/*
the following case produces suboptimal code:
everything compiled with -O3 only
*/
extern char testcase(int* d)
{
int w23;
w23 = d[2] < d[3];
w23 &= d[3] < d[2];
return w23;
}
/*
4.1.0 20050410 m68k-aout (cygwin host)
testcase:
link.w %fp,#0
move.l 8(%fp),%a0
move.l 8(%a0),%a1
move.l 12(%a0),%d1
cmp.l %a1,%d1
sgt %d0
extb.l %d0
neg.l %d0
cmp.l %a1,%d1
slt %d1
extb.l %d1
neg.l %d1
and.l %d1,%d0
extb.l %d0
unlk %fp
rts
notice that the same cmp.l is executed 2 times
the same happened on the 3.4.1 i386 cygwin build:
3.4.1 x86 cygwin
_testcase:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
popl %ebp
movl 12(%eax), %edx
movl 8(%eax), %ecx
xorl %eax, %eax
cmpl %edx, %ecx
setl %al
cmpl %edx, %ecx
setg %dl
movzbl %dl, %edx
andl %edx, %eax
ret
the x86 build for 4.1.0 optimized the 2. cmp away but...
4.1.0 20050410 cygwin
_testcase:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
movl 8(%eax), %edx
movl 12(%eax), %eax
cmpl %eax, %edx
setl %cl
setg %al
movzbl %al, %eax
andl %ecx, %eax
leave
ret
if we add a second block to make it more complex it returns.
*/
extern char testcase2(int* d)
{
int w23,w13;
w23 = d[2] < d[3];
w23 &= d[3] < d[2];
w13 = d[1] < d[3];
w13 &= d[3] < d[1];
return w23|w13;
}
/*
4.1.0 20050410 cygwin
_testcase:
pushl %ebp
movl %esp, %ebp
pushl %ebx
pushl %ecx
movl 8(%ebp), %eax
movl 8(%eax), %edx
movl 12(%eax), %ecx
cmpl %ecx, %edx ;1
setl %bl
movl 4(%eax), %eax ;2
cmpl %eax, %ecx
setg -5(%ebp)
cmpl %ecx, %edx ;1
setg %dl
andl %ebx, %edx
cmpl %eax, %ecx ;2
setl %al
andb -5(%ebp), %al
orl %edx, %eax
movsbl %al,%eax
popl %edx
popl %ebx
leave
ret
note: if in these redued function we replace the
w23 &= d[3] < d[2];
with
w23 = w23 && (d[3] < d[2])
we get the "correct"
return false;
but in my more complex project these double compares still
show up even with &&;
*/
--
Summary: missed-optimization setcc boolean compare
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: kellivoss25 at yahoo dot de
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21991
