http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50272
Bug #: 50272
Summary: A case that PRE optimization hurts performance
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: jiangning....@arm.com
For the following simple test case, PRE optimization hoists computation
(s!=1) into the default branch of the switch statement, and finally causes
very poor code generation. This problem occurs in both X86 and ARM, and I
believe it is also a problem for other targets.
int f(char *t) {
int s=0;
while (*t && s != 1) {
switch (s) {
case 0:
s = 2;
break;
case 2:
s = 1;
break;
default:
if (*t == '-')
s = 1;
break;
}
t++;
}
return s;
}
Taking X86 as an example, with option "-O2" you may find 52 instructions
generated like below,
00000000 <f>:
0: 55 push %ebp
1: 31 c0 xor %eax,%eax
3: 89 e5 mov %esp,%ebp
5: 57 push %edi
6: 56 push %esi
7: 53 push %ebx
8: 8b 55 08 mov 0x8(%ebp),%edx
b: 0f b6 0a movzbl (%edx),%ecx
e: 84 c9 test %cl,%cl
10: 74 50 je 62 <f+0x62>
12: 83 c2 01 add $0x1,%edx
15: 85 c0 test %eax,%eax
17: 75 23 jne 3c <f+0x3c>
19: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
20: 0f b6 0a movzbl (%edx),%ecx
23: 84 c9 test %cl,%cl
25: 0f 95 c0 setne %al
28: 89 c7 mov %eax,%edi
2a: b8 02 00 00 00 mov $0x2,%eax
2f: 89 fb mov %edi,%ebx
31: 83 c2 01 add $0x1,%edx
34: 84 db test %bl,%bl
36: 74 2a je 62 <f+0x62>
38: 85 c0 test %eax,%eax
3a: 74 e4 je 20 <f+0x20>
3c: 83 f8 02 cmp $0x2,%eax
3f: 74 1f je 60 <f+0x60>
41: 80 f9 2d cmp $0x2d,%cl
44: 74 22 je 68 <f+0x68>
46: 0f b6 0a movzbl (%edx),%ecx
49: 83 f8 01 cmp $0x1,%eax
4c: 0f 95 c3 setne %bl
4f: 89 df mov %ebx,%edi
51: 84 c9 test %cl,%cl
53: 0f 95 c3 setne %bl
56: 89 de mov %ebx,%esi
58: 21 f7 and %esi,%edi
5a: eb d3 jmp 2f <f+0x2f>
5c: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
60: b0 01 mov $0x1,%al
62: 5b pop %ebx
63: 5e pop %esi
64: 5f pop %edi
65: 5d pop %ebp
66: c3 ret
67: 90 nop
68: b8 01 00 00 00 mov $0x1,%eax
6d: 5b pop %ebx
6e: 5e pop %esi
6f: 5f pop %edi
70: 5d pop %ebp
71: c3 ret
But with command line option "-O2 -fno-tree-pre", there are only 12
instructions generated, and the code would be very clean like below,
00000000 <f>:
0: 55 push %ebp
1: 31 c0 xor %eax,%eax
3: 89 e5 mov %esp,%ebp
5: 8b 55 08 mov 0x8(%ebp),%edx
8: 80 3a 00 cmpb $0x0,(%edx)
b: 74 0e je 1b <f+0x1b>
d: 80 7a 01 00 cmpb $0x0,0x1(%edx)
11: b0 02 mov $0x2,%al
13: ba 01 00 00 00 mov $0x1,%edx
18: 0f 45 c2 cmovne %edx,%eax
1b: 5d pop %ebp
1c: c3 ret
