https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116546
Bug ID: 116546
Summary: Missed optimization of redundant comparison
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: bergner at gcc dot gnu.org
Target Milestone: ---
In the following test case, the "n & 4" test is redundant and should be
eliminated, since the "n &= 7;" statement limits n's potential values to [0,7].
The "n >= 4" test further narrows its potential values to [4,7], making the "n
& 4" test always true, allowing the elimination of the code path calling bar().
bergner@ltcden2-lp1:~$ cat test.c
extern long foo (void);
extern long bar (void);
long
test (long n)
{
n &= 7;
if (n >= 4) {
if (n & 4)
return foo ();
else
return bar ();
}
return 0;
}
Current gcc (-O2) does optimize this on powerpc64le-linux and removes the bar()
code path. However, if we change the "n >= 4" test to either "n > 4" or "n ==
4", then we fail to eliminate the code path to bar(), even though the "n & 4"
test remains redundant.
Using -O2 -mcpu=power10 (-mcpu=power10 allows tail calls making the resulting
asm easier to read, but it isn't required) produces for the "n >= 4" test:
test:
andi. 3,3,0x4
bne 0,.L4
li 3,0
blr
.p2align 4,,15
.L4:
b foo@notoc
Where as the "n > 4" test results in:
test:
rldicl 2,3,0,61
cmpdi 0,2,4
ble 0,.L2
andi. 3,3,0x4
beq 0,.L3
b foo@notoc
.p2align 4,,15
.L2:
li 3,0
blr
.p2align 4,,15
.L3:
b bar@notoc
Interestingly to me, the optimized code keeps the inner redundant "n & 4" test
(andi. 3,3,0x4) and removes the outer tests. I didn't expect that.
