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From: "schwab@linux-m68k.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/54555] New: (set (REGX) (CONST_INT B)) -> (set (STRICT_LOW_PART (REGX)) (CONST_INT B)) is pessimising
Date: Wed, 12 Sep 2012 09:37:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54555

             Bug #: 54555
           Summary: (set (REGX) (CONST_INT B)) -> (set (STRICT_LOW_PART
                    (REGX)) (CONST_INT B)) is pessimising
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: schwab@linux-m68k.org
            Target: m68k-*-*


Postreload (reload_cse_move2add) transforms

 (set (REGX) (CONST_INT A))
 ...
 (set (REGX) (CONST_INT B))

into

 (set (REGX) (CONST_INT A))
 ...
 (set (STRICT_LOW_PART (REGX)) (CONST_INT B))

which is a pessimisation on m68k if REGX is a data register and B is a small
constant in range for moveq which can no longer be used for the second SET;
instead moveb is used which is bigger and also slower on <= m68030.

$ cat moveb.c
void foo (void);
void bar (int a)
{
  if (a == 16 || a == 23) foo ();
  if (a == -110 || a == -128) foo ();
}
$ gcc -O2 -S moveb.c
$ cat moveb.s
#NO_APP
        .file   "moveb.c"
        .text
        .align  2
        .globl  f
        .type   f, @function
f:
        link.w %fp,#0
        move.l %d2,-(%sp)
        move.l 8(%fp),%d2
        moveq #16,%d0
        cmp.l %d2,%d0
        jeq .L2
        move.b #23,%d0             <--- should be moveq #23,%d0
        cmp.l %d2,%d0
        jeq .L2
        moveq #-110,%d0
        cmp.l %d2,%d0
        jeq .L4
.L13:
        move.b #-128,%d0           <--- should be moveq #-128,%d0
        cmp.l %d2,%d0
        jeq .L4
        move.l -4(%fp),%d2
        unlk %fp
        rts
.L2:
        jsr foo
        moveq #-110,%d0
        cmp.l %d2,%d0
        jne .L13
.L4:
        move.l -4(%fp),%d2
        unlk %fp
        jra bar
        .size   f, .-f
        .ident  "GCC: (GNU) 4.8.0 20120912 (experimental)"
        .section        .note.GNU-stack,"",@progbits

This transformation was introduced in r68532 (gcc 3.4).