<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/98462>98462</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] performance inconsistency at optimization level -O2
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          bi6c
      </td>
    </tr>
</table>

<pre>
    ```c
#include<stdio.h>
#include<stdint.h>
int32_t a = 1L;
uint64_t b = 18446744073709551615UL;;
uint64_t c() {
 uint8_t d[8];
  int32_t *e = &a;
  int f;
  for (f = 0; f < 8; f++)
    d[f] = 79;
g:
  *e = 1 ^ d[6];
  b--;
  if (b)
 goto g;
}
void main() {
  c();
  printf("%d\n", a);
 printf("%d\n", b);
}
```
Hello, we are using clang to compile the above code in Ubuntu 22.04.3 LTS.
We found that there is a performance hog when we compile it with llvmorg-18.1.8 and llvmorg-19-init at optimization level 2. 
When we compiled with llvmorg-17.0.6, the execution time was less than 1 second.
```console
$ ../compiler-builds/llvmorg-17.0.6_build/bin/clang -fsanitize=undefined -fsanitize=address -g -lgcc_s --rtlib=compiler-rt -O2 testcase.c -o exec
$ time timeout 5s ./exec
78
0

real    0m0.008s
user 0m0.008s
sys     0m0.000s
$ ../compiler-builds/llvmorg-18.1.8_build/bin/clang -fsanitize=undefined -fsanitize=address -g -lgcc_s --rtlib=compiler-rt -O2 testcase.c -o exec
$ time timeout 5s ./exec

real    0m5.002s
user    0m5.002s
sys 0m0.000s
$ ../compiler-builds/llvmorg-19-init_build/bin/clang -fsanitize=undefined -fsanitize=address -g -lgcc_s --rtlib=compiler-rt -O2 testcase.c -o exec
$ time timeout 5s ./exec

real 0m5.002s
user    0m4.994s
sys     0m0.008s
```
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzUVU2P5CYQ_TX0pWSEsWnbBx9mpreVw0p7SFY5jjCUbSIaWgbPZPbXR9j9OZOJkltWsmyoelX1eC5AhmAGh9gS8UjEbiPnOPqp7cxWbTqv31qyZeujCNsR9kB4YZyys0ZSPIWojacjKb584nTxxmtcLPhzBAmk2EH-lRSPq2M2Lm7L5wjd6qnLcluVJauKijVC5NtcfF_QHwIU4TXhDZDq5IHkqp8jaCIeayJ2lxiAc33CH3ApRPhW3vuhv5n3fgLC637BMlI8Qho-Qb0MCX9cnuYMh6VmT8RuCaiaS6qBFA9n1KV4DkR8WUK29zS7LLsl1ScO3bXO4KOH4YIg1W4dvHij4SCN-yDJWaWbrMfJuNgvZk640EQ8uWX4BPIO-Q_A7hZ4oXHpl3X6C1rrE_oVQU4IczBuAGWlGyB6UP5wNBYhjgiy8y8IymsE4-B7N7s4A-eUlbSAr7_9SteMvyP0fnYa4ihjCpwQTAAJR5x6Px2kUwijH-B1RJfKnmuYCK8mjmDty8FPQ5bXNKc1SKevpiYzzkSQEfwxmoP5IaPxDiy-oAVO4UThPrN-l7eijG7TmtOq8E9U85IkmgPCqwxgMYTE3kEOAZV3mr7TTnkXvMXzpiqBUsL3p3JT1s3G6kD4_r7k82InfN-lJtivImd9kM5E8wNJsZudxt441PdmqfWUOGUDZHZQ6jlAlk3Rmo4Uu0vVKUL2jUPEEJUMSBVkflneleayxPTycwQRILG-Iqp6_Z5aY31PKG3aO-zAKGN1OG3wgNM7U3gLcAWy8O_FWf7z_12c92oIyhi_VeODNQny38VYO_wnUuMzKUraNOXf9sa5Yy7baaPbQjdFIzfY5hVn20Y0eb0ZWyUrrFCzvOhkr4pa5F2FggnZy0IXvN6YljNesirP85yJvKJ5pfqqq3LV16yqEUnJ8CCNpUlg6qdhY0KYsW3qcss3VnZow3K3cr6InM5OsdtMbcJn3TwEUjJrQgzXDNFEu1zIa4TY3R1txqXTwYSITr19clJl3_hmnmw7xngM6fLhe8L3g4nj3FHlD6eGOH2y4-T_QBUJ3y_kU7-s_F9a_lcAAAD__wf0Yro">