[Bug rtl-optimization/23813] New: redundant register assignments not eliminated

[Bug rtl-optimization/23813] New: redundant register assignments not eliminated

[raeburn at raeburn dot org]

(CVS sources from ~6AM today US/Eastern, i.e., about 16 hours before submission)

Compiling source below with -O9 -fomit-frame-pointer, there are cases where registers are assigned 
multiple times without any labels in between; a register assigned zero, used in an inclusive-or, then 
overwritten; another register assigned zero and never used.

This 64-bit byteswap routine also exhibits the problems I submitted in 23810, 23811, 23812, but I 
think one or two of these might be separate problems from those.

typedef unsigned long long uint64_t;
typedef unsigned long uint32_t;
uint64_t bitreverse (uint64_t n) {
  /* Dr. Dobbs Journal 1983, reported in Sean Eron Anderson's "bit
     twiddling hacks" web site,
     http://graphics.stanford.edu/~seander/bithacks.html .  */
#define REV64_STEP(VAR, SHIFT, MASK) \
  VAR = (((VAR >> SHIFT) & MASK) | ((VAR << SHIFT) & (0xFFFFFFFFFFFFFFFFULL & ~MASK)))

  REV64_STEP(n,  1, 0x5555555555555555ULL); /* odd/even bits */
  REV64_STEP(n,  2, 0x3333333333333333ULL); /* bitpairs */
  REV64_STEP(n,  4, 0x0F0F0F0F0F0F0F0FULL); /* nibbles */
  REV64_STEP(n,  8, 0x00FF00FF00FF00FFULL); /* bytes */
  REV64_STEP(n, 16, 0x0000FFFF0000FFFFULL); /* halfwords */
  REV64_STEP(n, 32, 0x00000000FFFFFFFFULL); /* full words */
  return n;
}

assembly generated:

bitreverse:
        pushl   %edi
        pushl   %esi
        pushl   %ebx
        movl    16(%esp), %eax
        movl    20(%esp), %edx
        movl    %eax, %ecx
        movl    %edx, %ebx
        shrdl   $1, %ebx, %ecx
        shldl   $1, %eax, %edx
        addl    %eax, %eax
        shrl    %ebx
        andl    $-1431655766, %eax
        andl    $-1431655766, %edx
        andl    $1431655765, %ecx
        andl    $1431655765, %ebx
        orl     %eax, %ecx
        orl     %edx, %ebx
        movl    %ecx, %eax
        movl    %ebx, %edi
        movl    %ecx, %esi
        movl    %ebx, %edx
        shrdl   $2, %edi, %esi
        shldl   $2, %eax, %edx
        andl    $858993459, %esi
        shrl    $2, %edi
        andl    $-858993460, %edx
        sall    $2, %eax
        andl    $858993459, %edi
        andl    $-858993460, %eax
        orl     %edx, %edi
        orl     %eax, %esi
        movl    %edi, %ebx
        movl    %esi, %eax
        movl    %esi, %ecx
        movl    %edi, %edx
        shrdl   $4, %ebx, %ecx
        shldl   $4, %eax, %edx
        andl    $252645135, %ecx
        shrl    $4, %ebx
        andl    $-252645136, %edx
        sall    $4, %eax
        andl    $252645135, %ebx
        andl    $-252645136, %eax
        orl     %edx, %ebx
        orl     %eax, %ecx
        movl    %ebx, %edi
        movl    %ecx, %esi
        movl    %ecx, %eax
        shrdl   $8, %edi, %esi
        movl    %ebx, %edx
        shrl    $8, %edi
        andl    $16711935, %esi
        andl    $16711935, %edi
        shldl   $8, %eax, %edx
        sall    $8, %eax
        andl    $-16711936, %edx
        andl    $-16711936, %eax
        orl     %edx, %edi
        orl     %eax, %esi
            # we're about to copy this from esi to eax then clear esi;
            # wouldn't putting output in eax be better?
        movl    %edi, %ebx
        movl    %esi, %eax
        movl    %esi, %ecx
        movl    %edi, %edx
        xorl    %esi, %esi # esi set to zero
        # PRs 23810, 23811 look at this code:
        shrdl   $16, %ebx, %ecx
        shldl   $16, %eax, %edx
        andl    $65535, %ecx
        shrl    $16, %ebx
        andl    $-65536, %edx
        andl    $65535, %ebx
        sall    $16, %eax
        orl     %edx, %ebx
        andl    $-65536, %eax
        movl    %ebx, %edx
        orl     %eax, %ecx # output to ecx then move to eax, instead of just output to eax?
        movl    %ecx, %ebx # ebx only used to ior into edx, while ecx value still live
        movl    %ecx, %eax # assign eax twice??
        movl    %edx, %eax
        xorl    %edx, %edx # clear edx twice??
        xorl    %edx, %edx
        orl     %esi, %eax # esi still zero
        orl     %ebx, %edx
        movl    $0, %ecx # why? insn note shows REG_UNUSED
        popl    %ebx
        popl    %esi
        popl    %edi
        ret

A version of this swap routine that splits the 64-bit value into two 32-bit chunks, performs bit-
reversals on the chunks, and puts the two chunks back together in reverse order, comes out shorter, 
despite still showing the PR23810/23811 problems, but it does the same work as this function should.

I suspect at least some of these come from preserving DImode operations until fairly late, e.g., 
assigning a DImode to eax/edx, then shifting right 32 (eax:=edx, edx:=0), stuff like that.  I haven't 
figured out the esi and ecx bits yet though.

-- 
           Summary: redundant register assignments not eliminated
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: raeburn at raeburn dot org
                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=23813

[Bug rtl-optimization/23813] redundant register assignments not eliminated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-09-11 14:24 -------
This is the same issue as PR 23812.  GCC's RA likes to keep double sized variables in registers next to 
one another.

This is the normal subreg problem with the current RA.  There are a couple other bugs about this 
already opened.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23813

[Bug rtl-optimization/23813] redundant register assignments not eliminated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-09-13 20:31 -------
Confirmed, just the subreg issue.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |15792, 18427, 23812
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |missed-optimization, ra
   Last reconfirmed|0000-00-00 00:00:00         |2005-09-13 20:31:08
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23813