<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] optimizeCompareInstr can use the S flag from the BEXTR instruction which is undefined"
href="https://bugs.llvm.org/show_bug.cgi?id=40060">40060</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[X86] optimizeCompareInstr can use the S flag from the BEXTR instruction which is undefined
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>craig.topper@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
</td>
</tr></table>
<p>
<div>
<pre>With BMI1 this code will use the BEXTR instruction and use the S flag from the
BEXTR instruction directly. But BEXTR doesn't update the S flag with a
meaningful value.
define dso_local void @_Z3foojj(i32, i32) local_unnamed_addr #0 {
%3 = sub i32 32, %1
%4 = lshr i32 -1, %3
%5 = and i32 %4, %0
%6 = icmp sgt i32 %5, -1
br i1 %6, label %7, label %8
tail call void @_Z3barv()
br label %8
ret void
}
Output
foo(unsigned int, unsigned int): # @foo(unsigned
int, unsigned int)
shl esi, 8
bextr eax, edi, esi
js .LBB0_1
jmp bar() # TAILCALL
.LBB0_1:
ret</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>