https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91384
Bug ID: 91384
Summary: Compare with negation is not eliminated
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ubizjak at gmail dot com
Target Milestone: ---
Following testcase:
--cut here--
void foo (void);
void bar (void);
int
test (int a)
{
int r;
if (r = -a)
foo ();
else
bar ();
return r;
}
--cut here--
results in (-O2):
movl %edi, %r12d
negl %r12d
testl %edi, %edi
je .L2
The combine pass can't do anything, since it sees:
(insn 6 3 7 2 (parallel [
(set (reg/v:SI 82 [ <retval> ])
(neg:SI (reg/v:SI 83 [ a ])))
(clobber (reg:CC 17 flags))
]) "neg.c":9:9 464 {*negsi2_1}
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil)))
(insn 7 6 8 2 (set (reg:CCZ 17 flags)
(compare:CCZ (reg/v:SI 83 [ a ])
(const_int 0 [0]))) "neg.c":9:6 7 {*cmpsi_ccno_1}
(expr_list:REG_DEAD (reg/v:SI 83 [ a ])
(nil)))
Please note that the compare is with the original value (reg: 83), not with the
result of the negation (reg: 82). Tree optimizers give us:
r_3 = -a_2(D);
if (a_2(D) != 0)
goto <bb 3>; [50.00%]
else
goto <bb 4>; [50.00%]
This is a regression from 4.1.2, where the compiler is able to eliminate the
compare:
movl %esi, %ebx
negl %ebx
je .L2
