    8: NOTE_INSN_BASIC_BLOCK 3
   10: xmm0:DF=0.0
   23: [`y']=xmm0:DF
      REG_DEAD xmm0:DF
   11: NOTE_INSN_DELETED
   12: {flags:CCZ=cmp(ax:SI-0x1,0);ax:SI=ax:SI-0x1;}
   13: pc={(flags:CCZ!=0)?L15:pc}
      REG_BR_PROB 0x26ab

Constant load remains inside the loop. It looks that gcse2 pass cares only for
loads from memory, but I see no reason why constant load should not be
considered. It looks like an oversight to me.

The same happens with:

--cut here--
volatile long long y;

void
test ()
{
  int z;

  for (z = 0; z < 1000; z++)
    y = 0x123456789;
}
--cut here--

_.211r.gcse2:

   15: L15:
    8: NOTE_INSN_BASIC_BLOCK 3
   23: dx:DI=0x123456789
   24: [`y']=dx:DI
      REG_DEAD dx:DI
   11: NOTE_INSN_DELETED
   12: {flags:CCZ=cmp(ax:SI-0x1,0);ax:SI=ax:SI-0x1;}
   13: pc={(flags:CCZ!=0)?L15:pc}
      REG_BR_PROB 0x26ab

resulting in:

.L3:
        movabsq $4886718345, %rdx
        subl    $1, %eax
        movq    %rdx, y(%rip)
        jne     .L3

Reconfirmed as rtl-optimization (gcse2 pass) problem.

[1] 4.8.0 20130124 (experimental) [trunk revision 195417]