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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Dead Code Elimination Regression at -O3 (trunk vs 13.0.0)"
href="https://bugs.llvm.org/show_bug.cgi?id=52078">52078</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Dead Code Elimination Regression at -O3 (trunk vs 13.0.0)
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>theodoros.theodoridis@inf.ethz.ch
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>cat case.c
static int b = -1, e = 1;
static short c, d;
void foo(void);
short a(unsigned short f, int g) { return f >> g; }
int main() {
c++;
d = a(4294967295 + (c > 0), 1);
e ^= (short)(d * 3) / (unsigned)b;
if (!e)
foo();
}
trunk can no longer eliminate foo and it generates more complicated code:
clang-13 -v
clang version 13.0.0 (<a href="https://github.com/llvm/llvm-project.git">https://github.com/llvm/llvm-project.git</a>
d7b669b3a30345cfcdb2fde2af6f48aa4b94845d)
Target: x86_64-unknown-linux-gnu
clang-13 -S case.c -O3 -o /dev/stdout
...
main: # @main
.cfi_startproc
# %bb.0:
addw $1, c(%rip)
xorl %eax, %eax
retq
clang-trunk -v
clang version 14.0.0 (<a href="https://github.com/llvm/llvm-project.git">https://github.com/llvm/llvm-project.git</a>
424733c12aacc227a28114deba72061153f8dff2)
Target: x86_64-unknown-linux-gnu
clang-trunk -S case.c -O3 -o /dev/stdout
...
main: # @main
.cfi_startproc
# %bb.0:
pushq %rax
.cfi_def_cfa_offset 16
movzwl c(%rip), %eax
leal 1(%rax), %ecx
movw %cx, c(%rip)
xorl %ecx, %ecx
cmpl $32767, %eax # imm = 0x7FFF
adcl $-1, %ecx
movzwl %cx, %eax
shrl %eax
leal (%rax,%rax,2), %eax
xorl %ecx, %ecx
cmpw $-1, %ax
sete %cl
xorl %ecx, e(%rip)
je .LBB1_1
# %bb.2:
xorl %eax, %eax
popq %rcx
.cfi_def_cfa_offset 8
retq
.LBB1_1:
.cfi_def_cfa_offset 16
callq foo
xorl %eax, %eax
popq %rcx
.cfi_def_cfa_offset 8
retq
commit cfb6dfcbd13b417f83d8adca69b60ba4254b4233 introduced this regression</pre>
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