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* [Bug rtl-optimization/58295] New: The combination pass doesn't eliminates some extra zero extensions
@ 2013-09-02  8:39 uranus at tinlans dot org
  2013-09-02 22:02 ` [Bug rtl-optimization/58295] [4.8/4.9 regression] Missed zero-extension elimination in the combiner ebotcazou at gcc dot gnu.org
                   ` (14 more replies)
  0 siblings, 15 replies; 16+ messages in thread
From: uranus at tinlans dot org @ 2013-09-02  8:39 UTC (permalink / raw)
  To: gcc-bugs

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58295

            Bug ID: 58295
           Summary: The combination pass doesn't eliminates some extra
                    zero extensions
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: uranus at tinlans dot org

$ cat test.c
extern char zeb_test_array[10];

unsigned char ee_isdigit2(unsigned int i)
{
  unsigned char c = zeb_test_array[i];
  unsigned char retval;

  retval = ((c>='0') & (c<='9')) ? 1 : 0;
  return retval;
}

$ arm-eabi-gcc -v
Using built-in specs.
COLLECT_GCC=arm-eabi-gcc
COLLECT_LTO_WRAPPER=/home1/lhtseng/arm/4.9/libexec/gcc/arm-eabi/4.9.0/lto-wrapper
Target: arm-eabi
Configured with: ../../../../work/4.9/src/gcc-4.9.0/configure --target=arm-eabi
--prefix=/home1/lhtseng/arm/4.9 --disable-nls --disable-shared
--enable-languages=c --enable-__cxa_atexit --enable-c99 --enable-long-long
--enable-threads=single --with-newlib --disable-multilib --disable-libssp
--disable-libgomp --disable-decimal-float --disable-libffi --disable-libmudflap
--disable-lto --with-gmp=/home1/lhtseng/work/general
--with-mpfr=/home1/lhtseng/work/general --with-mpc=/home1/lhtseng/work/general
--with-isl=/home1/lhtseng/work/general --with-cloog=/home1/lhtseng/work/general
Thread model: single
gcc version 4.9.0 20130802 (experimental) (GCC) 

$ arm-eabi-gcc -O3 -S test.c
$ cat test.s
...
ee_isdigit2:
        @ Function supports interworking.
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        ldr     r3, .L2
        ldrb    r0, [r3, r0]    @ zero_extendqisi2
        sub     r0, r0, #48
        and     r0, r0, #255
        cmp     r0, #9
        movhi   r0, #0
        movls   r0, #1
        bx      lr
...

The instruction 'and r0, r0, #255' is a redundant instruction which cannot be
eliminated by the RTL instruction combination pass. This pass was able to
handle this case before this commit:
http://gcc.gnu.org/viewcvs/gcc/trunk/gcc/simplify-rtx.c?r1=191909&r2=191928&pathrev=192303
And the code was re-organized to line 643 ~ 656 after this commit:
http://gcc.gnu.org/viewcvs/gcc/trunk/gcc/simplify-rtx.c?r1=192006&r2=192186&pathrev=192303
For example, GCC 4.6.3 can handle it perfectly.

In GCC 4.9.0, reverting the two commits or simply commeting the lines mentioned
above can make the combination pass handle this case again:
$ arm-eabi-gcc-modified -O3 -da -S test.c
$ cat test.c.166r.expand
...
(insn 9 8 10 2 (set (reg:SI 120)
        (plus:SI (subreg:SI (reg:QI 118) 0)
            (const_int -48 [0xffffffffffffffd0]))) test.c:6 -1
     (nil))
(insn 10 9 11 2 (set (reg:SI 121)
        (and:SI (reg:SI 120)
            (const_int 255 [0xff]))) test.c:6 -1
     (nil))
(insn 11 10 12 2 (set (reg:CC 100 cc)
        (compare:CC (reg:SI 121)
            (const_int 9 [0x9]))) test.c:6 -1
     (nil))
(insn 12 11 13 2 (set (reg:SI 122)
        (leu:SI (reg:CC 100 cc)
            (const_int 0 [0]))) test.c:6 -1
     (nil))
...
$ cat test.c.197r.combine
...
Trying 9, 10 -> 11:
Failed to match this instruction:
(set (reg:CC 100 cc)
    (compare:CC (plus:SI (reg:SI 119)
            (const_int -48 [0xffffffffffffffd0]))
        (const_int 9 [0x9])))
Successfully matched this instruction:
(set (reg:SI 121)
    (plus:SI (reg:SI 119)
        (const_int -48 [0xffffffffffffffd0])))
Successfully matched this instruction:
(set (reg:CC 100 cc)
    (compare:CC (reg:SI 121)
        (const_int 9 [0x9])))
deferring deletion of insn with uid = 9.
modifying insn i2    10: r121:SI=r119:SI-0x30
      REG_DEAD r119:SI
deferring rescan insn with uid = 10.
modifying insn i3    11: cc:CC=cmp(r121:SI,0x9)
      REG_DEAD r121:SI
deferring rescan insn with uid = 11.
...

The insn 10 is generated by (define_expand "zero_extendqisi2" ...) of ARM's
machine description. Before the commits I mentioned above, the combination pass
successfully combines it with the insn 9. However, after those commits, the
combination pass never tries to do the combination '9, 10 -> 11.'

After reading the commit messages of the file 'simplify-rtx.c', we can
understand the commits, r191928, was trying to optimize x86 code generation,
but it led to the suboptimal code generation of the ARM's target.


^ permalink raw reply	[flat|nested] 16+ messages in thread
end of thread, other threads:[~2013-12-10 23:01 UTC | newest]

Thread overview: 16+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2013-09-02  8:39 [Bug rtl-optimization/58295] New: The combination pass doesn't eliminates some extra zero extensions uranus at tinlans dot org
2013-09-02 22:02 ` [Bug rtl-optimization/58295] [4.8/4.9 regression] Missed zero-extension elimination in the combiner ebotcazou at gcc dot gnu.org
2013-09-06  9:54 ` jasonwucj at gmail dot com
2013-09-06 10:18 ` jakub at gcc dot gnu.org
2013-09-06 12:15 ` uranus at tinlans dot org
2013-09-06 13:18 ` ebotcazou at gcc dot gnu.org
2013-10-16  9:49 ` jakub at gcc dot gnu.org
2013-10-30 13:35 ` rguenth at gcc dot gnu.org
2013-10-30 18:02 ` ebotcazou at gcc dot gnu.org
2013-10-31  3:31 ` npickito at gmail dot com
2013-10-31  6:58 ` ebotcazou at gcc dot gnu.org
2013-10-31  7:14 ` npickito at gmail dot com
2013-10-31  8:02 ` ebotcazou at gcc dot gnu.org
2013-12-10 22:58 ` ebotcazou at gcc dot gnu.org
2013-12-10 22:59 ` ebotcazou at gcc dot gnu.org
2013-12-10 23:01 ` ebotcazou at gcc dot gnu.org

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