item_cmp_type: code shortcut path #223
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The common case for this function is comparisions of the same type. In this case the result is the same type. As the compiler sees this enum with a potential value of INVALID_RESULT, the equality optimization isn't obvious. With this code change, the compiled code goes from: 0000000000000000 <_Z13item_cmp_type11Item_resultS_>: 0: 55 push %rbp 1: 89 f9 mov %edi,%ecx 3: 31 c0 xor %eax,%eax 5: 09 f1 or %esi,%ecx 7: 48 89 e5 mov %rsp,%rbp a: 74 3d je 49 <_Z13item_cmp_type11Item_resultS_+0x49> c: 8d 47 fe lea -0x2(%rdi),%eax f: 83 e0 fd and $0xfffffffd,%eax 12: 89 c2 mov %eax,%edx 14: 8d 46 fe lea -0x2(%rsi),%eax 17: 83 e0 fd and $0xfffffffd,%eax 1a: 83 ff 02 cmp $0x2,%edi 1d: 89 c1 mov %eax,%ecx 1f: 75 0a jne 2b <_Z13item_cmp_type11Item_resultS_+0x2b> 21: 83 fe 02 cmp $0x2,%esi 24: b8 02 00 00 00 mov $0x2,%eax 29: 74 1e je 49 <_Z13item_cmp_type11Item_resultS_+0x49> 2b: 83 ff 03 cmp $0x3,%edi 2e: 74 20 je 50 <_Z13item_cmp_type11Item_resultS_+0x50> 30: 83 fe 03 cmp $0x3,%esi 33: 74 1b je 50 <_Z13item_cmp_type11Item_resultS_+0x50> 35: 85 d2 test %edx,%edx 37: b8 01 00 00 00 mov $0x1,%eax 3c: 75 0b jne 49 <_Z13item_cmp_type11Item_resultS_+0x49> 3e: 83 f9 01 cmp $0x1,%ecx 41: 19 c0 sbb %eax,%eax 43: 83 e0 03 and $0x3,%eax 46: 83 c0 01 add $0x1,%eax 49: 5d pop %rbp 4a: c3 retq 4b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 50: b8 03 00 00 00 mov $0x3,%eax 55: 5d pop %rbp 56: c3 retq to: 0000000000000000 <_Z13item_cmp_type11Item_resultS_>: 0: 89 f8 mov %edi,%eax 2: 39 f7 cmp %esi,%edi 4: 74 2f je 35 <_Z13item_cmp_type11Item_resultS_+0x35> 6: 83 ff 03 cmp $0x3,%edi 9: 74 25 je 30 <_Z13item_cmp_type11Item_resultS_+0x30> b: 83 fe 03 cmp $0x3,%esi e: 74 20 je 30 <_Z13item_cmp_type11Item_resultS_+0x30> 10: 8d 57 fe lea -0x2(%rdi),%edx 13: b8 01 00 00 00 mov $0x1,%eax 18: 83 e2 fd and $0xfffffffd,%edx 1b: 75 18 jne 35 <_Z13item_cmp_type11Item_resultS_+0x35> 1d: 83 ee 02 sub $0x2,%esi 20: 83 e6 fd and $0xfffffffd,%esi 23: 83 fe 01 cmp $0x1,%esi 26: 19 c0 sbb %eax,%eax 28: 83 e0 03 and $0x3,%eax 2b: 83 c0 01 add $0x1,%eax 2e: c3 retq 2f: 90 nop 30: b8 03 00 00 00 mov $0x3,%eax 35: c3 retq In addition to the shorted path, there are no stack operations, and the common case, the branch at 4:, becomes easily predicted with 4 instuctions usually executed.
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The common case for this function is comparisions of the same
type. In this case the result is the same type. As the compiler
sees this enum with a potential value of INVALID_RESULT, the
equality optimization isn't obvious. With this code change, the
compiled code goes from:
0000000000000000 <Z13item_cmp_type11Item_resultS>:
0: 55 push %rbp
1: 89 f9 mov %edi,%ecx
3: 31 c0 xor %eax,%eax
5: 09 f1 or %esi,%ecx
7: 48 89 e5 mov %rsp,%rbp
a: 74 3d je 49 <Z13item_cmp_type11Item_resultS+0x49>
c: 8d 47 fe lea -0x2(%rdi),%eax
f: 83 e0 fd and $0xfffffffd,%eax
12: 89 c2 mov %eax,%edx
14: 8d 46 fe lea -0x2(%rsi),%eax
17: 83 e0 fd and $0xfffffffd,%eax
1a: 83 ff 02 cmp $0x2,%edi
1d: 89 c1 mov %eax,%ecx
1f: 75 0a jne 2b <Z13item_cmp_type11Item_resultS+0x2b>
21: 83 fe 02 cmp $0x2,%esi
24: b8 02 00 00 00 mov $0x2,%eax
29: 74 1e je 49 <Z13item_cmp_type11Item_resultS+0x49>
2b: 83 ff 03 cmp $0x3,%edi
2e: 74 20 je 50 <Z13item_cmp_type11Item_resultS+0x50>
30: 83 fe 03 cmp $0x3,%esi
33: 74 1b je 50 <Z13item_cmp_type11Item_resultS+0x50>
35: 85 d2 test %edx,%edx
37: b8 01 00 00 00 mov $0x1,%eax
3c: 75 0b jne 49 <Z13item_cmp_type11Item_resultS+0x49>
3e: 83 f9 01 cmp $0x1,%ecx
41: 19 c0 sbb %eax,%eax
43: 83 e0 03 and $0x3,%eax
46: 83 c0 01 add $0x1,%eax
49: 5d pop %rbp
4a: c3 retq
4b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
50: b8 03 00 00 00 mov $0x3,%eax
55: 5d pop %rbp
56: c3 retq
to:
0000000000000000 <Z13item_cmp_type11Item_resultS>:
0: 89 f8 mov %edi,%eax
2: 39 f7 cmp %esi,%edi
4: 74 2f je 35 <Z13item_cmp_type11Item_resultS+0x35>
6: 83 ff 03 cmp $0x3,%edi
9: 74 25 je 30 <Z13item_cmp_type11Item_resultS+0x30>
b: 83 fe 03 cmp $0x3,%esi
e: 74 20 je 30 <Z13item_cmp_type11Item_resultS+0x30>
10: 8d 57 fe lea -0x2(%rdi),%edx
13: b8 01 00 00 00 mov $0x1,%eax
18: 83 e2 fd and $0xfffffffd,%edx
1b: 75 18 jne 35 <Z13item_cmp_type11Item_resultS+0x35>
1d: 83 ee 02 sub $0x2,%esi
20: 83 e6 fd and $0xfffffffd,%esi
23: 83 fe 01 cmp $0x1,%esi
26: 19 c0 sbb %eax,%eax
28: 83 e0 03 and $0x3,%eax
2b: 83 c0 01 add $0x1,%eax
2e: c3 retq
2f: 90 nop
30: b8 03 00 00 00 mov $0x3,%eax
35: c3 retq
In addition to the shorted path, there are no stack operations,
and the common case, the branch at 4:, becomes easily predicted
with 4 instuctions usually executed.