| Issue |
147239
|
| Summary |
suboptimal code for switch statement
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
bhaible
|
In a `switch` statement with identical code in two branches, clang 20 (on x86_64) produces suboptimal code (with more conditional jumps than needed).
**How to reproduce:**
Save this file as foo.c:
```
int foo (int c)
{
int p = 0;
switch (c)
{
case 'c': case 's':
p = 1;
break;
case 'f': case 'F': case 'e': case 'E': case 'g': case 'G':
case 'a': case 'A':
p = 1;
break;
}
return p;
}
```
Run this command:
```
$ clang -O2 -c foo.c && objdump -d -r foo.o
```
Actual output:
```
foo.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo>:
0: 31 c0 xor %eax,%eax
2: 83 c7 bf add $0xffffffbf,%edi
5: 83 ff 32 cmp $0x32,%edi
8: 77 15 ja 1f <foo+0x1f>
a: 48 b9 71 00 00 00 71 movabs $0x7100000071,%rcx
11: 00 00 00
14: 48 0f a3 f9 bt %rdi,%rcx
18: 73 06 jae 20 <foo+0x20>
1a: b8 01 00 00 00 mov $0x1,%eax
1f: c3 ret
20: 48 b9 00 00 00 00 04 movabs $0x4000400000000,%rcx
27: 00 04 00
2a: 48 0f a3 f9 bt %rdi,%rcx
2e: 72 ea jb 1a <foo+0x1a>
30: eb ed jmp 1f <foo+0x1f>
```
Expected output:
```
foo.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <foo>:
0: 31 c0 xor %eax,%eax
2: 83 c7 bf add $0xffffffbf,%edi
5: 83 ff 32 cmp $0x32,%edi
8: 77 15 ja 1f <foo+0x1f>
a: 48 b9 71 00 00 00 75 movabs $0x4007500000071,%rcx
11: 00 04 00
14: 48 0f a3 f9 bt %rdi,%rcx
18: 73 05 jae 1f <foo+0x1f>
1a: b8 01 00 00 00 mov $0x1,%eax
1f: c3 ret
```
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