[Bug rtl-optimization/34503] Issues with constant/copy propagation implementation in gcse.c

["steven at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #7 from steven at gcc dot gnu dot org  2008-05-24 14:31 -------
I have found a test case for the issue mentioned in comment #4. And it comes
from gcc itself:

static int *free_phinodes[10 - 2]; /* was 'tree' */
static unsigned long free_phinode_count;
void
init_phinodes (void)
{
  int i;
  for (i = 0; i < 10 - 2; i++)
    free_phinodes[i] = ((void *)0);
  free_phinode_count = 0
}

When compiling this on powerpc-unknown-elf with r135815 and with the options
"-O2 -fdump-rtl-gcse1-slim", I get the following RTL at the end of the GCSE
pass:

   37 NOTE_INSN_BASIC_BLOCK
   36 NOTE_INSN_FUNCTION_BEG
   39 r164:SI=high(`*.LANCHOR0')
   40 r166:SI=r164:SI+low(`*.LANCHOR0')
      REG_DEAD: r164:SI
      REG_EQUAL: `*.LANCHOR0'
   75 r153:SI=r166:SI
      REG_EQUAL: `*.LANCHOR0'
   73 r165:SI=r166:SI+0x20
      REG_DEAD: r167:SI
      REG_EQUAL: const(`*.LANCHOR0'+0x20)
L46:
   42 NOTE_INSN_BASIC_BLOCK
   43 r156:SI=0x0
   44 [r153:SI]=r156:SI
      REG_EQUAL: 0x0
   45 r153:SI=r153:SI+0x4
   71 r157:SI=r166:SI
      REG_EQUAL: `*.LANCHOR0'
   50 r160:CC=cmp(r153:SI,r165:SI)
      REG_EQUAL: cmp(r153:SI,const(`*.LANCHOR0'+0x20))
   51 pc={(r160:CC!=0x0)?L46:pc}
      REG_DEAD: r160:CC
      REG_BR_PROB: 0x222e
   52 NOTE_INSN_BASIC_BLOCK
   56 [r157:SI+0x20]=r156:SI
      REG_DEAD: r157:SI
      REG_DEAD: r156:SI
      REG_EQUAL: 0x0

This is the dump *after* CPROP2, so the post-PRE-GCSE const/copy pass has run. 
Note the REG_EQUAL note on insn 71: "r157:SI=r166:SI {REG_EQUAL:
`*.LANCHOR0'}". This is a reg-reg copy insn, but CPROP2 does not record the
copy.  Instead it records that r157 is equal to the REG_EQUAL note value.  The
result is that CPROP2 fails to copy propagate r166 into insn 56 (where r157
reaches and dies, so the copy propagation would eliminate r157).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34503