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: unassig...@gcc.gnu.org
ReportedBy: sch...@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).
