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List:       llvm-bugs
Subject:    [llvm-bugs] [Bug 63625] Dead Code Elimination Regression when using __builtin_unreachable (trunk vs 
From:       LLVM Bugs via llvm-bugs <llvm-bugs () lists ! llvm ! org>
Date:       2023-06-30 16:44:09
Message-ID: 20230630164409.2bd2e046dd109ff4 () email ! llvm ! org
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<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63625>63625</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Dead Code Elimination Regression when using __builtin_unreachable (trunk \
vs 16)  </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    https://godbolt.org/z/oecxca7hK

Given the following code:

```C
void foo(void);
static char e = 3, i;
static unsigned f = 6;
static int g, h = -1L;
static char(a)(char b, char c) { return c == 0 ?: b / c; }
static short(d)(short c) {
 if (!(((c) &gt;= -1) &amp;&amp; ((c) &lt;= 0))) {
        __builtin_unreachable();
 }
    return 0;
}
int main() {
    char j = 0;
    if (0 &gt;= f) {
        e = 0;
        for (; e; ++e) {
            foo();
 j = a(1, j) != g;
        }
        d(4);
    }
 d(i);
    d(h);
    a(0, -1L);
    f = 0;
}

```
clang-trunk -O2 does not eliminate the call to foo:

```asm
main:                                   # @main
# %bb.0:
	pushq	%rax
	cmpb	$1, f(%rip)
	jne	.LBB0_3
# %bb.1:
	xorl	%eax, %eax
	.p2align	4, 0x90
.LBB0_2: # =&gt;This Inner Loop Header: Depth=1
	movb	%al, e(%rip)
	callq	foo@PLT
	movzbl	e(%rip), %eax
	incb	%al
	jmp	.LBB0_2
.LBB0_3:
	movb	$1, f(%rip)
	xorl	%eax, %eax
	popq	%rcx
	retq
.Lfunc_end0:
```

clang-16.0.6 -O2 eliminates the call to foo:

```asm
main: # @main
# %bb.0:
	movb	$1, f(%rip)
	xorl	%eax, %eax
	retq
.Lfunc_end0:
```

Bisects to 946b32680311f43a349d0199f9e286f385cd9847 (@floatshadow )
</pre>
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